TH400 slipping out of third gear

[Ethan Phillips]

Recently my TH400 started slipping out of third gear. I just had it rebuilt as it was doing the same thing before the rebuild. (Only has less than a thousand miles on the rebuild currently) Before the rebuild it hesitated to shift into third and was sluggish doing so, and I had to ease back on throttle for it to shift. Currently I have 24" of mercury of vacuum pressure going to the modulator. The VRV is adjusted all the way to the maximum towards the radiator. Could this be the reason it started slipping out of third gear before and now?

 

[Barrman]

It reads as if your rebuilder didn’t do something. The vacuum moves the transmission through the gears during normal driving. Put the truck into low range without locking the front hubs in and drive it. The engine should wind out and slam into second and then do the same with third. That shift action is a mechanical process controlled by the governor.

The only reason it should go back to second is when the governor determines the transmission needs to go to a lower gear. The TH400 has the electric activated downshift. The CUCV has an actuator for that on the throttle pedal assembly.

 

[Ethan Phillips]

It reads as if your rebuilder didn’t do something. The vacuum moves the transmission through the gears during normal driving. Put the truck into low range without locking the front hubs in and drive it. The engine should wind out and slam into second and then do the same with third. That shift action is a mechanical process controlled by the governor.

The only reason it should go back to second is when the governor determines the transmission needs to go to a lower gear. The TH400 has the electric activated downshift. The CUCV has an actuator for that on the throttle pedal assembly.

Ok thank you for the info. It shifts into second at low rpms at around 10-15 mph. I had the rebuilder install a shift kit into it as well. Do you mean shift manully?