New to me H3, 180,000 miles. Otherwise very nice condition.

When warm, the transmission does not shift down into 1st, even when stopped. PCM (according to Torque Pro) is commanding 1st gear, but it is not shifting, even when selecting 1st from the gear selector. When cold, it DOES shift into 1st. All shifts seem firm, and it does not appear to be slipping in any gear.

I changed fluid this weekend - fluid was very dark, but no burnt smell. No abnormal metal particles - just the normal dark sludge on the magnet. NO shiny particles. I also added a can of Seafoam Transmission Conditioner.

Service manual seems to really point to dirty or contaminated fluid, but to me that seems like it would be a persistent problem, and not one that is dependent on temperature. Unless the valve is binding in the valve body? Apparently the TFP could be an issue, but I have heard these almost never fail.

Has anyone encountered this particular problem on their H3s that could weigh in?

 

maybe something to read
https://cpt4l60e.com/4l60e-shift-solenoid-guide/

 

 

That is a lot of miles on an old AT.

If you feel lucky, I would change the fluid again since you did not flush out all the dark go go juice with the last change. If that saves the day temporarily, you can hold off on a rebuild for a short while.

 

I hear a transmission shop getting excited....

 

I appreciate the comments. bronxteck, the reading was good, and lead me to a few other articles with similar issues with details on the issues - accumulators, valves, etc.

Doc, I appreciate your opinion on the miles, but I'm not yet convinced the entire transmission is toast. Everything else works fine - shifts are firm, no slipping, etc. It *could* be that it needs a rebuild, but I want to dig a little deeper before that happens. My other H3 has 330k on the engine and transmission, and it is still going. Lucky maybe, but I change the fluid often (40k miles or so).

After putting a few more miles on the new(ish) fluid with conditioner, it seems to be shifting into first better and longer. Hummerz, I will be changing fluid again, and I'll see about a flush.

With just such a specific problem, I hope that it is an identifiable (and fixable) problem without a rebuild. Sounds like it is not a very common problem, so I'll continue to dig and update as I find issues (or it get's rebuilt).

 

the article i linked to mentions a sticking 1-2 shift solenoid. maybe check that out and its harness plug for a loose fit if your in that part of town again.

 

That is exactly my plan this weekend. I have been pouring over the diagrams for the transmission, and it seems that a sticking or only partially closing 1-2 shift solenoid could cause my symptoms. Here's to hoping for that.
Changing it is fairly cheap (~$35), and it will also mean changing the fluid again, which I wanted to do anyway.

If that doesn't solve it, I think I'm down to at a minimum taking out the valve body and cleaning it (if I believe it is just dirty fluid), or possibly replacing it with a rebuilt valve body (~$300 from my little searching). But then I'm getting into the Doc and Happy Hummer realm and leaning toward a full rebuild with the number of miles.

OR, just keep driving it, as it does drive well except for the 2nd gear start when warm. I haven't had it on the trails yet as I haven't had it long, but I'm sure it would be extremely frustrating with no 1st gear.

 

Same experience on my '07 a few years back with NO 1-2 automatic shift however I was able to manual shift with the gear shifter starting in 1 then shift at the proper mph shift points.
I have flushed replaced maintained the fluid after purchasing in 2012 which still looked clean & full when the shifting issue started/
I installed a new set of 1-2 / 2-3 A & B shift solenoids along with a new filter/pan gasket & MORE FRESH DEXVI!
Still the same after driving with new solenoids installed.
I knew my transmission shifts perfectly in the shift points around 8 seconds 0-60 when manually shifting!
That's when I found the CURE that worked for my 4l60e:

 

Circling back on this:

Replacing the 1-2 shift solenoid completely solved my issue. As a recap:

Symptoms:
2nd gear start. Would downshift into 1st under part throttle when just barely moving, but only when not hot. When hot, the transmission would tend to stay in 2nd gear for all starts from a stop.
Also would downshift from 4th to 3rd at 40-50mph when throttle was closed (going down a hill for example).
All of these were UNCOMMANDED shifts - the PCM was commanding 1st (or 4th) but the transmission was not complying.

Solution:
Replaced the 1-2 shift solenoid, which solved the issue. I BELIEVE that in my case the solenoid was NOT fully closing. This allowed the 1-2 shift valve to move to the N.O. position with lower pressure. When rpm's would go up, enough pressure/volume was available to keep the 1-2 shift valve closed.

Note that my solenoid checked out good resistance-wise (20 ohms). So I do not believe my problem was electrical, but rather internal to the solenoid valve itself.

Notes:
The work was very easy, BUT, if you do this yourself, hold the solenoid in when pulling the clip: The 1-2 valve is under spring pressure, and my solenoid along with the valve itself came flying out when I pulled mine. Everything turned out ok, and I was able to check over the valve, but watch out.

Part was $30 from AutoZone, but note that it does NOT come up on their computer under Hummer. Ask for SKU 201790 - picture below.

Valve body diagram below. 1-2 shift valve is 366 and the solenoid is 367A. You can see how it is held in with the clip. This is all accessible by dropping the pan - you do not need to remove anything else.

My Hummer is now shifting happily, and I am super relieved it was an easy fix!

BTW, for $30, I changed the 2-3 solenoid as well. Identical part and just as easy.