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It's happened AGAIN ! grrr...

lovejoy

Regular user
Hi to all you knowledgeable C4 enthusiasts, I had this problem last year, and posted the thread below about it:

I thought the problem was solved, but last Friday, went for a run to a local venue, parked for a few hours, NO START

again. Fires up immediately, then dies instantly. Had been starting & running fine since last September.

I had the vette recovered back home, but as before, cannot read any codes by linking the A & B connectors on the ALDL 12 pin connector, below steering wheel,

I am wondering if it's an intermittent connection in the ignition switch for the key resistor, the VATS security system?

Will an OBD II reader give me any info if I can connect it to the OBD1 port with an adaptor cable?

Failing that, has anyone the details of how/where to bypass the VATS with a resistor below the steering collumn?

Any help much appreciated, and I hope, if finally sorted, this will help others who might suffer the same issue,

I am losing faith in being able to rely on such an enjoyable vette (when it is running!!!)

Thanks for any guidance, once again...
 

Attachments

Last edited:

lovejoy

Regular user
Hi Phil, it fires up, so enough fuel gets to fuel rail to start,
supposedly about 40 psi to do that, but I don't have access to a
pressure gauge to try on the schraeder valve, you can hear the pump
run for 2 seconds when ignition turned on... it just immediately dies again...
 

lovejoy

Regular user
there is supposed to be a test point, for the fuel pump feed, "near the battery",
as it says in the Factory Service Manual, of which I have a copy, but I can't see it
anywhere !
 

PhilV8

CCCUK Member
Have a look at 4vetteregistry.com/vrforums/c4-general-discussion/my-88-cranks-runs-for-5-seconds-then-dies/444194/
Guy with the same problem, talks about pressure switches etc, might be of some use.
 

teamzr1

Well-known user
OBD-II does use same protocol as OBD-I so cannot mix the 2 types of scanners
If you have a spare Ign key try that in case the VATs resister on key is the issue
Best via tuning the engine calibration via the E-prom is turn VATs off
See if this helps you, esp of the oil sender or HEI is the issue

L-98 Engine Start Sequence

When an L-98 engine is in startup process, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.

Fuel Rail Pressurization:

When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails.

There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure.
The reading will go to 38-40 pounds nominal once the engine is running.test by attaching a fuel pressure gauge to the fuel rail at the shrader valve, on TPI and LT1 engines its located on the pass side fuel rail.

Initial Crank Action:

If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.

Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run. (Note that you should have a black oil pressure switch/sender.
It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad units that stayed in the GM parts pipeline for some time).

The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter.
These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.

ECM Reaction:

If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods.
(In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).

The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to start and run unless you can’t make at least 4 PSI oil pressure.
This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).

The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54 volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts during start and at idle from the TPS is very important to both starting and run performance.)

Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.

Engine "Catches":
When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how much air is being pulled through to the intake manifold.

The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms nominally so as to attain a proper air/fuel mixture to insure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).

The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.

If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be mis-adjusted.
Alternatively, there may be a leak in the intake manifold or another vacuum leak may be present. Listen for hissing sounds---there should be none.

ECM Mode:

The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.

Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.

Note that prolonged idling can force the computer back into open loop mode.

Note:

In 1990, the MAF was eliminated from the engine in favor of a speed/density system.
This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold.

This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.

Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system.

Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly.
A multitude of things can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.
 
Last edited:

teamzr1

Well-known user
Summary:

If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.

A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems.

Contact us for one of the best scanners on the market or visit our frontstore.

Most of the C4 Corvettes used a MAF (Mass Air Flow) sensor to determine how much air is being pulled into the intake manifold.
The exceptions are the 1984 Corvette that used a speed density system--a sort of predictive method of measurement---and the 1990 through 1993 C4 models which were also speed density based.
In 1994, Corvette went back to the MAF based system but used the speed density approach as a back up. (1989 Bosch MAF installation shown at right).

