Transmission Woes
 

My 2007 hummer H3 has recently been dropping out of OD into 3rd and back again frequently on flat level ground at highway speeds. Today it quit. Completely. I towed it home and it seems to go into 1st then gradually lose power after a few hundred feet. I can lock it in low 4 and it runs fine and shifts through all the gears normally. Tranny was rebuilt less than 20k ago. Anyone got a clue ? I’m about 99% sure it’s electrical but I’m stumped.

What is the level and quality of the Trans fluid?

Electrical? Maybe, but that would likely (not always) throw a code. Have you scanned it?

A rebuild is only as good as the rebuilder. Human error is always possible.

Hello Doc. I was hoping you’d pop up. Yes I scanned it. The only codes were for the cam position sensors. It’s really very odd. If I leave it in normal 4 high non lock or 4 high lock it will go into 1st and acts weird. Barely moves and only will shift into second at high rpm and feathering the gas. If I put it I. 4L it will run through all the gears perfectly without issue.


fluid is good and clean. The shop I had rebuild it is extremely reputable. Were this a motor issue I gotcha covered. I’m not a transmission guy tho.

Could a bad encoder motor cause it Doc ?

I might add the speedo is acting (or was) erratic when the problem of shifting outta OD started. Bouncing around from 5-7mph

Did you check line pressure?

I have not. I would assume pressure is adequate since it is shifting normally in 4L.

The plot thickens
 

Well. I have confirmed that mechanically the tranny is sound. It shifts through all gears in 4L. Now it seems the transfer case is partially stuck in low 4. There my be a 2007 H3 for sale in the near future 🤬

First, don't ever drive above 30mph in 4L! Second, driving in 4L increases TV position. Checking line pressure is 1 way to diagnose.


https://cimg8.ibsrv.net/gimg/www.hum...7c0469272f.jpg

Ummmmm ok. At 30 in OD the RPMs were not anything of significance to worry about. And it’s not like I went to town like that. It was just down my driveway to verify it was cycling through all gears. Which it is. I’ll check the pressure tomorrow. Thanks.