TH400 reverse question


May 24, 2001
TH400 in one of my GNs works great going down the track, and the Pro tranny brake works perfectly. What's odd is that it rapidly slips in and out of gear when backing up. What is the likely culprit?


87 GN, original owner, 9.61 @ 141
87 GN, semidaily driver, 11.02 @ 124
01 Chevy Silverado K2500HD Duramax/Allison, semidaily driver and tow machine.
90 454SS, Whipplecharged, for sale.
ck the modulator valve. They stick sometimes

On passenger side of the trans by the cooler lines there will be a solenoid. Remove the solenoid and then remove the valve {you will need a small magnet or a small hook to pull the valve} . The valve may have {most cases they do} a small spring in front of it. Use a scotch brite pad, or some emery cloth to clean up the valve. Then Reinstall in reverse order {spring, valve,solenoid}. Also, ck the solenoid, it may be jamming up too.
Who's Brake? Is it neutral and push button for reverese or is it just put in reverse and it backs up?

I was thinking along the same lines as Bruce here.

The TCI brake, for example, has a Reverse in the Reverse shifter position but at a greatly reduced line pressure. I could exhibit similar symptoms as yours.

A much stronger Reverse may be had by putting the shifter in Neutral and depressing the trans-brake switch.
Thanks for the replies. It's a Coan-built transmission and they installed the brake - I dont know what brand. I put the shifter in reverse, and it does not engage immediately... goose the throttle and car will move, then kind of slip in/out as car backs up. I never tried holding the button in neutral.

I'll call Coan for brake info, etc., if the ideas recommended above don't pan out. Your responses have already shed some light.