A Mass Air Flow sensor has an extremely fine wire inside its bore.
The 1985 through 1989 C4 engines used a Bosch MAF sensor that heated the wire to 100 C. The 1994 and later C4 models used a AC/Delco MAF that heated the wire to 200 C. The amount of current required to reach the temperature is measured in each case.
(Note: the LT-5 engine used in the ZR-1 used a speed density system and continued to use that system in 1994 and 1995 since the engines had already been made prior to the last two years of production. The ZR-1 therefore has no MAF even after Corvette went back to the MAF based system).

Theory of Operation

As the air travels past the heated wire enroute to the intake manifold, it will cool the wire and additional current is added to again heat the wire to the design temperature.

Since the amount of air moving past the sensor is directly related to the amount of cooling experienced by the heated wire, a feedback condition is established whereby the exact amount of moving air is directly related to the amount of current passing through the wire and the intake air is therefore precisely measured.

Once the amount of air is known, the computer controlling the engine can add or subtract fuel as required to maintain the magic 14.7:1 air-fuel mixture resulting in the cleanest burn possible from an emissions (pollution) standpoint.

It does this by varying the "on time" of the fuel injectors. The injectors are pulsed on and off and the width of the pulse is lengthened or shortened as required.

When you first start a typical engine, the pulse width is around 4 milliseconds but as soon as the engine "catches" the pulse width is shortened to about 2.2 milliseconds for idle. During operation, the measured air flow through the MAF will cause the computer to increase or decrease the pulse width as explained above.

MAF Operating Conditions

The Bosch MAF is more complex than the AC/Delco version. Both measure the air flow but the Bosch MAF has a circuit called the 'burn-off circuit' that cycles on for about 2 seconds when you shut the engine down.

This circuit heats the wire to a high enough temperature to burn off any residue that may have collected on the wire during operation.
If you are in a quiet area, you can hear the relays click on and then off on a 1985-1989 C4 as the burn-off cycle occurs.

There are two relays involved with the Bosch MAF: A power relay that passes current to the MAF wire during normal operation and the burn-off relay that provides the current for the cleaning cycle.

Both are located on the firewall in the engine compartment, just behind the battery on the drivers side. Bad MAF power and burn-off relays can cause hard starting problems and should be changed periodically as preventative measure and any time you experience hard starting conditions.

The AC/Delco MAF has a power relay but no burn-off relay. For this reason, you should pay even closer attention to the condition of your air filter on a later model C4 than normal since a contaminated wire in a AC/Delco MAF is going to stay contaminated for the most part and cause false signals to be passed to the computer.

Also, the Bosch MAF outputs its information as a analog signal to the computer but the AC/Delco sends its signal as a digital component of varying frequency.
For this reason, you cannot measure it's operation directly.

A scan tool is generally the best way to troubleshoot engine problems and with the 1994 and later Corvette, it is virtually mandatory. (An oscilloscope will also work on the AC/Delco MAF but a regular test meter will not).

MAF Problems

Faulty MAF sensors will normally light the check engine light on the drivers information center if the problem is constant and store a trouble code.
If intermittent, a trouble code will still be stored as long as the battery is not disconnected.

Normally, the problem is a poor connection at the sensor and wiggling the wires, unplugging and reinserting the connector will often cure the problem.

A faulty MAF will normally cause a no start or difficult start condition and although you can eventually get the car into the "limp-home" mode in most cases, you need to attend to the problem ASAP.

This flow chart might help.

Fuel System Layout

AC/Delco sensors can become intermittent or give false readings if the wires become contaminated as explained above.

The MAF is a critical part of the emission control system and as such will cause the computer to react to problems very quickly, setting trouble codes and reducing performance in ways that cannot be ignored for long.

MAF Mods

The Bosch MAF is often modified by removing the two screens that are present in the front and rear of the cylinder. Removing these screens significantly increases the air flow through them and this results in more horsepower.

Removing the screens is an old trick from the Corvette Challenge days in 1988 and 1989. It does work but is illegal in many states so be advised not to do anything that will get you arrested for a pollution violation.

The AC/Delco MAF is not readily modified. It is what it is but since it is a larger diameter than the Bosch, it responds well to changing the air filter to a free flowing type such as the K&N filter.

