Hybrid transmission

Discussion in 'Hybrid General Discussion' started by mtown27, May 13, 2018.

  1. mtown27

    mtown27 TYF Newbie

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    I’ve got a 2008 Yukon hybrid and just rolled over 200,000 miles. Seems like my transmission is acting up, clunky noises during shifting or pulling light loads. I’m curious if anyone has had problems with these transmissions or have ever had it replaced/rebuilt.
     
  2. canuc

    canuc TYF Newbie

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    hi there i have a 2009 yukon hybrid with 249000 kms on it i dont have any clunking noises but i had one hell of a winter up here in canada with my truck once the temperature went down to minus 20 or so for over 4 months every day once i warmed the truck up for 10 min or so and drove slowly away and approached a stop sign u could not get this truck to want to slow down and the drivetrain almost locked the rear wheels wmen trying to slow down and when stopped the engine was revving up to 3000 rpm with foot on brake in gear had to put in neutral or give gas to drop idle down, when trying to pull away from dead stop the truck would slightly hesitate then stop go stop u get the picture i mean sudden jerking coffee down defroster outlet get the hint, once u were at steady speed of 20 km steady speed ok slow down look out i mean dangerous cars behind u were pretty ticked this went on for over 4 months repeated trips to the dealer did no good no codes showed up for 8 visits and when taken out for road tests never acted up once even when i was a passenger with service adviser behind the wheel. now that its warm weather no issues because auto stop comes on and i start moving on electric motors which do not operate in very cold temperatures. i read a forum on line that was dated dec 14 bump and jerk for hybrids from 2008 to 2009 models tahoe and yukons that may have defective auxillary pumps on the transmissions which control all slow movements of the vehicle it wasnt a gmc bulletin, but one of the techs that checked my truck out suggested this pump i know have to wait until the fall again until this comes back, this pump could possibly be your problem, let me know what u find out. William
     
  3. braveheartwallace

    braveheartwallace TYF Newbie

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    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.
     
    Last edited: Jun 7, 2018
    Marc Marc and swathdiver like this.
  4. mtown27

    mtown27 TYF Newbie

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    That’s a lot of info. I appreciate that. I know the engine has its issues, AFM, etc. My concern is the aux trans pump. I’ve already swapped out the aux transmission control module. Some days to change the pump, some say you’d know if it was bad. The dealership in Memphis isn’t sure what it could be, they don’t see a lot of these trucks. I think the pump is a few hundred bucks plus the new wiring harness. Have you had yours swapped out yet?
     
  5. braveheartwallace

    braveheartwallace TYF Newbie

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    I have not had my aux pump or control module replaced.
     

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