Engine/trans idle and shifting problems

[ScottMH]

Working on a GMPP 430hp LS3/4l65e connect and cruise package. Package was installed last year and has been running fine up until recently. Battery died while going down the road so customer brought it in. Apparently he has been driving it like this because he said he would just swap batteries and start it up and go. So while investigating this issue I notice the ECU has code P0700, the connect and cruise packages do not have the TCU wired into the OBD port so I have to plug in my laptop with the gm software on it. Read the codes and came back with P0758 (2-3 shift solenoid fault) and P1860 (tcc solenoid fault). Do some troubleshooting and learn that these codes are common when the TCU loses power. I cleared them out, put a fresh battery and the car ran normally. Monitored battery voltage vs ECU voltage(scanner) for an hour. Nothing out of the ordinary. Took the car on a test drive, MIL light came back and car wouldn't shift. Bring it back to shop scan the TCU has code P0758. Cleared the code on the tcu and ecu. Ohm out solenoid through wiring harness, within spec. Inspected wiring harness for damage, found nothing. Took the car for another test drive. Drove and ran perfect. Get back to shop turn off and immediately turn it back on still fine. Drive up the rack. Still good. Turn it off let it cool off. Come back turn it on still good. Let it warm up to approx. 150f turn off. Turn back on. Engine starts runs the slowly dies. Try again engine starts and runs fine. Let warm to 210f. Turn off. Turn back on. Engine starts then slowly dies. Try again. Engine starts the slowly dies. Put a fuel pressure gauge on the rail. Key on fuel pressure is 60psi start fuel pressure drops from 60psi to 0 after its running. Hook up scanner. Watch fuel pump engagement on scanner. Fuel pump is commanded on. Start car, fuel pump on then commands off fuel pressure drops slowly car dies slowly. So the ecu is turning the fuel pump off. Check for codes. P0700 is present. Clear code. Try to start. Car starts normally. Turn car off. Walk away to get opinion of another tech. Talk about it approx 5mins. Start car. Starts normally. Turn off and on again several times with no strange happenings. Cuss. let car run for a few minutes. Turn it off and back on. Fuel problem is back, P0700 is back. Car won't stay running. Yell, see I'm not crazy!!

I tried to monitor when the tcu throws the code and it seems to do it at start up. If it starts without throwing the code the car runs and drives normally. Otherwise it won't stay running or it won't shift.

So is it possible the ECU is seeing the P0700 and is commanding the fuel pump as some kind of protection?

Is there a way to wire the TCU to the ODB on these connect and cruise kits?

Do you think I have a damaged TCU? I am thinking having the battery die while the car is going down the road may have damaged it. I do not know how many times this happened. He was vague. I've checked the harness as best I can without removing it from the car, I don't believe it's the harness.

The TCU gets its power from the ECU. Is it possibly for the ECU to cut power to the TCU for any reason?

Is it possible I've fallen into the twilight zone?

[GHuggins]

PO700 will cause a engine control safe mode in most applications - if or why they have this setup to kill the fuel pump is beyond me - I would think that would be dangerous... PO700 can be caused by a faulty tcm, trans component, solenoid or wiring problems in general... If it's had the voltage dropped and re-applied to it several times, then yes it's possible it got messed up - even more likely it got zapped with being jumped off incorrectly or something of that nature...

Now the more important problem that still needs to be addressed - why is the battery dying while it's going down the road - this needs to be addressed - obviously electrical charging or wiring problem...

[ScottMH]

Yes that's is definitely going to get sorted out. But I can't reliably drive the car currently to even get to the point of trying. I've inspect the wiring from alternator to bat, bat to starter and ecu fuse box to bat and found no shorts or breaks. That's a problem for another day.

[ScottMH]

I also have a suspicion that the battery was eaither jumped off backwards (positive to negative and vice versa) or there was a attempt to install the battery with the polarities reversed. the positive post on the battery had a large gouge and looks like may have shorted enough to melt a piece of the post. Could have shorted it to ground i suppose but this looks like it was more violent than that. I wish I would have snapped a picture of it

[GHuggins]

Sounds like a module or modules may have gotten damaged then...

[ScottMH]

Just want to give an update. We figured out the problem and it was not anything we expected it to be. The issue is with a aftermarket steering column from IDidIt. They have a key switch system that works off of micro switches and relays. The problem is when you go from off to crank to run is hits the IGN relay 4 times in the process. So the TCM was essentially being turned off and on 4 times in the start up process. The TCM could not complete the start up self check so it would throw a code and put the trans into limp in mode. We replaced the IGN relay that came with the IDidIt column with a Time Delay relay that holds power for .75 seconds. This seems to have fixed the issue. The only downside is when the car is switched off it holds for .75 seconds after the key is moved to off. Its hardly noticeable. If you call IDidIt they are aware of the issue and will send you the time delay relay with instructions on how to wire it. I hope this helps someone in the future.

[GHuggins]

Well, I definitely wouldn't have figured an ignition cycling too many times too quickly issue - interesting that it only happened after a year...

[ScottMH]

The IDidIt column was not in there at first. It was installed at a later date. The issue didn't begin until after that. Just didn't put 2 and 2 togeather