Welcome to C4 vette codes it is very ....repeat very important that if you are not savvy of working on your Vette ...you would be better off - taking your car to a dealership for repairs on your trouble codes.

However if you feel that you want to dive right in ..than you have come to the right place. First locate your car's alcl this component is located just below the instrument panel and to the left of the center console.
 

teamzr1

Well-known user
Remove the plastic cover the first two slots to your right are the A & B slots for a drawing of the alcl module's picture is added below.

The A slot is the diagnostic slot and the B slot is the ground slot. insert the computer key into these slots (with the engine off) this is very important...now only put the ignition key to on ( not start !!!) the check engine light will display a code 12 which is one flash followed by two flashes. This code will be flashed three times ..followed by the trouble code stored in your car's computer.

What ever the code is it will be flashed three times.
Have a paper and pencil ready and write down the code.

code 13 =1 flash followed by 3 flashes =>oxygen sensor
code 14 =1 flash followed by 4 flashes =>coolant sensor
code 15 =1 flash followed by 5 flashes =>coolant sensor
code 21 = 2 flashes followed by 1 flash =>throttle position sensor
code 22 = 2 flashes followed by 2 flashes=> throttle position sensor
code 23 = 2 flashes followed by 3 flashes=> manifold air temp sensor
code 24 = 2 flashes followed by 4 flashes=> vehicle speed sensor
code 25 = 2 flashes followed by 5 flashes=> manifold air temp sensor
code 32 =>egr system
code 33 =>map sensor
code 34 =>maf sensor
code 35 => idle air control
code 41 => cylinder select error
code 42 => electronic spark control
code 43 => electronic spark control
code 44 => lean exhaust
code 45 => rich exhaust
code 51 => PROM
code 52 => fuel calpak
code 53 => system over voltage
code 54 => fuel pump circuit
code 55 => ecm
code 62 => oil temp

Please remember that if you have the computer key installed in the alcl and you start the engine (you will ruin the engine's computer) only put the ignition to on (not to start).

If you should get a check engine soon display.. you can use the above procedure and codes to buy the right part or at the very least to keep from getting taken for a ride and be made to pay hight prices for some inexpensive module that you could have installed yourself.

If your engine displays a trouble code ... your engine will go into limp mode ..it will still run but very poorly.

You might be able to reset the computer if it will not start (just to get home) by disconnecting both battery cables and re-installing them ...this is not recommended ..but if you are stranded it might help unitl you get your car home or to a repair shop..good luck.

1985 TO 1991:

Code #12: Normal No Codes.
Code #13: Open Oxygen Sensor Circuit.
Code #14: Coolant Sensor Circuit Low.
Code #15: Coolant Sensor Circuit High.
Code #21: Throttle Position Sensor High.
Code #22: Throttle Position Sensor Low.
Code #23: Manifold Air Temperature Circuit High.
Code #24: Vehicle Speed Sensor.
Code #25: Manifold Air Temperature Circuit Low.
Code #32: EGR System Failure.
Code #33: Mass Air Flow Sensor High.
Code #34: Mass Air Flow Sensor Low.
Code #36: Mas Air Flow Sensor Burn-Off Function Fault.
Code #41: Cylinder Select Error.
Code #42: Electronic Spark Timing.
Code #43: Electronic Spark Control.
Code #44: Lean Exhaust indication.
Code #45: Rich Exhaust Indication.
Code #46: Vehicle Anti Theft Fault.
Code #51: Faulty Mem-Cal.
Code #52: Fuel Calpak Missing.
Code #52(1990-91 Corvette Only): Engine Oil Temperature Sensor Low.
Code #53: System Over Voltage.
Code #54: Fuel Pump Circuit Low Voltage.
Code #55: Defective ECM.
Code #62: Engine Oil Temperature Sensor Circuit High.

