I've got an 89 GMC Suburban with the th350 tranny. Yesterday I was hauling my buddies Crawler back to his shop and during a long climb it would not downshift. Once we topped out on the hill it would down shift.

I know I am supposed to check the Vacumn Modulater and the governer. While turning the srew in the vacumn modulator what results will I get by turning the screw in either direction?

Any other things I should check? Thanks for your help.

i will guess that its not the modulator, as it is vacuum operated, and if you have problems with it, it will not upshift until you get full line pressure.

the other option would be the governor... if its got a worn out gear, replace it. check for movement of the spool valve inside.

It sounds like your kickdown cable is incorrectly adjusted. That cable that rises from the right side of the transmission case above the pan is the kickdown cable. When pulled under full throttle conditions it allows throttle pressure to override governor pressure effecting a down shift. Check the cable to make sure it is connected to the carburator/ TBI body. The plastic clip/ connector sometimes breaks and viola, no downshift. I would adjust it so it is in full detent mode when the throttle is fully depressed. Make sure your are achieving full throttle travel when the accelerator pedal is depressed from inside the vehicle. Cable adjustments are either a guillotine snap, a slide/ bolt or a detent clip at the attaching point on the Carb/ TBI. Hope this helps....PS. the modulator assist in controlling line pressure so you don't have exceedingly high pressure when you don't need it. Disconnect the modulator line and you will have exceedingly high and hard upshifts. If you turn the modulator adjusting out (counterclockwise) you will lower the pressure and have soft (slippy) shift points. Hope this helps...

i will guess that its not the modulator, as it is vacuum operated, and if you have problems with it, it will not upshift until you get full line pressure.

the other option would be the governor... if its got a worn out gear, replace it. check for movement of the spool valve inside.

Upshifting is not the problem. THe vehicle would not "downshift" till I got to the top of the hill(when I really didn't need it). I am suspecting it is a vacumn issue since at the lower rpm of lugging up the hill the tranny would not downshift. Once the motor was at a higher rpm it would downshift. I will sure check both the governer and the modulator.

Thanks for your help.