Hybrid transmission

[BG1988]

What’s the procedure for pulling the trans on a hybrid? I’ve installed some TH400 and 700R4 10+ years ago but those were on older trucks. I’m guessing disconnect battery, drain fluid and go.

disconnect both batteries

 

[BG1988]

I just have to say this Run it in M4 mode

Pull the gear shifter all the way down to M.

make sure it's set to M4....Tow mode off

drive it around for a few miles to reset the computer..

I was able to reproduce the result since there is a spot that i drive to and I always run into the shaking
and make it go away by doing that

 

[Nightskyisme]

Hi Tom,

My hybrid just rolled 200,000 last December and is currently in the shop getting a complete engine overhaul (oil pump/hydraulic lifters issues).
Around 150,000 miles the car shutoff completely and wouldn't start while displaying the dreaded "Service hybrid System" message. My chevy dealer "serviced/cleaned" the hybrid unit to fix the problem.

Before and after that service I have noticed the transmission seemingly slipping/lurching at strange times.
(Slipping being a loss of power at acceleration with increased engine rpm.)
(Lurching being the car suddenly pulls hard without warning. This has happened in stop in go traffic and while the car automatically switches in and out of auto stop.)
Clunky noises can be heard during several of these events.

I'll ramble and explain everything I have learned about the hybrid in the last year of working on mine. Hopefully it'll broaden your understanding of the inner workings of the hybrid. I wish I had know this before mine started acting up.


Now lets dive into whats going on/some common issues:

1. Some clunks originate from the system starting the gas engine. The hybrid variant of this particular 6.0L engine doesn't have a standard starter, instead the electric motor cranks the engine directly from the transmission assembly to the crankshaft. This allows the gas engine to start and stop quickly in traffic, but often results in, shall we say, inopportune starts. (Like a power lag when you need to accelerate or are towing a load the automatic system doesn't anticipate.) This type of clunk isn't harmful.

2. Shifting on the hybrid Tahoe/Yukon/Escalade transmission is quite different from the standard GM transmission found in other tahoes/suburbans/yukons/silverados, etc...
The hybrid transmission unit contains two electric motors, 4 planetary gears, and somewhere its all connected by a CVT (continuously variable transmission). These parts work together to make what appears to the driver a transmission that functions as a generic transmission with 6 speeds.

Lets expand this transmission more; it really only has 2 modes (hence the 2-mode hybrid name):​

​

First mode--at low speed and light load, the vehicle can move with either the electric motors alone, the internal combustion engine (ICE) alone, or a combination of the two. In this mode, the engine (if running) can be shut down by the computer under appropriate conditions and all accessories as well as vehicle locomotion continue to operate exclusively on electric power. The hybrid system will restart the ICE at any time it is deemed necessary. One of the motors, actually better described as motors/generators (M/Gs) acts as a generator to keep the battery charged, and the other works as a motor to propel, or assist in propelling the vehicle.​

​

Second Mode--at higher loads and speeds, the ICE always runs, and the hybrid system uses technologies such as cylinder deactivation (GM calls it Active Fuel Management; Chrysler calls it Multi-Displacement System) and variable valve timing to increase its engine's efficiency. In the second mode, things get a little tricky as the M/Gs and planetary gear sets phase in and out of operation to keep torque and horsepower at a maximum. Basically, it works like this: At the threshold of second mode, both M/Gs act as electric motors to give full boost to the engine. As the vehicle's speed increases, certain combinations of the four fixed ratio planet gears engage and/or disengage to continue multiplying engine torque, while allowing one or the other of the M/Gs to switch back to generator mode. This dance among the two M/Gs and four planet gears continues as vehicle speed and/or load fluctuates across road and traffic conditions.​

SO HERES WHERE THE CLUNKING/LURCHING COME IN!
All standard transmissions have a gas motor driven fluid pump. This lets the transmission open and close/engage on various gears.. GM hybrids have the same gas motor driven transmission pump, but when the gas engine shuts down, an electric auxiliary pump kicks in to take over the work of holding fluid pressure, and holding those clutches on the gears where they need to be on when on electric mode.

There is a lag between the time it takes the system to switch from the motor driven pump to the electric auxiliary one. This lag results in a drop of hydraulic pressure which results in gears engaging when they shouldn't. This produces lurching in severe conditions and clunks in less severe conditions.

Yes, this is a design flaw and can sometimes be fixed by replacing the auxiliary transmission fluid pump, but that may not solve the issue as it could also be computer related.
There is an active NHTSA investigation into this, but we are unlikely to see a recall.
I've thought about offering my Hybrid Tahoe to the NHTSA as a guinea pig since I have not replaced my transmission aux pump and the lurching problems are easily reproduced.