ECM CODES 1992 TO 1993:

Code #12: Normal No Codes.
Code #13: Left Oxygen Sensor Circuit.
Code #14: Coolant Temperature Sensor Circuit High.
Code #15: Coolant Temperature Sensor Circuit Low.
Code #16: Opti-Spark Ignition Timing System.( Low Pulse)
Code #21: Throttle Position Sensor Circuit High.
Code #22: Throttle Position Sensor Circuit Low.
Code #23: Intake Air Temperature Sensor Circuit Low.
Code #24: Vehicle Speed Sensor Circuit.
Code #25: Intake Temperature Sensor Circuit High.
Code #26: Quad-Driver Module #1 Circuit.
Code #27: Quad-Driver Module #2 Circuit.
Code #28: Quad-Driver Module #3 Circuit.
Code #32: Exhaust Gas Recirclation Circuit.
Code #33: Manifold Absolute Pressure Sensor Circuit Low.
Code #34: Manifold Absolute Pressure Sensor Circuit High.
Code #36: Opti-Spark Ignition Timing System. (High Resolution Pulse.)
Code #41: Electronic Spark Timing Circuit Open.
Code #42: Electronic Spark Timing Circuit Grounded.
Code# 43: Electronic Spark Control Circuit.
Code #44: Left Oxygen Sensor Circuit Lean.
Code #45: Left Oxygen Sensor Circuit Rich.
Code #51: Mem-Cal Error.
Code #52: Engine Oil Temperature Sensor Circuit Low.
Code #53: System Voltage.
Code #55: Fuel Lean Monitor.
Code #56: Vacuum Sensor Circuit.
Code #61: Secondary Port Throttle Valve System.
Code #62: Engine Oil Temperature Sensor Circuit High.
Code #63: Right Oxygen Sensor Circuit Open.
Code #64: Right Oxygen Sensor Circuit Lean.
Code #65: Right Oxygen Sensor Circuit Rich.
Code #66: A/C Pressure Sensor Circuit Open.
Code #67: A/C Pressure Sensor Circuit. (Sensor or A/C Clutch Circuit Problem)
Code #68: A/C Relay Circuit Shorted.
Code #69: A/C Clutch Circuit.
Code #72: Gear Selector Switch Circuit.

CODES 1994 TO 1996:

DTC #11: Malfunction Indicator Lamp Circuit.
DTC #13: Bank #1 Heated Oxygen Sensor #1 Circuit.
DTC #14: Engine Coolant Temperature Sensor Circuit Voltage Low.
DTC #15: Engine Coolant Temperature Sensor Circuit Voltage High.
DTC #16: Distributor Ignition System Low Pulse.
DTC #18: Injector Circuit.
DTC #21: Throttle Position Sensor Circuit Voltage High.
DTC #22: Throttle Position Sensor Circuit Voltage Low.
DTC #23: Intake Temperature Sensor Circuit Voltage High.
DTC #24: Vehicle Speed Sensor Circuit.
DTC #25: Intake Air Temperature Sensor Circuit Voltage Low.
DTC #26: Evaporative Emission Canister Purge Solenoid Valve Circuit.
DTC #27: EGR Vacuum Control Signal Solenoid Valve Circuit.
DTC #28: Transmission Range Pressure Switch Assembly Fault.
DTC #29: Secondary Air Injection Pump Circuit.
DTC #32: Exhaust Gas Recalculation.
DTC #33: Manifold Absolute Pressure Sensor Circuit High.
DTC #34: Manifold Absolute Pressure Sensor Circuit Low.
DTC #36: Distributor Ignition System High Pulse.
DTC #37: Brake Switch Stuck On.
DTC #38: Brake Switch Stuck Off.
DTC #41: Ignition Control Circuit Open.
DTC #42: Ignition Control Circuit Shorted.
DTC #43: Knock Sensor Circuit.
DTC #44: Bank 1 LF Heated Oxygen Sensor #1 Circuit Lean.
DTC #45: Bank 1 LF Heated Oxygen Sensor #1 Circuit Rich.
DTC #47: Knock Sensor Circuit Or Module Missing.
DTC #48: Mass Air Flow Sensor Circuit.
DTC #50: System Voltage Low.
DTC #51: EEPROM Programming Error.
DTC #52: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #53: System Voltage Low.
DTC #55: Fuel Lean Monitor.
DTC #58: Transmission Fluid Temperature Sensor Circuit Low.
DTC #59: Transmission Fluid Temperature Sensor Circuit High.
DTC #62: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #63: Bank 2 RF Heated Oxygen Sensor #1 Circuit Open.
DTC #64: Bank 2 RF Heated Oxygen Sensor #1 Circuit Lean.
DTC #65: Bank 2 RF Heated Oxygen Sensor #1 Circuit Rich.
DTC #66: A/C Refrigerant Pressure Sensor Circuit Open.
DTC #67: A/C Pressure Sensor Circuit Sensor or A/C Clutch.
DTC #68: A/C Relay Circuit.
DTC #69: A /C Clutch Circuit.
DTC #70: A/C Clutch Relay Driver Circuit.
DTC #72: Vehicle Speed Sensor Loss.
DTC #73: Pressure Control Solenoid Circuit Current Error.
DTC #74: Traction Control System Circuit Low.
DTC #75: Transmission System Voltage Low.
DTC #77: Primary Cooling Fan Relay Control Circuit.
DTC #78: Secondary Cooling Fan Relay Control Circuit.
DTC #79: Transmission Fluid Overtemp.
DTC #80: Transmission Component Slipping.
DTC #81: Transmission 2-3 Shift Solenoid Circuit.
DTC #82: Transmission 1-2 Shift Solenoid Circuit.
DTC #83: Torque Converter Solenoid Voltage High.
DTC #84: 3-2 Control Solenoid Circuit.(Auto Only).
DTC #84: 2nd And 3rd Gear Blockout Relay Control Circuit.
DTC #85: Transmission TCC Stock On.
DTC #90: Transmission TCC Solenoid Circuit.
DTC #91: One To Four Upshift Lamp(Manual Only).
DTC #97: VSS Output Circuit.
DTC #98: Tachometer Output Signal Voltage Wrong.
 

