GM hybrid issues

[Ken]

Hey guys,

I have an 08 Tahoe that Im having trouble getting a good diagnosis on. It is stalling out or sometimes hesitating to accelerate when I try to move from a stop. It happens when the truck is running off the engine and also when running off the HV hybrid batteries. It is also just running poorly in general, feels like the engine is misfiring, or trying to die sometimes but no misfire codes. All of this only occurs when it is hot, and the AC has been cranking. Im unsure if its related to the temperature itself on the hybrid components, or the fact that the extra draw from the electric AC compressor (not belt driven) is putting a higher load on the HV batteries. The codes pulled by the GM dealership tech were P0AC4 (HPCM request MIL) P0BBD (Hybrid batt pack voltage variation high) P061B (control module torque calculation performance) U0073 (AC compressor control module) The dealership seemed unsure, and recommended replacing the hybrid battery (out of warranty and $4200). The Hybrid repair shop thinks a transmission issue. Please help!

Topic

[Ken]

Here is a link that explains the hybrid system in depth, especially the transmission. it seems odd to me its only stalling/hesitating when the AC is cranked up.

http://www.atraonline.com/gears/2009/2009-04/2009_4_34.pdf

[Ingvar]

As a Toyota hybrid owner, with electric compressor, have my word – ac comp draws no power off engine. Traction battery is heavily watched by ECM and hybrid management systems and when its voltage drops down to a specific level, ICE turns on. Well functioning system simply does not let traction battery drain itself. How much is GM hybrid drive different, that I am not sure. Mine does not have conventional transmission. Yours might. It depends how they arranged it.
But either way, ICE function should not be influenced by hybrid drive.
Why don’t you run a can of Seafom through tankful, 2 times in a row? Just to give fuel system good cleanse? Clean throttle body too. How is air filter?
Btw, are you in CARB state? If yes, lucky you, hybrid drive is covered for 150 000 miles.
Personally, I love my hybrid system. But it’s Toyota, you know. I wouldn’t dare to buy something that complicated from GM or Ford.

[Ken]

Thanks ukrkoz,

I ran a bottle of fuel system cleaner through it last month, changed the plugs and wires and 12V battery. We just bought the vehicle used, and it needed some TLC, the battery had a bad cell and still had the original spark plug wires, and I assume spark plugs at 106,000 miles! Anyway, A new air filter is going in it today, and I will remove and clean the throttle body, I’ve read lots of GM trucks/SUV having sticking blades so its worth a shot.

According to the article I read about this GM 2ML70 transmission, the HV batteries run the electric AC compressor at all times. It is odd that I only have this stalling/stumbling/hesitation issue on hot days. Its common on GM truck hybrid threads over on greenhybrid.com, yet no one has seemed to find a good fix, not even GM. The hybrid tech working on my car didn’t seem very confident in his diagnosis. I wish I was in a carb state for that added warranty.

[Jim]

I just kinda skimmed through the codes and definitions, bulletins through GM’s service information. It could be battery related, meaning the big guy under the back seat. There is a test that can be done to located faulty batteries within the module. I also found a bulletin stating to make sure the latest and greatest software has been installed as well. If you took your vehicle to a dealer and they were unsure how to proceed, they may not get many hybrids through the door. I would probably look for a dealer, any GM dealer that is larger and gets a lot of these sorts of vehicles. There are special tools to properly test the systems, but not all the dealers have them. Plus, since this system carries enough power to kill you instantly, some techs aren’t too excited to deal with them.

Just take it to a bigger dealer with the experience to handle this vehicle, don’t try to fix or test it on your own. Like I said, it could just be a programming issue.

[Author]

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