View Full Version : TH400 - Erratic downshift when cold

12-29-2012, 11:25 PM
I have a 55 Bel Air with a 350/290 HP crate motor and a TH400 trans. The car needs to be driven for about 30 mins in order to be warmed up enough to downshift correctly. If the car isn't completely warmed up and I try to downshift, it won't, or if I try to stay on the gas while getting on the freeway, it will shift back and forth between 2nd and 3rd gear, until I back off the throttle. Any suggestions? I only drive 10 minutes to work, so I rarely drive the car while it's warmed up enough to shift properly.

12-30-2012, 04:26 AM
Hi Rick
There are many things that can cause that !

Have you checked the vacuum line to the modulator, from the intake manifold at both ends? A vacuum leak here will cause the transmission to not shift.

Some basics about the TH400.

The vacuum modulator and governor work together to control shift points and shift firmness. If there is no vacuum supplied to the modulator, the trans will shift late and hard. As long as the vacuum modulator is connected to a vacuum port in the intake, it should work ok. The adjustment screw is a fine-tuning thing, it won't prevent it from shifting.

The kickdown on a TH350 uses a pull cable. The kickdown on a TH400 uses an electric switch. You don't need to connect the kickdown wire on the 400, but it will shift early and soft on D full throttle shifts if you don't.

Fluid level in automatic transmissions is critical. Too low is bad, too high is bad. Check it with a known matching dipstick and tube, with the trans warmed up from driving several miles, and with the engine running, trans in park. I make it a habit to apply the brake and run the shifter thru the gears first. Pull the dipstick, wipe it off, put it back in and pull it again, and read the level carefully, looking at both sides of the dipstick. (engine running!) If it's above the ADD mark, you should not need to add any fluid. Note: the aftermarket Lokar type dipstick setup is a real PITA to check and fill correctly.

also it sounds like the valve body could be mess up, the valves have to move (although the sleeves don't), so if it's a shift valve that's stuck, that could explain why it's not shifting.
Also a dirty tranmission filter can also cause problems .


03-03-2013, 03:06 PM
Thanks for your reply. I hooked up a test light to ensure the kickdown box was working properly, and it was not. I opened it up and saw that one of wires connected to the switch inside the box was loose and just barely disconnected. So I replaced the wires and connectors and resealed the kickdown box and now it works great. Thanks!

03-03-2013, 04:42 PM
Your welcome


03-03-2013, 05:10 PM
Sounds like Sid had the cure and it was easy. I was going to say you had an internal problem with the trans but your symptoms were backwards from the usual.