I've got a 1999 Chevy Suburban. The CEL came on and gave me a code of " transmission module unstable". (It just started shifting hard from 1st to 2nd today and then the CEL came on). Anyone know where this module is located? Nothing in my Haynes manual on it.

old style burban and a 350 right?
that one should run off the ECM that under the hood to control the trans.

what is the code number? go through a pinpoint test and see what it says. been having lots of bad luck with those year ECM's. specifically in vans.
just remember you have to program those with a tech2 if you replace it, and might have to reprogramm the pats also, as the last 2000 chevy wit ha 6.5L i did i had to get the pats system working correctly before the ECM would program.

Yes, old style Sub with a 350. Code pulled was 1870(transmission module unstable). Dont know what a tech2 is but I know I dont have one. Is this something normally done by a qualified mechanic?

Its in the valve body. Very likely your 1-2 solenoid is taking a crap. If you dont know exactly what you are doing or where its at, Id highly reccommend that you take it to a trans shop. It can be done from the bottom of the trans, but IDK if the valve body has to be removed or not (cant remember, its been a few years). You probably have the 4L60E with the 350. Not the best trans in the world but it is better than previous models. If it needs rebuilding, you're lookin at about $1400-1800.

Ive got a 99 2500 Suburban myself. Great truck, bad on gas....

The chevy dealer said it is located in the VCM and must be re-programmed after install. So it looks like it's off to the dealer I go.

The chevy dealer said it is located in the VCM and must be re-programmed after install. So it looks like it's off to the dealer I go.

ya, thats what i was referring to. have fun programming......

Thanks, BurnedB. You hit it right on the head. I won't be doing any programming.