The Stovebolt Site Forums The Shop Area Engine Shop t400 auto trans diagnosis

My 70 longhorn has an auto t400 trans. It is the first truck (out of about 20) that has an auto trans and I know nothing about them. I have put about 5k miles on it recently with no problem. Yesterday after about 30 miles and a stop at the gas pump it had trouble shifting. Fluid level is fine. It revs way up in first and then jumps into second. It does the same between 2-3 but not as hard. Where do I begin?

The T400 doesnt use a downshift cable, so it can't be that. Check the vacuum modulator at tranny. It has a vac line that goes to manifold vacuum and controls upshift. See if line is connected, and the modulator isnt leaking tranny fluid. It'll do that if the modulator diaphram is shot. TH400s use a downshift switch, check switch and wiring.

You can pull pan and check for metal or crapola in pan. Could be a sticking valve in valvebody, worn clutch packs, plugged filter, wrong fluid, band adjustment, shifter adjustment, govenor sticking, or bad torque converter. Hear any buzzing, whining or other noises? Find any serious metal in the pan, and its curtains for that build....

good luck

chip

Thanks Chip. Auto trans stuff always scrares me. I almost did not buy this truck because of it. Anyway I climbed under the truck and immediatelly found a vacumn tube to the trans melted. I put new headers on the 454 about a month ago. The vacumn line was loose enough to just touch in the right circumstances. Replaced line and I'm back in action.

TH400 is a real strong tranny. But don't get it hot and don't run it low on fluid. Caddy used them behind their monster v-8s in the late 60s and early 70s.

There is also another reason a TH400 or a TH350
won't shift. That is a stuck governor or the nylon gear is worn out and the governor is not turning with the output shaft. Most of the time people will not push the engine in rpms to get it to shift out of first when the governor or gear is bad but it would eventually shift.

Brent

Right on Brent. I had a governer gear strip out on a TH 400 I had in a motor home. It would not shift out of 1st. Turned out the motor home had sat a while and some small rust spots developed on the output shaft gear that drives the governor. Then it ate the governor gear.

Small correction here- - - - -
With a bad modulator, or missing vacuum connection, the trans will shift, but at high RPM. An apple-cored governor gear locks it up in 1st. gear, and it won't shift at all. Been there, done that, several times!
Jerry