2008 hummer h2 automatic transmission 121k. so when driving low speed accelerate get THUD feel my truck, (only low speed accelerate changing gears) highway no problem . can you help me out find the issues. thanks folks

 

I'm thinking low line pressure in the transmission. Could be a couple different reasons, all buried inside the case. I'm assuming you have the 6L80 in the 2008? It's an expensive enough transmission.... Should be worthwhile to have a transmission shop do a proper diagnosis on it. Some items can be addressed with it in the vehicle.

Definitely make sure your transmission fluid isn't overfilled or underfilled. Both scenarios can cause issues.

 

thanks, you think low pressure related to the transmission filter clog?

 

I Have a 2009 H2 Lux. with the 6L80. I'll get a little klunk every now and then but it sounds like "U" joint play or transfer case slop.I have abot 78 K on the clock. There is so much gack between the flywheel and the road wheels by the time the power hits the road 5 Bazillion gears and mecks have to move. Otherwise. Starting out in first with a steady pedal ...it goes thru all the gears nice and smooth. No Klunks.

It will Klunk when I shift to reverse when stopped and back again. But that's the only time. If I hit it hard from 6th to pass it will drop down and scream and go thru the gears ok. Since you have a lot of mileage, It it were me ...I have all the "U" joints checked. They can ware and still drive the truck but pick up sloop and make noise.

Just My $.02 worth. There are many on the H2

 

Are you talking about tip in clunk? As in coasting or slowing down and then stepping on the gas again? Or when you press the gas and the truck downshifts? If so that’s just a characteristic of the truck. It will do it no matter what. It’s a combination of having the sloppy 2500 front diff in an awd application it isn’t designed for and the crappy tuning and design of the 6l80 transmission.

If your clunk is really bad check all your u-joints and the front differential bushings. I’d be willing to bet all your front diff bushings are bad. Again. Crappy gm design flaw. The bushings they used are too small and too few for the front diff on a 7500lb truck.

these are the bushings I replaced mine with. Wasn’t the worst job in the world. There was nothing left of the drivers side upper bushing at all. Reduced clunk by about 50% but was still really annoying.
https://www.outfitterdesign.com/h2-f...shing-kit.html


I just posted a thread a few weeks ago about doing a part time 4wd t-case swap on my 08. Eliminates all of the clunk. Drives 1000% better.
https://www.hummerforums.com/forum/h...er-case-48529/

 

No, I doubt it's the filter. The filters in the pan are more like screens, never really get plugged. (Which is why I run an inline hydraulic filter)

​​​​​​Wouldn't hurt to check your u-joints but I sort of doubt it. You'd be hearing the clunk when you take it out of park. Same for the transfer case chain.

​​​​​​

 

If your running a stock trans tune then this may help, if it has not already had the updated program installed, If I remember correctly I've looked at the modified tables in the tune for this issue and the piston slap tables also.

Neal,

TECHNICAL

Bulletin No.: 08-07-30-012

Date: March 13, 2008

Subject:
6L80 Automatic Transmission - Tip In Low Speed Clunk in First Gear When Throttle is Applied and Released, Engine Braking Sensation at Low Speeds (Reprogram TCM)

Models:
2008 HUMMER H2, H2 SUT

with 6L80 6-Speed Automatic Transmission (RPO MYC)

Condition

Some customers may comment that the vehicle has a "Tip In" low speed clunk in first gear when the throttle is applied and released in normal "D" drive. Other customers may comment on an engine braking sensation at low speeds in normal "D" drive.

Correction

A revised transmission calibration has been developed to address these issues. Reprogram the transmission control module (TCM) with updated calibration files using the TIS2WEB Service Programming System (SPS) application. As always make sure your Tech 2(R) is updated with the latest software version.

Warranty Information

 

It wouldn't surprise me if GM does have an update for that. All of the aftermarket tuners when it comes to the transmission, bump up the line pressure a little. Gives firmer shifts, produces less heat and extends transmission life.

 

So That's for 2008 Models of the H2? My 2009 seems fine?

 

Yep, I think the 09 has the modified tables for this issue, I haven't downloaded the 09 OEM tune to look it over so Its a guess no my part.

Neal