c5 fault showing pcm to tac module serial data circuit

[nickowen1969]

hi all, im looking for a garage in the north west uk. ideally as close to wigan or possible some help in diagnosing a fault. my car fails to start, once this code (pcm to tac) is cleared it starts and runs fine for the rest of the day. the next day the code is back and needs clearing off, my local garage is in experienced with corvettes so im looking for some help. thanks all.nick

 

[Oneball]

Can’t help with garage recommendations but what’s the battery like?

A quick Google shows that that fault can be caused by a low battery which would tie in with it coming up in the morning but ok whilst using it.

 

[nickowen1969]

Can’t help with garage recommendations but what’s the battery like?

A quick Google shows that that fault can be caused by a low battery which would tie in with it coming up in the morning but ok whilst using it.

hi thanks, i have only just purchased the car so it is still in the car sales but there struggling to get it started first thing in the morning, it has been there for a few months so yes the battery could be goosed. i will have a chat tomorrow to see if they can get it tested, will google it myself so thank you for the help. nick

 

[Yellowshark]

Hi if this is still a problem can you tell us what the DTC code is please

 

[nickowen1969]

Hi if this is still a problem can you tell us what the DTC code is please

Hi the code is p1518
Hope this helps. Once the code is wiped the car starts fine and runs brilliant, the next day the code is back and fails to start until cleared again then runs ok. Bit like groundhog day. Thanks nick

 

[Yellowshark]

Hi Nick, OK maybe the quickest way forward is for me to email the 4 pages in the FSM that comprise the process for determining the cause of the failure. If the garage has a good auto electrician, or access to one, they could sort it. Yes P1518 can be caused by a low voltage in the battery although that is not something I have ever seen in my 19 years with my C5, but the code can also be set by a short to the TAC module's ground circuit. Also if the TAC module's ignition feed circuit is shorted to a B+ supply circuit the TAC module will remain powered up when the ignition is tuned off. So when the ignition is turned on again the TAC module is powered up before the PCM and P1518 is set because the TAC sees no communication from the PCM which has not started up yet (normally the PCM powers up first). Also checking the fuses for the cruise control and brake circuits which share the TAC module harness would be useful. Let me know if you want the diagnostic pages, send me a pm with your email. Out of interest are you seeing P0606 too, as well as reduced engine power message and the MIL turning on? Oh also you say the car starts fine - does that mean there is no problem with the battery?

 

[nickowen1969]

Hi Nick, OK maybe the quickest way forward is for me to email the 4 pages in the FSM that comprise the process for determining the cause of the failure. If the garage has a good auto electrician, or access to one, they could sort it. Yes P1518 can be caused by a low voltage in the battery although that is not something I have ever seen in my 19 years with my C5, but the code can also be set by a short to the TAC module's ground circuit. Also if the TAC module's ignition feed circuit is shorted to a B+ supply circuit the TAC module will remain powered up when the ignition is tuned off. So when the ignition is turned on again the TAC module is powered up before the PCM and P1518 is set because the TAC sees no communication from the PCM which has not started up yet (normally the PCM powers up first). Also checking the fuses for the cruise control and brake circuits which share the TAC module harness would be useful. Let me know if you want the diagnostic pages, send me a pm with your email. Out of interest are you seeing P0606 too, as well as reduced engine power message and the MIL turning on? Oh also you say the car starts fine - does that mean there is no problem with the battery?

