98B

First i want to say i feel bad asking for help since I'm always on here but dont contribute but i really could use help from the experts! I read a bunch of the threads on anything electrical and have tried every fix Bill's and other diy threads make that has to do with my issue but I still have a few questions about tac/pcm issues. Had the car in my shop to wrap it and the battery went low so I charged it and drove it around fine and then a few days later I go to move it and I get reduced power, lights, no fuel pump priming and the car will crank but not start. Go to pull the codes and DIC is not working. Since I know these cars don't like low voltage I replaced the battery just to make sure that wasn't my culprit and it wasnt. I borrowed a scanner and got p1518 and p0601. P0601 cleared but p1518 would never go away. After testing fuses, voltages, cleaning grounds and ignition switch with no sign of a problem I bought a good working tac from vettenuts. Installed today and nothing has changed. Still the same codes and p1518 won't go away. I know most have to replace the pcm with the P0601 code but I've heard that's not always the case and with the battery draw issue and dic suddenly not working I want to make sure it's not something else. If the pcm is bad would it still throw p1518 even if it's a new working tac? What would cause tac/pcm to go bad after just a few days of sitting in the shop (no corrosion in the battery area)? Any other causes I might have missed that would cause these codes/dic not working/battery draw? And if I do have to replace the pcm can my tune still be pulled even though it's bad (i dont have access to the tune anymore but would really like to keep it) Any help would be greatly appreciated!

Greg_E

I would start with the power and ground circuits for the TAC module You want to be sure you have good power and ground connections to the TAC module. If those are good you should check the connections and wiring for the serial data lines between the TAC and PCM.

Here is the troubleshooting procedure for a '98. (See attached pdf)
G106 is the ground for the TAC PCM and numerous other components. S120 is where all those grounds are gathered and a single wire is brought to G106. I think that is done in the wire harness but I've never looked at it so I can't say for sure. They did not use a splice pack for G106 like they did with some of the other grounds for less critical components.
Locations:
S120 Engine harness, approximately 15.0 cm (5.9 in) from the main harness in the branch leading to the Powertrain Control Module (PCM) connectors (CKT451)
G106 Engine harness, RH side of the engine, above the starter (CKT 451)

98B

Sorry the car is a 98. As far as mods if it is asked I do have an a&a kit, cam, etc. but all have been on the car for years without problem and nothing electrical or mechanical has been added recently.

Thanks I'll inspect those grounds you mentioned again and make sure everything is good there then run through troubleshooting as there were a couple of steps on there I hadn't tried

Greg_E

I uploaded the the troubleshooting procedure for a 1998 and deleted the other one.

Odd that the IPC stopped working at the same time. Have you checked the fuses and power to the IPC?
Have you checked for voltage to the TAC module when the key is off?
'99 Diagnostic Aids, Bullet 5:
"DTC P1518 sets if the TAC module ignition feed circuit is shorted to a B+ supply circuit. The TAC module stays powered-up when the ignition switch is turned off. When the ignition switch is turned on, the TAC module is powered-up before the PCM. DTC P1518 sets because no communication is detected by the TAC module from the PCM. Inspect related circuits for being shorted to a B+ supply circuit." That would account for the P1518 and the battery draw you mentioned.

I'm going to post the troubleshooting procedure for the '99 again as it seems to have some troubleshooting info that is not contained in the one for the '98.

BTW I am far from being an expert but I'll try to help if I can.

BTW I would think if you can communicate with the PCM through the OBDII port you would be able to upload the tune to your PC with tuning software. Obviously your PCM has some functionality or you would have many codes, not one.

98B

Thought I'd update for anyone having similar issues. Unfortunately ended up needing a new pcm as well. Did a lot of searching on here and had some conversations with some tuners and this code combo just seems to be a bad sign on 98's. Not sure what goes wrong but both were lost. But I put the new tac and pcm in and she'll be on her way to get tuned in the next week or two. Thank you for the help!

Greg_E

Did the tuners that you spoke to say if you will be able to save the tune in the old PCM, or do you have to start over? It might be worth bringing the old one with you. They probably have a bench tuning harness they can use to check, without putting it back in the car.
Thanks for the update.

98B

The tuners I've talked to said they can't. Wish I knew someone in the dfw area that might have a bench harness to try because I was comfortable with my old tune but I'm ready to drive it again

Greg_E

The P0601 code is a problem with the PCM memory. I know you said the code had cleared but I suppose the data stored in that location could be corrupted. Just a guess on my part. They have probably seen it often enough to know. Good luck with your new tune.

K-Spaz

Not very far down the road I'm afraid...

Greg E above referred me to this thread. I too seem to have the same set of codes. I don't have to have the car working really so I will probably mess with it a bit. I find this hard to believe that I killed a PCM and a TAC module with what I've done recently. I did do a engine/trans swap, but I'm pretty sure this all began after I sent the first new tune to the PCM. Not sure when around that time, but this is all new.

I am not thrilled with the idea that the car needs a new PCM and TAC. This whole process for me has been one hurdle after the next.

abarbi61

Have a 1999 Corvette and as been sitting at a chevy dealer 2 months now waiting for a TAC module I was told is being build by some race car company and GM bought the rights to all the modules when finished building them. So I am still waiting. Anyone else heard this?

helga203

Have not heard about this. What is the price?? When mine went bad i got one from a junkyard for 250. Had it in 3 days shipped.

abarbi61

My car at a Chevy dealer. Cost for part and installation $700. Part will be new with warranty. No warranty with used ones.

abarbi61

I took your advice and ordered a Tac module used with 15,000 miles on it from Vettenuts and delivered to dealer in 3 days and back on the road again price $245 with a 30 day warrenty. Cannot believe it sat at that dealer for 2 months waiting for a new Tac module to be built. still would have been waiting till mid Oct. I am learning...Thanks!

helga203