Automatic transmission, engine stalling while driving?

M0N5T3R H3

MEMBER
2007 HUMMER H3
3.7L
5 CYL
4WD
AUTOMATIC
146,000 MILES

I’ve done tests on the alternator and it’s not what’s causing the issue. My H3 drives fine and there’s really no issues besides when I hit my brakes, the headlights will dim, the RPMs will drop to like 400 and jump back up to 1,000 and then repeat a few more times. Sometimes it recovers, sometimes it completely stalls out. It’s an automatic, so this shouldn’t be happening. I lose all power steering which is dangerous in a state with busy round-a-bouts. I have to quickly shove it into neutral and start it again and put it into drive.

Any idea what might be causing this besides an alternator?

2 Replies

KEN L

MASTER CERTIFIED MECHANIC
42,904 POSTS

It sounds like the torque converter control solenoid is not releasing causing the engine to stall out, but we should run the codes to see what comes up. This guide will help, and I have included the location for the control solenoid in case you need it. Also, if the transmission has not been serviced, I would change the filter as well when doing the solenoid. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem.

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Monday, February 6th, 2023 AT 6:24 PM

BRENDON S

MECHANIC
653 POSTS

Hello,

Normally I would say check your vacuum brake booster because I have had vehicles stall out because the diaphragm inside the booster is allowing unmetered air into the engine. But I believe your brake booster is electronic. It uses an electric pump to increase the pressure in the system using a piston.

They are usually pretty reliable; however, everything breaks, eventually.

Do you have any trouble codes? If so, please send them over. Please check before you disconnect the battery or the codes if there are any will be lost.

Let's try this. Check all your engine and body grounds. If any are rusted, disconnect your battery, remove the ground, clean to bare metal, bolt it back on, and paint over it. I do this on my own vehicle, it ensures you won't have a problem again. Don't use Dielectric grease because it's non-conductive and you might not get a proper ground.

You could also disconnect the battery and unplug the booster. Zip tie it out of the way and to make sure it won't touch anything, ever or you could short something out. Once secured, reconnect the battery and see if it does it again. Take it for a ride around the block, come to a full stop and try to replicate the issue. Make sure your lights are on and you can even turn your heat on high to add more load to the system. When you unplug this unit, you will probably have a red brake warning light and an orange abs light. So, keep in mind you will have no brake assist and no abs. So, please use caution. If it doesn't do it with it disconnected, you found your culprit.

Make sure you disconnect the battery before reconnecting the module.

To me, at least at this point, it does sound like an electrical issue. Let's see what we can come up with.

Let me know how it goes.

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