TH400 Downshift

[Guest]

I just picked up a 70 GS with a 455 and TH400 tranny. I can not shift it down into low 1. Is there an adjustment on some linkage that can be made or do I need to do something else. When the car is in drive, everything is fine, but say I am doing 15-20 and want to kick her down to first, she does not down shift and I can not manually put it down into first. First of all, should it automatically drop down to first and secondly, why can't I shift it down myself. Any suggestions would be greatly appreciated.

[Guest]

check the vacuum diaphragm. these trannies shift by vacuum. the diaphragm has a little screw in it when you pull the hose off. screw that all the way in. i used to have this same problem till i replaced the vacuum lines to the tranny.

[Guest]

Sorry, but the ST400 and TH400 transmissions downshift via a 12 volt detent solenoid inside the transmission (there's an electrical plug on the driver's side).<P>If the solenoid is bad or it's not getting 12 volts, the trans won't downshift.

[Steve B]

Check the solenoid as 462 cat described, but you should be able to manually pull the TH 400 into low below 40 MPH. Check to see there is nothing in the way of the shift linkage under the hood or where the linkage connects to the shifter arm. Make sure the bushing and spring are on the shifter arm that goes to the trans. There is a linkage adjuster for the gear selector where the linkage connects to the shifter arm on the trans. Loosen the bolt, push the linkage up through the adjuster a little bit at a time, and tighten the bolt. Repeat the process until the trans will click into low when you shift it there. You may need to adjust the neutral safety switch if the back up lights do not come on in R, the starter won't engage in P or N, of if the starter will engage in R or D. Then go have fun in low. Bet your tread life will never be the same!