TH400 Shifting/Gear retention problem - any ideas?

[MTMike]

At Divide Ride this past summer, a new and strange problem cropped up with my TH400. The first day, I had an overheating problem and the truck got pretty warm a couple of times. On the way back, the truck would only shift into 3rd and hold 3rd with essentially no throttle. As soon as I give it any gas, even enough to just try to maintain speed, it will kick down to 2nd gear. It does it at highway speed in high-range, and it does it too at lower speeds in low range too. Here's the kicker. In the morning, heading out the trail, it would run 5-7 miles just fine in 3rd gear when it was cool. As it warmed up, it would suddenly kick down to 2nd. If I let off the gas, it'll upshift to 3rd until I get on the gas again.

Fluid is at the correct level, not burnt at all and is a good color.

Any ideas? A few people have told me it's the vacuum modulator, but I'm not sure.

I'm swapping in a 465 before I take the truck out again, but I want the tranny in good shape when I pull it out and sell it.

Thanks for any and all input!

 

[MTMike]

Anybody?

 

[dyeager535]

I'm a little suspect that holding in third with no throttle is not related here, but what about the kickdown switch on the pedal? Did it maybe get misadjusted?

Vacuum modulator shouldn't be the issue as I see it, that "tells" the transmission how hard to shift, the governor governs the shift RPM, imagine that.

If it were the governor, I would expect the problem to be consistent regardless of temperature, they tend to stick from junk in them. When it's acting up, you can try manually shifting the transmission between the gear that doesn't work and next lower, if the governor is sticking that sometimes fixes it.

In my case a sticking governor didn't allow upshift from 2nd. I upshifted/downshifted continously for about 5 minutes while driving, finally it upshifted and worked properly ever after.

 

[MTMike]

Thanks for your input! From what you're saying it sound smuch more like a sticking governor than the VM....

I drove it around a lot while it was doing that - probably a total of 200-250 miles that week, manually shifting the transmission, as well as driving both in and out of low range. Conditions were highway speed all the way down to crawling level 4+ trails, and it never un-stuck, and it always did it when warm. As it was warming up (not quite cold, but not all the way to temp) I did notice the 2-3 shift sometimes took 3 full seconds to complete, but as it came up to temp it would just quit holding 3rd all together. Additionally, the 1-2 shift was and still is neck-snappingly solid.

Perhaps the governor needs to be replaced?

 

[dyeager535]

With your temperature related issues, I'm almost wondering if you don't have something going on in the valve body. As it warms up the expansion is affecting something.

You could try replacing the governor, but I'd only do that if I could find one I didn't have to buy. You can replace it in all of 5 minutes. You could also pull it out (drivers side rear, under the dome shaped stamped steel cover) and blow it out with compressed air, maybe work it while submerged in some solvent. There isn't anything complex about them, it's usually the gear that strips if there is a failure there (it's nylon IIRC) if it's not that, perhaps a particle or burr that is messing with operation.

 

[MTMike]

Thanks for the input. I do appreciate it!

 

[dyeager535]

Edited my response, I mean that I'd swap governors if I could find one I could try for free that I was reasonably certain worked.

If you pull yours and it's obviously messed up, of course fix it and see if the problem goes away!