th400 shifting problems

[big dan]

My brother in law just had his th400 rebuilt in a 79 3/4 ton and now it slips between gears.

When you put the truck in gear from say park or neutral it shifts into gear good and runs good, no slipping, until it shifts gears. When it shift from 1st to 2nd and from 2nd to 3rd its like it goes into neutral. If you let of the gas for just a second and then get back on it, it will go just fine, no slipping.

Reverse also works just fine, no problems. You can also hold it in gear and as long as you keep it in that gear it doesnt ever slip. It will only slip when shifting between gears.

Any ideas on what it could be? My guess was the vaccum modulator and I told him to replace it when he got a chance but my knowledge of automatics is lacking so I hope someone can help. Thanks.

 

[diesel4me]

Check first to see if the modulator has vacuum at the hose going to it with the engine running...if it does,it might have a ruptured diaphram,to check that you can remove it (one bolt with a fork like dewhickey holds it in ),and if any fluid drips out of the modulator when you tip the hose connection downward,its likely ruptured..

My experience with TH350's and Th400's that have a bad modulator or no vacuum going to them will refuse to upshift until the truck is going a lot faster than when it would normally shift,and it'll slam in harder than usual...also when you let off the gas,it'll feel like it slips into neutral and coast,instead of having an engine braking effect.....but shifting it manually or downshifting it manually,it'll act "normal"...

It could be a valve body problem also,but I'd suspect the modulator first...

 

[big dan]

It does take a little longer to shift up. Forgot to mention that but it is one reason I suspected the modulator.

 

[76zimmer]

Dan, I had a similar sit. with my T400. would drop in gear nicely, and the 1-2 shift was great, the 2-3 was only noticeable by watching the tach slowly drop in rpm (when held at a steady speed) It ended up being the direct drum clutch pack not being set up properly and the piston was binding in the drum. But by all means check out external possibilities first. In my limited experience a malfunctioning modulator just wouldn't allow normal upshifts. the stock valve body is programmed to make an upshift from 1-2 at about 5K I believe even if you hold it in 1st gear.

 

[Greg Ducato]

Make sure the vacuum going to the modulator is coming directly from the intake manifold and not a ported vacuum outlet on the carb, too much vacuum will lower line pressure in the trans potentially causing slippage between shifts. Your vacuum line should be steel and the ends good solid rubber, if the ends collapse they can store vacuum in the line with the same high vacuum results.