teamzr1

Well-known user
The resister pellet on key would measure one of the resistance below
You will have to determine the resistance your system is using.
A ohm meter across the pellet in your key will soon tell you that.

There is 15 different resistances used.

Pellet Resistance Code Ohms
1 - 402
2 - 523
3 - 681
4 - 887
5 - 1130
6 - 1470
7 - 1870
8 - 2370
9 - 3010
10 - 3740
11 - 4570
12 - 6040
13 - 7500
14 - 9530
15 - 11800
See if this helps

  1. Is there a constant blinking security light on the dash?
    1. Yes – Most likely VATS
    2. No – Check to make sure the light bulb for the Security light, and all fuses in the entire car are good.
  2. Turn your key to the ON position, but do not start/crank.
  3. Do you hear a soft whirring sound behind the passenger seat area? This is the fuel pump priming.
    1. Yes – The lock cylinder and key most likely works (for now)
    2. No – The lock cylinder or key may have failed.
      • Try bypassing the lock cylinder
      • Try reprogramming the ECU
  4. Try starting the car, does the Starter engage?
    1. Yes – The Start Enable Relay is most likely good.
    2. No
      • The Start Enable Relay may be at fault. Either replace or bypass.
      • Using a set of jumper cables, you can attempt to connect the positive battery terminal to the positive post on the starter while turning the key to the Start position. If the engine turns over and starts, it’s most likely the Start Enable Relay.
      • The Starter could also be bad, have a local parts store check it.
  5. No Security Light, Fuel Pump primes, Starter engages, but still no start
    1. You may have some other issue that is not VATS related.
    2. Various VATS components may be functioning, but the BCM or something else in the VATS system may not be sending the “all clear” code to the computer. If you’re still convinced this is a VATS issue, your only other option is to have the computer reprogrammed.
    3. If your car has an older EPROM style chip, you will need to contact me to have a new EPROM chip programmed with all of VATS removed.
    4. If your car has a newer style of re-programmable computer, you can visit any performance shop or dealership that handles General Motors vehicles and they can remove all of VATS for you. If this doesn’t take care of it, then it most likely wasn’t VATS.
    5. Wiring maybe one of the two below
9091universaldeterrent.jpg9091passkey.jpg
 

teamzr1

Well-known user
It may be a possible that the thin white wires are near broken off or are broken going into the lock cylinder
If your C4 has the older passkey 1 design which it should

Locate the red wire under the driver's kick panel that comes from the steering column. Inside this will be two white wires and if you follow them you'll come to wiring connector. Insert your key in the ignition and read the resistance on the two white wires (inside the red wire) coming from the steering column.

