SUPERCRUZ

I started a thread about a month ago after my car failed to start after it had been sitting on a battery tender for about 6 weeks. The car ran fine prior to this failure. DIC displays Reduce Engine Power, Service Traction Control System. All lights and gauges seem to be working fine. Codes present are PCM P0606, P1518, TCS C1276. Car is in complete shut down mode. Car will crank fine but will not run. No spark and no fuel. After a lot or research and help from folks on this forum, decided the PCM had probably failed so I shipped the car to VA Speed in VA Beach.

Ed Hutchings, my tuner, attempted to connect his Laptop to my car and received a data serial port error. He flashed my tune to a spare PCM he had around the shop and installed it in my car. Car fired right up and ran perfect. He started it multiple times over several days. Thinking we had resolved the issue, the crew at VA Speed then went on to install a new clutch for me but upon completion of the work received the same DIC messages and codes when they went to start the car after the clutch install. This is the second PCM failure. Ed has a brand new PCM from GM ready to go in but he is hesitant to do so until we solve the issue causing the PCM failures. He has indicated to me a P0606 DTC has no diagnostic tree and indicates replacement of the PCM.

Battery is an optima red top that is about 4.5 years old. Always maintained on a tender and no signs of leakage. PCM and TAC module wiring appear intact and in good order. There do not appear to be any blown fuses in the fuse panel under the hood. Cleaned grounds in the engine compartment and none appeared to be in poor condition.

Any ideas on where to look next or what may be causing the PCM failures? Are there any possible sources for high voltage spikes from on board systems to the PCM that would cause them to fail? Now I am getting worried. Ed is going to continue to trouble shoot the problem tomorrow. Thanks in advance for your replys.

dgrant3830

P0606 PCM Internal Communication Interrupted

I found the following but not sure it'll help:

P0606 - Electronic Throttle Control System Fault in combination with other DTCS, or Powertrain Control Module Internal Communication error for DTC Alone P0606 alone, indicates register readback (PCM internal Communications) error, for P0606 in combination with other Electronic Throttle Control DTCS, indicates a system fault. If the other DTCS are repaired, then should also be corrected. Damaged, or defective PCM
Electronic Throttle Control system fault, DTC dependent

I'm thinking that you're missing a significant ground somewhere and whichever ground that it is, is somehow reverse drawing the ground through the PCM itself, which is cooking it. The PCM basically controls lots of things with signaling grounds out for the various devices, such as the fans and so on. I would get hot onto looking at every ground in the car.

Bill Curlee

Heres what I recommend:

Find the Serial Data Buss STAR connectors. There to the LEFT of the BCM. There two thin Grey connectors. One of the connectors has only a few wires. Thats one connects the SEAT MODULE, Right DOOR MODULE and LEFT DOOR MODULE to the serial data buss. Disconnect the top of that serial data Starr connector and those modules will be out of the serial data buss circuit. Those are the MOST common serial data buss failure items.

Next:

Read the 12 VDC fuses that supply the BCM with power.. There are several. The ones that are HOT in Run and Start are the ones in question. There are TWO small slots on top of the fuses. Read those to ground. The voltage reading you should see is exactly the same as battery voltage. If its LOW,, the ignition switch is your problem.

Follow my ignition switch rebuild post to repair it.

Bill

SUPERCRUZ

Guys - Thanks for your input. I will pass this on to my Tech. John

SUPERCRUZ

Update - I received a call from my Tech today and the culprit appears to be the battery itself. Ed indicated the battery was pulled out and load tested when the car first came in and showed it needed a charge but seemed okay other than that. Keep in mind, this battery has always been maintained on a battery tender. Battery was brought up to full charge and everything appeared to be good. As I indicated in the title, this car killed the original PCM and another used one at the shop that was installed for testing purposes. A third used PCM was put in the car and the car ran 5 minutes before killing that PCM. All kinds of tests were done on chassie grounds, ground circuits and the wire harness' leading to the PCM and TAC modules. Everything was testing out OK. Ed decided to test the battery again and again it showed low charge. Something did not seem right so Ed dropped in a new battery and so far the car is running fine. Not sure exactly why the battery was causing the PCM failures. Car cranked fine and there were no outward malfunctions with any of the accessory systems in the car. I guess stranger things have happened with these cars and their sensitivity to batteries. I hope Ed will post and try and explain the cause for the failure in more technical detail. I am not going to start an Optima Red Top bashing party here. They have been good batteries for me up until this point and I have run them in a number of my cars with no issues. This battery was almost 5 years old. Never the less, I am switching to the Exide Orbital for my replacement. Wish me luck.

