[price]

I have a 89 Chev 1/2 ton with a 400 turbo. It started going into high gear and immediatly going back into second when the truck was not warmed up. Then it started doing it into 1st gear at slow speeds anytime.If I stopped and let it shift up again it seemed to be ok. We put in a new modulator but same problem. We pulled the kickdown wire and it has not done it since. Could the computor be doing this ? The tranny seems real good (for a lot of miles) and the pan had a minimum of crap ...
Thanks .Clint

[briscoe]

On this year 400 the passing gear is controlled by the computer.

The computer monitors the map sensor [ vacuum ] and grounds kickdown relay, for passing gear, when vacuum is low.

If you hook a test light to ground and to terminal "A", of the relay,then with the engine running, nail the throttle for just a second, the test light should come on, briefly.

Yours probably won't light up [ you disconnected the kickdown and trans works good ]. So if no light, with engine off, key on, disconnect the computer and ground the computer harness at terminal A7.

If the light comes on, suspect a problem with the map sensor.

If the light stays off, check for 12 volts at the relay, terminals D&E.

If you have voltage at D&E, replace the relay.

If you do not have voltage at terminal D and this is a C or K series truck, check for a misadjusted brake light switch, or bad fuse.

This procedure is for a no kickdown situation.

YOu can circumvent all of these diagnostic procedures and just install a kickdown switch and bypass the computer, if you like.