Get a resistor that is very close to the same value. Just a couple of percent either way will be fine. Insert the resistor in the plug that you disconnected earlier. Insert it in the end of the plug that doesn't go to the steering column.

Your car will now think that it has the correct key in it all the time. The anti-theft feature will be gone but your car will start every time.
 

lovejoy

Regular user
Hi Team ZR1, we talk again ! Thanks for your massive posts, if you click on the "old CCCUK" link
that I posted last year, you will see I did investigate this at some length, I don't think it is the VATS
system that is the problem, I don't have a problem with "SECURITY" lite at left of instrument panel.
What I AM suspicious of is the OIL PRESSURE/SWITCH/sender, virtually inaccessible by hand,
the fault cleared itself last year, I know not how, after weeks of wiggling and unplugging connectors,
and swearing, I went away 200 miles for 3 weeks, came back, and it started right up !!!
WTF? and has been running almost every weekend since last september. Till last week...
So there is an intermittent problem.
We do not have the luxury of knowledgeable dealers where I am, unfortunately, which is why I search
the multitude of USA forums for an answer, hopefully.
Why would the various relays underhood, & the "SES" lite on the DIC, pulse at 1 second intervals
with just ignition "ON", before I turn the key to "START", which it does fire up instantly, for a wee
moment, then stops..... I have tried my spare second key, same result, both 3,000 ohms on my
test meter, so #9.
I AM hoping an OBD II reader will give me some info, via the 12 pin to 16 pin adaptor I have
ordered from the internet, but did you state that will not work?
It is not possible to read ANY fault codes from the usual paperclip test, not even code 12
start of sequence; I am unable to access underneath, and wonder if there is a bad ground
where G106 bolts to the block above the oil filter, there are at least FIVE terminals routed
to that point: OR G107, more earths (grounds!) on the bellhousing nearby...
I tried a starter jump lead from battery NEGATIVE to the block, no difference,
and also checked ohms resistance with a DVM, good short circuit, so earths seem OK.
I read that they are known to give problems....
It's a good one.... surely can't be anything major, the vette was running a week ago !
 

teamzr1

Well-known user
If you cannot even get a code 12 when you short the 2 pins
1. Could the light on dash be dead or wiring problem for it ?
2. You have a second controller ( behind the radio) which is the CCM. It is the boss. All ECM data exchange goes
through the CCM. If CCM has a problem ECM cannot function correctly
3. Can you jumper the 12 pin diag port to talk to the CCM or HVAC to see CCM is talking
4. To protect engine ECM will not allow engine to continue to run of oil pressure is not putting out correct signal
5. You mention the pellets of keys are correct but if there is a problem with either 2 wire off Ign switch CCM will
not allow VAT relay to stay engaged
6. OBD-I - GM quit using it as 1994 base Corvette moved from a ECM to a PCM and began using OBD-II as GM had to be ready to comply with EPA federal law for OBD-II rules of 1996.
1 and II are total different design, signals and codes so cannot just use a cable that goes 12 pin to 16 DTC
7. Check fuse for the CCM ?
8. Any auto parts by you that can let you use a OBD-I scanner ?
9. If in doubt could you get a 3K Ohm resister and jumper the 2 white pins of connector I mentioned above post or
get to Ign switch so see if it is not allowing 12 volts constant to start system ?
10. Yes, crappy grounds if for CCM, ECM or relays can cause big problems

11. One other thought, due to the ECM mounted high where engine bay temps are high it is know for the ECM
has a multi layer motherboard, over time/heat layers start lifting and cause odd problems

1. Maybe open hatch bottom of ECM and pull Memcal out (unclips) and see if problem changes. ( check all grounds to ECM)
2. If you believe VATS is not the problem for sure then I'd go to oil pressure sensor, it's wiring, the CCM or it's wiring at fault
, grounds, or ECM/Memcal faults

