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TH400 slipping out of third gear

Ethan Phillips

New member
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Illinois
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

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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

New member
9
2
3
Location
Illinois
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?
 

Tow4

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Orlando, FL
The TH400 has the electric activated downshift. The CUCV has an actuator for that on the throttle pedal assembly.
I had this happen on and old Pontiac. The actuator got broken off and jammed. The car would only upshift near redline. Unplug the actuator and see if it shifts normally.
 

Barrman

Well-known member
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Location
Giddings, Texas
I hadn’t thought of that. Good idea.

The shifting at redline thing is what I was suggesting by going through the gears in low range. I just didn’t write it correctly.

If it shifts by a mechanical method only and stays in gear then. That means you are back to the vacuum parts or maybe the governor with the transmission probably good. Which in my mind brings up your rebuild. What exactly was done and was anything found to be wrong? To have the same issue before and after a rebuild just seems like you got ripped off.
 

CARC686

Well-known member
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Location
Las Cruces, New Mexico
I hadn’t thought of that. Good idea.

The shifting at redline thing is what I was suggesting by going through the gears in low range. I just didn’t write it correctly.

If it shifts by a mechanical method only and stays in gear then. That means you are back to the vacuum parts or maybe the governor with the transmission probably good. Which in my mind brings up your rebuild. What exactly was done and was anything found to be wrong? To have the same issue before and after a rebuild just seems like you got ripped off.
Not necessarily. Vacuum system diagnostics are beyond the scope of a transmission rebuild. If it sounds like a cricket chirping under your hood, you've got a vac leak, probably in the pump. Contrary to popular belief, they are not unobtanium. You can pull the diaphragm pod off the very available Detroit Diesel belt-drive vacuum pump accessory. They are direct bolt-ons for the CUCV vacuum pump. Make sure your vac hoses aren't leaking. Try swapping the modulator. If none of that clears you up, clean and adjust the throttle position thing on the injector pump. The one with the vac lines going in and out of it. I assume you already unplugged the pedal switch.
 
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