1981 Corvette Odd Alarm (anti-theft system) issue

V8Adam

V8Adam

Busy user
Hi everybody,
I have an odd factory alarm issue that has popped up on my 1981 Corvette. The issue is that when the factory alarm system is armed and I unlock the drivers door with the key, the alarm goes off. If I unlock the passenger door first, it does not go off. If I turn the key counter clockwise in the drivers lock first, it does not go off. If I turn the key counter clockwise in the drivers door, it shuts the alarm off.

There is a post in this forum that indicates that this is normal behaviour with user dskopp stating 'Deactivation occurs when you use the key and turn it counter-clockwise first, then clockwise to open the door' but I do not believe this is normal behaviour. Both the owners manual and factory manual simply state that unlocking the door with the key will disarm it.

I did change the t-top mount plates before this issue occurred. After I did this I would lock the car (arming the alarm) and as soon as I shut the door, the alarm would go off after a couple of seconds. I then realised that the new plates were slightly different and the t-top pins were not pushing the switches to the open position. I therefore tried disconnecting them but the issue then changed to what I now have. I then reconnected them but used a zip tie to keep the switches in the open position, again issue remained.

Any thoughts?
 
johng

johng

CCCUK Member
If you unlock the passenger door first does the alarm still go off if you then unlock the driver's door?
 
V8Adam

V8Adam

Busy user
No. I armed it, unlocked the passenger door but did not open the door. Then unlocked the driver door and the alarm did not go off.

Further, I have realised that this only happens if I lock with the power locks. If I lock with the manual internal locks, then it is fine, I can unlock the driver door as normal and the alarm does not go off.

Thinking on this further, there is currently still a small fault with the power lock switches. The driver side one locks and unlocks both doors but the passenger side one locks the doors but does not unlock them. I was going to replace the switch sometime soon to see if it was that. Could that be contributing somehow, especially as they are wired in series.

It's just odd that it only seemed to crop up when I replaced the t-top modules. I am sure that it was working fine even after I worked on the power lock modules as that was weeks ago.
 
V8Adam

V8Adam

Busy user
If I use the internal locks to arm the system, then just unlock the drivers door, when I open the door the alarm goes off. If I turn it counter clockwise first (locking it, although it is already locked) then unlock it, the alarm does not go off.

To make it even more confusing, I have now 'fixed' it. I fitted the old t-top module and realised that even on the old one, the t-top pin was not opening the switch. I therefore fitted the new one again but this time did not jam it open with a zip tie. Now everything works fine. This is of course not correct as the alarm should be sounding as it is as if the t-top is not fitted.

This is my theory. At some point the t-top stopped opening the switch. Rather than fix this problem, somebody has done something to the wiring to get around it, effectively reversing that switch. When I then put it as it should be, the alarm is going off.

Extremely confusing but it is working now and I am not fussed about having the alarm function, so will leave it alone. If anybody knows what somebody may have done to 'reverse' the t-top sensor, so that it deactivates the alarm when in the closed position, rather than the open, please let me know.
 
teamzr1

teamzr1

Supporting vendor
Hi Adam
Have you used an OHM meter and checked that left and right switches report the same when switches are open or closed states ?
Same with ringing out the wires to the switches as if wiring problems or wired incorrectly
Is it possible the switch is not aligned properly, so the pin of the top is not fully hitting the switch contact

Best is if you could find a C3 GM service manual and then make an image of wiring drawing that includes the wiring for alarm
and post here, so we can debug the alarm setup ?
 
johng

johng

CCCUK Member
This is the circuit for a 78 in case your problem re-appears (although not sure if it's the same for an 81)
alarm circuit.jpg
 
V8Adam

V8Adam

Busy user
I have not taken any readings.

I am pretty certain that the switch is making contact as when I force the switch open (such as with a zip tie) I get the results mentioned before, the alarm goes off or has the odd door unlock sequence.

Attached is a picture of the diagram from the factory manual.
 

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teamzr1

teamzr1

Supporting vendor
Adam,
so you feel there is no problem dealing with the alarm on right side and only on left side ?
 
teamzr1

teamzr1

Supporting vendor
Does this have power switch in doors to open/close door, or have to do that manually ?

Cleaned up image to see better

Adamalarm.jpg
 
johng

johng

CCCUK Member
It appears that the 81 alarm circuit is significantly different to the 78, so my one is of no use.
 
A

Alben78

Well-known user
I can confirm that the 78 alarm system is quite different to the 81 system. Having said that the 78 one was complicated enough and I had endless problems with mine particularly around the earthing arrangements which gave some odd effects at times.
 
V8Adam

V8Adam

Busy user
Thanks for replying. To try and summarise:
  1. Never had the alarm go off or any issues with it.
  2. Installed new t-top mount plates, locked it using the power locks, shut the door, alarm went off (unexpectedly of course!) Disarmed it, locked car using key as it was late.
  3. Next day, tried again, same result.
  4. Looked at new plates and realised the t-top pins were not pushing the mounting plate switches to the open position. Great, I thought, there is the problem.
  5. Disconnected the blue cable and thought I would come back to it later to see if I could make the pin engage the switch somehow (perhaps by trying to adjust the pins on the t-top).
  6. Locked it with the power locks, shut the door, no alarm. Great I thought.
  7. Unlocked the door with the key, opened the door, alarm goes off. Turn the key counter clockwise (locking the door), alarm silenced.
  8. Locked it with the power locks, shut the door and this time turned the key counter clockwise first, then unlocked, then opened, no alarm.
  9. Locked it with the power locks, unlocked the passenger side like normal, opened passenger side, no alarm.
  10. Locked it with the power locks, unlocked the passenger side like normal (did not open door), unlocked driver side like normal, opened driver side, no alarm.
  11. Reconnected the t-top mount switch and jammed it in the open position using a zip tie. Issue remained.
  12. Connected old t-top mount switch and noticed that even this one was not opened by the t-top. This meant that before I changed anything, these had always been in the closed position when the car was locked. Very odd as this should have set the alarm off each time from my understanding as the switch being in the closed position is as if the t-tops are not fitted.
  13. Installed the new t-top mount plates again, this time made sure they were in the closed position, everything working perfectly.
  14. The only thing I can think is the following - at some point the t-top pins stopped opening the mount switches. Rather than fixing this, somebody, somehow made it so that the alarm would not go off when the mount switches were closed, almost reversing the behaviour. When I installed the new mounts, initially one was slightly open?? Why the behaviour changed from the alarm going off straight away to only when I opened the driver door normally, I have no idea.
It is working fine now, so I am not overly fussed, I just have no idea why it is working as it is and what really caused the initial behaviour.

Only other thing to note is that it did have an aftermarket Clifford alarm when I got it but my workshop removed it.
 
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