3. Also common is the fuse panel below ECM the fuse's connectors get corroded, clean them
 

teamzr1

Well-known user
If your country lacks professionals who do not even understand OBD_I which has been in GM cars since the 1984 C4,
then maybe if you have a laptop that runs MS windows
buy a full OBD-I scanner such as the Datacat scanner
here : Datacat OBD-I Scanner
Cost with software and OBD-I cable is like $135 US
This not only would allow scanning ALL controllers and what DTC (error codes) but also all sensors car has
Also would allow to go into record mode that collects all the info/data and then later replay but also then can
email the recording (like me) who then can also replay and tell you what they see

If you could get a 12 pin DLC at one end and like RS232 other end to pull into a laptop I could maybe
email you a scanner that would not be licensed but allow to connect XX amount of times

Different world you have in your country as here we were hacking into our C4s for tuning and scanning back then
in 1980s :-(
 

teamzr1

Well-known user
Here is a OBD-I code reader, like $20
OBD1 Code Reader

I think I understand what you were saying about a 12 to 16 pin DLC connector

IF you had a scanner tool that supported OBD-I and II but only came with the 16 pin DLC
then using a cable like this might work ? not sure depends on the scanner tool
12to16 DLC
 

Redlevel

CCCUK Member
Starts but no idle? Thats the IAC valve. Easy fix.. Dont bother trying to resurrect the old one, just replace it.
Big can of carb cleaner and a stiff brush to clean the inside of the plenum once the TB is off. Then set your throttle position sensor.
Shouldnt take more than an hour .
 
Last edited:

lovejoy

Regular user
Hi Redlevel, interesting suggestion, but why would that cause a flashing "SES" light,
and relays to click on and off around the engine bay at one second intervals ??
I don't think Euro carparts will have an IAC valve on the shelf either; quite honestly,
I suspect the oil pressure sender is telling the ECM " no oil pressure", after start-up,
and the ECM shuts engine down immediately.... how on earth do you get to it though?
The oil pressure gauge is indicating 550 (80psi) even with ignition off, that can't be right?

I will try and get a replacement first, would Keef at Eurovettes be my best bet?
I tried an OBD2 reader this morning, connected with an adaptor cable to the OBD1
port; didn't even light up, which it does in a daily Hyundai I tried.
Teamzr1 seems to indicate they use a different operating protocol, but it was worth a try!
 

teamzr1

Well-known user
IAC being a idle RPM only function will not cause the VATs and or passkey system issue
While at this check out the keyless security system as I replied to
Anti Theft/Vats issue

You guys in the UK really need to put a stake in the ground and maybe get your club to buy and then lend out
a OBD-I and OBD-II code reader and a full scanner, all else is a time wasting guessing game and why GM designed such tools
as BTW I worked at GM Tech Center in Warren MI back in 1980s and we designed such tools back in mid 1980s called CAMS

And yes, OBD-I used RS232 signalling at slow BAUD rates where OBD-II started with what GM VPW protocol and then since 2005
Uses a higher bandwidth protocol called GMLAN and EPA for generic testing using a CAN protocol lower speed.
 

lovejoy

Regular user
Hi Teamzr1, both my 1990 vert, AND the new poster "Flyafts" 1990 vert
(which I had seen before he bought it) do not have keyless passive entry!

I do not have a VATS issue, but can you throw some light on the "oil pressure
sensor" situation please, it has only ONE tan wire connection, I can just about
see it buried behind the distrubutor at back of inlet manifold....