Good morning, thank you the pages would be good. The auto electrician is going again so the information could help him. Reading your quote a few times i understand what you are saying. My email nickowen1969@aol.com thanks nick

 

[Yellowshark]

Hi Nick you should have my email now with the FSM pages you need. HTH Pete

 

[nickowen1969]

Hi Nick, OK maybe the quickest way forward is for me to email the 4 pages in the FSM that comprise the process for determining the cause of the failure. If the garage has a good auto electrician, or access to one, they could sort it. Yes P1518 can be caused by a low voltage in the battery although that is not something I have ever seen in my 19 years with my C5, but the code can also be set by a short to the TAC module's ground circuit. Also if the TAC module's ignition feed circuit is shorted to a B+ supply circuit the TAC module will remain powered up when the ignition is tuned off. So when the ignition is turned on again the TAC module is powered up before the PCM and P1518 is set because the TAC sees no communication from the PCM which has not started up yet (normally the PCM powers up first). Also checking the fuses for the cruise control and brake circuits which share the TAC module harness would be useful. Let me know if you want the diagnostic pages, send me a pm with your email. Out of interest are you seeing P0606 too, as well as reduced engine power message and the MIL turning on? Oh also you say the car starts fine - does that mean there is no problem with the battery?

hello, thank you for your help, i have now got the car but it is still showing faults, could you possibly let me know where i could get, purchase a copy of the FSM manual needed, there is lots showing on the internet but not sure which one to buy. thanks again nick owen.

 

[Yellowshark]

Sure, well a quick check shows me that Zip and Mid America are both carrying the manuals, others will as well. Prices between the different years vary but will likely be in the 300 usd to 450 usd range. Plus there will be a hefty freight charge; there is a reason they are expensive they are huge, running to three volumes.

https://www.mamotorworks.com/corvette/category/1997-2004-c5-gm-service-repair-manuals-2195

https://www.zip-corvette.com/instantsearchplus/result/?q=shop service manual

There is also the option of getting the manual on CD, which at around 30 usd is somewhat cheaper! These are I believe full copies of GM’s printed versions but I have no idea of what they are like and how good they are to use, in particular the searching capability. There may be people in the club with CD versions.

 

[Stingray]

I don't recognise the specific fault but confirm C5 electronics can go crazy if the battery is weak. A battery can be too weak even if it will still start the car.

During cranking the high current drawn always causes some voltage drop. It's why in the old days your headlamps would go dim if left on during cranking. These days if the battery is weak voltage will drop below what's needed to run the electronics properly (say, 9 volts) and all sorts of errors can occur.

An early sign of "weak battery" on my C5 was the car defaulting to kilometres instead of mph when the engine was started.

 

[nickowen1969]

thanks, the auto electrician has been out and it needs a throttle position sensor. Any idea on a uk supplier, only just got the car so new to the corvette scene. Thanks nick

 

[Yellowshark]

Hi Nick,

No doubt we all have our favourites. In the UK I use Keith at Eurovettes who is helpful, honest and reliable. If Keith does not have an item in stock he will find it for you and give you a price and delivery date. I am not sure what the position with Covid is but Keith normally has a monthly delivery schedule from the States. The club magazine also lists a variety of suppliers.

If I order direct from the States I tend to use either ZIP, Cultrag Performance or RockAuto, the latter who seem to be competitive and use the Global Shipping Programme which seems to be well priced on freight too and pays the customs up front so you get the total price up front and there is no waiting around and no collection in the UK. There can be differences in prices and indeed cost of freight so it is worth checking around before hitting the buy button.

 

[nickowen1969]

Hi Nick,

No doubt we all have our favourites. In the UK I use Keith at Eurovettes who is helpful, honest and reliable. If Keith does not have an item in stock he will find it for you and give you a price and delivery date. I am not sure what the position with Covid is but Keith normally has a monthly delivery schedule from the States. The club magazine also lists a variety of suppliers.

If I order direct from the States I tend to use either ZIP, Cultrag Performance or RockAuto, the latter who seem to be competitive and use the Global Shipping Programme which seems to be well priced on freight too and pays the customs up front so you get the total price up front and there is no waiting around and no collection in the UK. There can be differences in prices and indeed cost of freight so it is worth checking around before hitting the buy button.

Hi. Thank you for this. I ordered from rockauto yesterday morning at 10.00am and it has been shipped via fed ex. Checking with tracking and it is already at stanstead airport uk. Cant get a better service than that, thanks nick