Tom Johnson

71 350/270 400 Auto Before I had the tranny rebuilt it would not downshift after 50. I had the tranny rebuilt and its doin the same thing. Could it be the detent switch not working? It just seems strange that it works before 50, but not after. I would think that it works or it doesn't. Any thoughts?

ZD75blue

i think there is a built in throttle switch for wot so it doesnt down shift, though i think at 50 you wouldnt be at wot... might wanna check the vaccum modulator watcha doodle

ZD

Techno

Do you mean that your driving 55 and you down shift manually? Or you floor it to get it to down shift?

Tom Johnson

When I floor it.

Tom Johnson

The tranny shop found my problem. It was a bad wire going to the tranny. At lower rpm's the kick down is operated by the vacuum modulator. At higher rpm's the kick down is operated by the detent switch. It cost me $160.00 to find this out, but now I know how passing gear works in these 400 trannies.

Jvette73

On the Th400 the vacuum modulator will cause a kickdown on part throttle. The electric pedal detent switch will cause a kickdown only on full throttle if its working properly. Heres how you can checkyour pedal detent kickdown.

1. If your electric detent is working you could cruise at 35mph in 3 gear, then punch it, and the tranny will drop to 1st gear.
2. If the detent is not working, cruising at 35mph, punch it,
the tranny will only drop to 2nd.
3. On the highway at 55 or 60mph cruising in third gear, If the detent works, punch it, it will drop to second.
4. If the detent is not working, it may not downshift to second at 55 or 60mph.

The way the detent works is: When you floor the throttle, the detent switch under the gas pedal opens the circut to the solenoid inside the tranny pan. The last time I fixed a 400 detent it needed the pedal switch adjusted and a new solenoid which required removal of the tranny pan.

Its a shame it cost that much to fix it. I bet its worth it now that it works right though.

Tom Johnson

Thanks for the info Jvette73, When I drove the vette home from the tranny shop the downshift is still not working properly. The only difference I noticed is it will not downshift after 45. Before it would not downshift after 50. I picked it up at closing time, so I'll have to wait till tomorrow to call them.

Tom Johnson

Picked up the vette from the tranny shop. The carpeting and floor mat was keeping the gas pedal from being able to be pushed down all the way. The guy at the tranny shop bent the acclerator pedal rod a bit. Im gonna check the carpet and see if I can get a tighter fit. The floor mat still gets in the way