1991 v2500 trans problem, TCC


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So i bought a ls swapped 1991 suburban 2500 4x4. From the factory it has a 4l80 transmission. When i got it the 5.3 was already installed and running good but the torque converter won’t lock up. So i took it to a mechanic friend in Louisville ky who specializes in ls swaps. He did his magic and cleaned some stuff up.

Here’s the kicker, the speedo works about 40% of the time. When i take off it always works up to 15-20 mph, then it bounces around and if it stays working once i hit 30-35 it sticks and works like normal on up to whatever......but if it doesn’t stay working and drops to zero as i speed on up to whatever the torque converter wont lock up. I know its an electronic speedo but i dont know what causes it to bounce and flutter like that. My mechanic friend is a bit stumped by this (he doesn’t do many 4x4 swaps).

Any help will be much appreciated.



Check to make sure all grounds were put back on. you should have two coming off the back of the block some coming from the Block to the frame and there should be a couple behind the dash even add a couple for extra safety. Anytime I've ever ended up with some weird electrical Gremlins that's always the best place to start


Unless you have an aftermarket speedo, your 91 Burb has a cable driven unit with a speed sensor built into the speedometer itself.
Erratic is usually cable in my experience, but dropping out is gears... I have one in an old dump truck that just reads way slow occasionally. Haven't figured that out yet either, but I haven't looked into it at all.

Having the speedo drop to zero means the computer also thinks you're going zero, so it's no surprise it's not telling the converter to lock. Check the cable. Lube it. Check the speed pickup and the gear in the back of the t-case.

Edit: Nevermind. I'm evidently smoking my lunch too often. Perhaps it's the vans that had a cable unit with the speed sensor in the speedo, or was it the 87 trucks?? I don't remember anymore...
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I owned a '91 Suburban 2500 4x4 5.7l with factory 4l80 and spent $2600 on a trans rebuild. Was having the same Speedo problems as you are, I even changed the speed sender at the transfer case. The engine would even go into limp mod.

After trading the burb off. A year later I found out that there is an electronic module up under the dash that has something to do with Speedo/gear change/engine management. The current owners replaced with a remanufactured module and it runs awesome, fixed all of the problems.

Only 1991 electronic Speedo have this module. Hopefully this helps.
That's a drac module. That's why I was asking what original poster was using for trans control. It's also a likely culprit. And asking so somewhat calibrate able to correct for tire size asking London gear changes