dgrant3830

A battery that old is certainly possible for it to be having issues. The car's computers require a minimum of 12.5vdc to run properly. Anything below that, its simply not going to run or run correctly. Remember that we're driving a computer(s) on four wheels, so yes, I can see the battery causing this. Do yourself a favor now, clear all codes and put it through some restart cycles, drive it and so on and make sure you pull the codes again. Yes, you can pull the codes while driving it too...fyi. Look for anything that isn't working correctly. I would suspect that if all is normal, then the other two pcm's are probably still good.

SUPERCRUZ

Fortunately, the car is still down in VA Beach at VA Speed's shop. Ed is going to run it through it paces before he gives the car back to me to make sure everything is good to go. We shall see.

kh400

I've had two cars , both late model GM's , in a forty plus year auto repair career that would crank fine , but not start. Both were corrected with the installation of a new battery. Go figure!!!!!

SUPERCRUZ

Update - Battery is not the culprit. Car has now fried 3 PCMs, two shop spares and one Reman. Funny thing is the car will only run on one particular PCM out of 4 we have tried. For some reason, this one PCM is unaffected. New symptom that has manifested itself is that the passenger door window will not go down from the drivers door switch but will go down from the passenger door switch. No BCM or Communication Codes, only those noted above. I wonder if Bill C was on to something above. We continue to troubleshoot the issue.

Note - I know this may sound like we are just throwing PCMs at this car in hopes that they will fix the problem. Nothing is further from the truth. Because this issues completely shuts down systems delivering fuel and spark, we are trying to diagnose the problem in a state where the car will run. We are looking at grounds, harness connections, everything. Just have not found the culprit yet. Every time we think we have the problem solved i.e. bad PCM, bad battery, the problem comes back after 2 - 3 key cycles. Man this is frustrating.

Bill Curlee

I had a very strange feeling that you would be reposting with this info. Im involved in a couple of things this morning but, I will PM you my number. Call ne this evening around 5PM EST. PM SENT!

SUPERCRUZ

Well, it has been several months since I last updated this post but wanted to let everbody know what we finally found as the issue. It appears the TAC module was bad.

After extensive testing which included checking all grounds, pinning and scoping every lead in the main PCM harness and finding nothing unusual, Ed replaced the TAC module along with a new PCM and the car has run perfectly for dozenns of starts. Ed theorized the TAC was somhow corrupting a certain table in the PCM which would cause damage to the main chip. He checked my EFI Live tune to make sure that table had not been tampered with by the previous tuner and it had not. This is only a theory and the best cause we have right now. I guess it all boils down to black box type stuff.

This has to be one for the record books. Almost 3 months in the shop. I am just glad the car is fixed and I am going to get it back and be able to drive it again. I have not driven it since December. Do not even ask what my bill is going to be. I want to say thanks to Ed at VA Speed. He has been very kind to me and did not hurt me too badly on the bill given the amount of time he has in it.

lucky131969

Interesting outcome.....but confusing to me. Too bad it's not possible to have the TAC module in question tested. While I do not have intimate knowledge of the internal circuitry of the PCM, I can't help but wonder how the TAC could corrupt a microprocessor in the PCM via the class 2 serial bus.

Hope it works out for you!

Bill Curlee

Mike

Ive seen the TAC module fail a couple different ways. On one C5 the TAC module was sending out voltage spikes to the PCM and BCM and MAF when ever the car was accelerating. The high mount break light and the HVAC AC ON indicator would rapidly flash when it happened. Figure that one out. Took quite a while to figure that the TAC was the culprit. Once we saw that it happened with the key ON engine OFF as the accelerator was depressed, we nailed it down easily, New TAC module and the car was good to go.

I had a bad TAC in my 98. It would just cause a Reduced Power Mode issue out of the blue. There was a TSB that described the issue for 97 and early 98 C5's Forgot what DTC it would throw but you could clear the DTC on the fly and she would be happy again. Got very proficient at clearing that PCM DTC in a hurry.

They don't fail very often but when they do, you don't go anywhere quickly if the engine runs at all.

BC

lucky131969

Hmmm.....so an induced voltage on the serial data line.....that's funky.