(Oh by the way, the NHTSA has issued a recall for the airbag systems installed in GM hybrids!)

Here is a video talking about the aux transmission pump and where it is:

And here is a video showing what an aux pump sounds like when it needs to be checked by a mechanic: (I have heard my hybrid make this noise before.)

Here is an article describing the bump/jerk/lurch:
http://www.autoserviceprofessional.com/article/94828/gm-hybrid-bump-jerk

Here is a detailed look at the hybrid transmission functions:
https://www.searchautoparts.com/mot...uction-gms-2ml70-two-mode-hybrid-transmission

And here is a workshop manual with info you may need to replace the aux pump (part numbers, etc.)
https://workshop-manuals.com/chevro...d_transmissions_service_auxiliary_fluid_pump/

Hope this helps. I'll watch this thread incase you want anything explained more/differently and I wish you the best of luck with your hybrid system.

Hello and what an excellent thread on these Hybrid Tahoes, I am a new member on this site and I have an 09 Tahoe that I purchased in December from somebody who had just put in a new engine and Hybrid battery, the vehicle has a hundred and fifty thousand miles on it and is actually in excellent shape, except I keep getting a recurring poc2b code and request to illuminate the check engine light, I am also now getting a lot of bump and jerk, as per the suggestion of a few other people on this site, I checked a couple of the large wiring connectors near the main battery of the vehicle and there's no corrosion, everything looks good, at this point I would like to trade in the vehicle but of course can't do that with a check engine light issue, I was suggested to look at the transmission harness wiring which I did not do as of yet, I'm just curious if you have any further suggestions as to what I can look at to take care of this issue, since this is the only code that I get, I am assuming that the auxiliary pump control module is probably okay? I have no problem replacing it if that's the issue as well, but since this is the only code, then I'm assuming as another poster indicated, that there might be a feedback wiring issue somewhere, any suggestions you can give to what I should look for would be greatly appreciated, thank you for your time!

 

[mtown27]

Bringing back an older thread. Has anyone swapped the aux trans fluid pump and wiring harness themself? Im ordering the replacement this week, just dont know the technical procedure for this. Truck has north of 225k miles on it now. I've already changed the trans pump control module a few years ago.

 

[braveheartwallace]

Bringing back an older thread. Has anyone swapped the aux trans fluid pump and wiring harness themself? Im ordering the replacement this week, just dont know the technical procedure for this. Truck has north of 225k miles on it now. I've already changed the trans pump control module a few years ago.

What year is your hybrid? They redesigned the connector on the wiring harness but not the actual pump for year model 2010 and later.
I replaced it myself on my 2009 and it was all pretty uneventful. The new aux pump from GM Part Direct came with a new gasket and the adapter for the wiring harness to fit my 2009 without me asking for or ordering the parts separately.
There was a core charge. I forgot what it was though and kept my original pump.

Aux transmission pump part number (redesigned for 2010): 19207983
Additional wiring harness part number required for vehicles prior to 2010: 24259588 (may be included at no extra charge like mine was, may not be. Inquire before ordering.)

Pump + Harness were $317.22 from gmpartsdirect.com according to my notes from 2019.

 

[mtown27]

As far as the removal and install, did you have to drain the oil or just remove the pump and bolt on the new one? Trying to gauge if I can do it at home or if I need to put it on a lift.

 

[BG1988]

As far as the removal and install, did you have to drain the oil or just remove the pump and bolt on the new one? Trying to gauge if I can do it at home or if I need to put it on a lift.

according to the dealer the transmission has to be drained if you're going to replace the fluid see if it's dirty first if it is filter the fluid(coffee filter) and reuse it

they told me it's 12qt (confirm with dealer some places state it's 13qt for the 2ML70 i'll check the user manual)go to o'reilly auto, and you can get it for $5.86/qt tell them to price match walmart

Valvoline Dexron IV​

 

[mtown27]

If I've already replaced the control module (2010 version), do I need to buy the additional wiring harness or is the new 2010 version aux pump pigtail long enough to reach the control module?

 

[BG1988]

If I've already replaced the control module (2010 version), do I need to buy the additional wiring harness or is the new 2010 version aux pump pigtail long enough to reach the control module?

was it 4 inline blades? in a square shape connector? and when you mean control module you mean the pump power supply correct?

 

[mtown27]

This is what I have already replaced. Aux pump control module is what it is. I guess that’s the power supply, although I’ve never heard it named as that. It’s been so long I don’t remember which connector went to what.