It seems they WERE a known issue, back in the day..... if one fails, and the ECM
gets a "no pressure" indication, that surely would shut the engine down to protect it?
 

teamzr1

Well-known user
Interesting for C4s I have owned, 84,87,89, 93 and a 94 ZR-1 and they all had a standard feature the passkey system and VATS

Refer to start seq below, as to engine protection the ECM has to see from oil pressure sensor at least 4 PSI on engine cranking

C4 Corvette Start Sequence for L-98 Engines
When starting an L-98 Corvette engine, a sequence of events take place that precedes the engine running.
Knowing these sequences will help you troubleshoot no start issues with your C4 Corvette.
  1. Fuel Rail Pressurization
  2. C4 Corvette Start Sequence begins when you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails. There is a Schrader valve on the passenger side fuel rail near the rear of the engine and proper fuel pressure after the pump runs should be between 40-42 pounds of pressure. The reading will drop to 38-40 pounds after the engine is running.
  3. Initial C4 Corvette Start Action: Once rotating the key to the start position if the anti-theft system has not disabled the starter the engine will rotate.
  4. Once the oil pressure has reaches at least 4 PSI, the oil pressure switch contacts will close allowing the fuel pump to run. Note that you should have a black color oil pressure switch/sender. This sender is mounted behind the distributor on the driver’s side. If its not replaced it with one that's only black. If the car fires then stalls replace this switch.
  5. The distributor outputs rotational frequencies to the ECM (Engine Control Module) as a result of the engine being rotated by the starter. This frequency continues as long as the engine turns (both starting and running) if no frequency output is present the engine will not run.
  6. ECM Outputs : Once the ECM sees oil pressure greater than 4 PSI and the reference frequency from the distributor the ECM will command the injector drivers which will begin pulsing the injectors open for 4 ms (milliseconds) periods. (The L98 injectors all fire on one side of the engine at the same time followed by all injectors on the other side firing at the same time. The LT-1 fires the injectors individually per the firing order).
  7. The ECM also pulls the fuel pump relay which makes the contacts and parallel's it electrically with the oil pressure switch. This way if the fuel pump relay fails you can still normally get the car to run. This will only happen if you make at least 4 PSI oil pressure. This “limp home mode” feature was put in place to allow for a fuel pump relay failure.
  8. The ECM requires an input from the TPS (Throttle Position Sensor which is mounted on the throttle body assembly) required voltage is .54 volts DC. Greater than 0.54 volts the ECM will assume the engine is flooded. This logic is triggered by the driver when pressing the accelerator pedal to the floor. The flooded condition would clear as a result of restricted fuel flow. The output of . 54 volts during start and idle from the TPS is very important in both starting and run performance. Its important to have a good voltmeter to troubleshoot something with such low voltage. Poor quality voltmeters can be inaccurate up to .2 volts.
  9. Assuming the ignition module has spark of sufficient intensity to ignite the air-fuel mixture the engine will start.
  10. C4 Corvette Start Sequence of Engine "Initial Start" for 1985 through 1989 L-98:
  11. When the engine initially starts, the MAF sends a signal to the ECM reporting that air is flowing at "XXXX" hertz frequency through it and into the intake manifold. The ECM refers to its Airflow Table for ECT conditions of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms this supplies a proper air/fuel mixture for combustion. For information on the 1990 and 1991 L-98 see below.
  • Note 1990 and 1991 L-98 : The MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.
  • GM used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system calculates air flow indirectly. Errors and vacuum leaks can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.
  • Initial engine idle speed of should be around 900-1100 RPM and then slowly decay to around 600-700 RPM unless the air conditioner is on at which case it will run at around 800-1000 RPM depending on pressure altitude. If this does not happen, the Idle Air Mixture Valve or IAC may be plugged or faulty. You could also have an intake manifold leak or another vacuum leak. Check for vacuum leaks.
Engine Running
  1. ECM Open Loop Mode: In "Open Loop" mode the ECM is controlling the air/fuel mixture by referencing generic values stored in the ECM Flash memory. These values do not benefit from from the O2 sensor monitoring and lean rich correcting factors.
  2. After the C4 Corvette Start Sequence is finished the Oxygen Sensor (mounted in the exhaust pipe) indirectly is considered to have reached operating temperature of several hundred degrees when the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) is 160 degrees. This would be a Boolean logic type of result. In simple terms X+X+X= Output "Y" The computer then switches to "closed loop" mode meaning the Oxygen sensor’s output is being referenced and being used along with the MAT and ECT outputs for the ECM to adjust the injector pulse widths (more “ms longer” or less “ms shorter”) to maintain a 14.7:1 air/fuel mixture.
 
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