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Discussion Starter · #1 ·
Hey Gang,

I finished up this truck for a gentleman and we're having an issue I can't seem to figure out and is stumping the local LS guys. Hopefully you guys can help. Here's the stats:

It's a 51 Chevy 3100 with a full take out 5.3 Vortec, 4L60E, and factory harness, cables, and computer from an 2006 Chevy Tahoe. The package also came with the plastic gas pedal, TAC module, and all other needed components. I sent the harness off to a guy in Arkansas that reflashed the computer, went through the harness and eliminated what I didn't need, and did everything else needed to make the harness stand alone.

The car starts and runs fine. Driving to about 40mph, the engine goes into REP mode (reduced engine power, or limp mode) and can be remedied by disconnecting the battery for five minutes to remove the MIL codes. An LS guy came out and swapped computer with ones that are known to be good thinking it was the computer. It wasn't. While driving and a laptop hooked up, the car would revert to REP and he would clear it with the laptop, then we'd continue to drive. The only code it is throwing is U0107, loss communications with the throttle module. OK, we replaced that thinking maybe it was bad. Truck got to 65mph, then it did it again. New battery, all grounds are good and connections are super tight.

So after doing some research, it seems GM put out a program that dealers or anyone that had access to the tools, could reprogram the computer with this new program to eliminate that problem as it was specific to 06-07 years. Anyone know anything about this? Is this something I need to ask the dealer about and have them reflash the computer?

This is pretty frustrating since I know zip about EFI, and supposedly, the DBW throttles are basically a wolf in sheep's clothing. Thoughts on this?:confused0006:
 

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The 06-07 Have a unique TAC and pedal vs the early unit. Easy to install and run the wrong ones, seems that maybe the case here.

What is the part number on the TAC box installed now?

Verify you have the correct TAC and 6 pin pedal for the 06-up units.

Also who you send the PCM and harness to?
 

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Discussion Starter · #3 ·
I sent the harness to a guy named Leonard who goes by Chevythunder. He was recommended to me by someone who's used him and had good results. The TAC module and pedal are all out of the 06' Tahoe. It was a package, from what I was told, and yes, it is the 6 pin pedal. We replaced the module with a new one from the dealer, but as I said that didn't help matters. I don't have the part number handy, but can get it maybe tomorrow night to update the thread. If not, then Thursday for sure.

What about that program issue I see? Some sort of download fix the dealer does?
 

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Yes there is a TSB on this, I will have to see if I can find it but thought its just a software update in the TAC. It won't fix a physical problem. Since this is a swap I would double check some things first.

Check the ground for the TAC itself. Should be a black or black white wire. Pin 15 on the large connector. Also it could be a power supply issue to it as well.
 

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Discussion Starter · #5 · (Edited)
OK, thank you for the help on this. I'll check that ground PIN and power supply PIN too. Can I tap into that ground PIN and run a dedicated ground to the chassis off of it for grins? I'll be looking over the truck Thursday, so I'll post an update then but in the meantime, I did find this:

"PCM Reprogramming Instruction (if indicated from Diagnostic Aids) Reprogram the PCM with the latest calibrations if the above concern is encountered. An updated PCM calibration is now available to address this concern. This calibration, or any that follow, is designed to address this concern. Refer to Service Programming System (SPS) using the appropriate Service Information (SI) procedures. The new PCM calibrations will be available to dealerships as part of TIS2000 incremental satellite update version 8.5, which was broadcast to dealers on 08/20/2006." I attached the document, as it's the GM Bulletin that I quoted it from

That was contained in a doc I found on the problem and it specifically says to check the ground on the back of the head. If that's found to be OK, then this program will fix it. That sound right?
 

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Remove the that plug and check the ground. If its good them it might be OK. I am not sure were they placed the ground for it when they swapped the harness. So it could just be cutting out under vibrations. But you could run a ground for to troubleshoot. Double check all grounds should be one point were they tie together. Make sure its grounded to battery, motor, frame, body, more grounds the better!



Also check the power to make sure it is off with the key off and on key on. If they happened to place it on the batt buss then it will have power before the PCM which will give that code as well. But typically it just will not start. Now it could be losing power as well under vibrations.


Also inspect the TB wire it self near the TB itself. Wiggle it while its idles and see if it throws a code.
 

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Discussion Starter · #7 ·
Gotcha. You replied pretty fast, so check my above post. I think that's the service bulletin on this problem.
 

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Yeah but a lose of ground or power will do it too. Check that first if all good then may need to reflash the TAC firmware, as it my have been corrupted from the reflash of the PCM. Basically that what the TSB does is reflash the TAC with latest update.

But since its a swap its most likely a wire issue. If it was in a reg truck then I would be reflashing it.
 

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Discussion Starter · #9 ·
Gotcha. I'll check all that tomorrow and report back with what I find. Thanks for the help!
 

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Discussion Starter · #10 · (Edited)
OK, just did all the things you suggested. All grounds are clean, super tight. Battery is tight, good charge, cables are all good. TAC ground is good, power lead is good (only power when IGN on) and I went and test drove the truck. It seems as though when the pedal hits a certain range (pedal throw) is when it goes into REP mode. It's not speed specific, nor does it seem that it's RPM specific. It just appears to me as though it's throttle pedal position. I had the truck up to 35, then 45, then 50. But a hard pedal push and goes into REP. Only code it's throwing is U0107. While idling, no codes are being thrown when I move any connections. Also, I can just shut the truck off when it happens and restart then be on my way until it happens again. No need to unhook the battery.

So, saying all this, you think it's that program I'm referring too? Tuesday I'll be taking the truck to a local service place here to try and diagnose this further. Any more thoughts since I've exhausted all other options?
 

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Discussion Starter · #11 ·
Reflashing the computer didn't help. Next step is cleaning the throttle body and TPS and see what happens, with possible replacement. There was a lot of crude in there when the motor arrived, so maybe a flat spot or the throttle blade is sticking? Just trying to rule out everything here. I'm also going to recheck the wiring to the TB at that connection as I'm finding that there may be breaks in that connector not visible and all too common. I've read several posts where just cleaning the TB and MAF sensors helped that. I'll keep you posted!
 
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