TH400 slipping out of third gear

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?

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.

Barrman said:

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.

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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?

Does it stay in 3rd if you shift manually?

Barrman said:

The TH400 has the electric activated downshift. The CUCV has an actuator for that on the throttle pedal assembly.

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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.

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.

Barrman said:

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.

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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.

Barrman said:

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.

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It does not shift into third at all mechanically. As for what was done in the rebuild and what was wrong, the guy who rebuilt it said that it had lost hydraulic pressure. The torque converter also went out too and the truck wouldn't move at all. I had him install a shift kit also and a new torque converter. And now, when I put it in reverse, theres a delay of about a second and feels like its jamming into gear, and when in drive, it barely moves at an idle. Before the trans rebuild, it had decent speed at and idle.

Looking in the TM9-2320-2890-34 section 2-35 it talks about your issues. Check and make sure the fluid is at the proper level is about every other line. Then it goes through the vacuum and governor issues for shifting trouble. I goes into replacing the main pump once you get to not moving status.

I think you got a bad rebuild.

What does "slipping out of 3rd" mean? Does 3rd gear slip?

I once combined two th400s. 3rd slipped and burned up, I think that's called the direct clutch pack. I put new clutches in it, new seals on the piston, and in short order it did it again. I found that the direct drum was not sealing to the pump, those 3 steel rings on the pump have to seal into that "drum" of the clutch pack. Mine didn't seal because of a thrust issue, or to say another way, the drum wasn't close enough to the pump, probably caused by swapping guts and cases combined with lots of youthful ignorance.

When I drag raced that trans, I left the shifter in 1st gear and put 12 v on the "kickdown solenoid", that forces upshifts on the governor. I had a governor from a 70 GTO that shifted at 5500 RPM.

There is an adjustment on the "Modulator" . Take off the vacuum line and you will see a small "straight" slot for a screwdriver. Turn this in or out to fine tune your shift point.
The modulator is the transmissions way of knowing how much throttle you're using. So if your pressing down hard on the throttle, your transmission will stay in a lower gear giving you the ability to build up speed. Then when the "governor" pressure reaches a high enough point to overcome the modulator pressure it will shift into the next gear.
Conversely if you're just lightly pressing on the throttle the governor pressure will easily overcome the modulator pressure, and you will have quick upshifts.
I agree about first checking the fluid level. Then tune the modulator. If that still doesn't work, then you have a more serious problem.
You can check out my post of my TH400 rebuild I did about 6 years ago.
Also about the throttle "kickdown" lever. That is just to give you a quick way to downshift when your needing extra speed or power like trying to pass someone.
Sometimes this lever does stick (again check out my post) and you will have no upshifts since it is draining all the oil out of the governor circuit. The lever just actuates a switch which controls an "on-off " valve on the governor circuit.
I was a "Transmission" specialist for decades and rebuilding the TH400 was one of my specialties.
I have been posting on the rebuilding of the 303M HydraMatic in the M135 truck section lately and I have not been keeping up with other posts lately. Sorry I came to this party late.