Hey guys n gals,


I've got an 08 Tahoe Hybrid 4WD 105,000 miles I just bought from a Chevy dealership and already having issues....just my luck. Anyway, when at a stop, I press the accelerator to start going and the truck hesitates and stumbles, not all the time, only intermittently, seems to happen more often when the AC is on which may just be a coincidence... maybe heat related? Poking around I found a bad cell in the 12V battery and replaced that since it was only supplying 220 cold cranking amps, but that didn't resolve the issue. It did need to replaced anyway so no hard feelings bout that.


Every once in a while when it gets really bad, the truck will stall out completely and throw codes on the ECM : P0AC4 and P061B. I've seen some others on here talk about the bulletin related to said codes, and have read through it. I'm starting to wonder if I need to replace the auxiliary trans pump below the bell housing, or if perhaps the aux pump control module under the 12V battery is the culprit. Looks like the aux pump from GM parts is about $250 which isn't bad and looks like an easy fix...unless I'm not considering some aspect of the hybrid system that should keep a DIY guy from doing a simple job like that?


The local dealership has one hybrid tech, I figure I should let them diagnose it first before I start replacing anything, but I'm afraid that they wont be very good at diagnosing, considering some horror stories I've read on here about GM hybrid techs.


Another member mentioned that the cooling fan for the hybrid batteries could cause similar issues, possibly blocked ventilation passages to the rear seat area? Ill have to figure out how to check this cooling fan out to see if its operating properly.


Any additional insight on the matter is greatly appreciated!

 

Return to the dealer where you got it and have them pull diagnostics.

 

I would normally do that but I bought it out of state, so I'm willing to pay the $70 diagnostic fee locally if need be

 

P061B can indicate a HV battery issues However, get more data in the form of the 20 Volt Meters on the internal HV battery cells from a good scan tool.

 

Well, I really thought that my issue may have been the cause of the vents being blocked near the floor on the back seat (for the squirrel cage fan), but after ensuring we removed the high chair cover from that area, the problem still persists.

I have noticed it only happens when its hot out... typically first thing when getting in the car after its been sitting out in the sun in 90+ degree temps. Once the AC is on for a while it seems to run fine again.

I've also noticed that sometimes before the truck has the hesitation/stalling issue, or right after, the engine seems to run poorly. At a stop, I can feel it chugging, as if the engine isn't running smooth.

 

Check fuel pump? Just saying!

 

" get more data in the form of the 20 Volt Meters on the internal HV battery cells from a good scan tool."

There are also 5 temperature sensors in the HV battery pack. Let's see just how hot it is. Can you hear the fan runing under that back seat? The scan tool is able to command the fan to test it. There is an exhaust duct going out throught the floor that is easy to remove and check for obstructions and flow.

Any Check Engine Light brings with it stored codes. A P061B will set Freeze Frames that will allow a "skilled" tech to look at other vehicle parameters and make an informed decision on what to do next.

You are asking a forum to answer a question that could have hundreds of possible answers that may include fuel pump, slipping clutches, Aux Pump, HV battery, and sophisticated traction drive motor computers.

 

Thx Hillbilly,

I did verify the fan was running under the rear seat with the radio/ AC off at auto stop I stuck my head back there and heard it. Didn't even thin about checking exhaust for obstructions. It definitely seems heat related, only happening on really hot days.

I was hoping to save money and find a simple fix, but I'm now sitting at the dealership. The tech here sounds pretty experienced with the system, he pulled two bulletins related to the codes and symptoms. He's in the diagnostic process now, which he said he has to take snapshots and send them to GM before going further.

 

Bad news, the tech just came in and told me the Hybrid battery needs to be replaced, a $3400 price tag just on parts... I feel the Chevy dealership who sold me this car two months ago should be held responsible somehow.... But we all know that won't happen.

He followed procedures outlined in two bulletins and sent the scanned data and snapshots to GM. The rep he was getting help from insisted that because the code P0BBD was present, he was sure that it's the hybrid battery. The tech said that it could also be the trans aux pump or aux pump module but he can't test them with the hybrid battery in its current condition. He said it is possible that they could replace the hybrid battery, only to find that the trans aux pump or module is the culprit, which doesn't give me a warm and fuzzy about speding $4000 on the battery.

As far as temps, he said one cell in the battery was showing a higher temp than the others, so he suspects that it's a bad cell.

 

Can you still start the vehicle? If you can, you're still in luck if they can pull any other codes besides the battery.