TH400 shifting issue (modulator)
Thread Starter
Racer
Joined: Apr 2007
Posts: 361
Likes: 0
From: Sacramento (Citrus Heights) California
TH400 shifting issue (modulator)
Just replaced my modulator because I had a bit of an issue with shifting when cold and I've also topped up the fluid.
Here's the problem:
I can get First and Second, but only while the shifter is in "1" or "2" but in Drive I can't get it to stay in gear under any kind of load, though if I have the modulator vacuum disconnected and capped it stays in gear with the shifter in Drive. Is this just a modulator adjustment or could there be a deeper problem?
I'm a little afraid of overadjusting the modulator and moving my shift point too far up the RPM band.
Here's the problem:
I can get First and Second, but only while the shifter is in "1" or "2" but in Drive I can't get it to stay in gear under any kind of load, though if I have the modulator vacuum disconnected and capped it stays in gear with the shifter in Drive. Is this just a modulator adjustment or could there be a deeper problem?
I'm a little afraid of overadjusting the modulator and moving my shift point too far up the RPM band.
Safety Car
Joined: Oct 2004
Posts: 3,923
Likes: 733
From: Youngstown Ohio
The problem may other than the modulator....even if you adjust it to maximum (or minimum), the difference in shift points only varies by about 3 mph or aprox. 500-600 rpm. But to rule out the mod, I'd check for vacuum leaks along the hoseway; I had a shifting problem once that was solved by fixing a vacuum leak. For peace of mind, you can replace the mod for around $25.00.
Thread Starter
Racer
Joined: Apr 2007
Posts: 361
Likes: 0
From: Sacramento (Citrus Heights) California
The problem may other than the modulator....even if you adjust it to maximum (or minimum), the difference in shift points only varies by about 3 mph or aprox. 500-600 rpm. But to rule out the mod, I'd check for vacuum leaks along the hoseway; I had a shifting problem once that was solved by fixing a vacuum leak. For peace of mind, you can replace the mod for around $25.00.
Replaced the modulator valve already, but it would seem to me that the modulator would work BETTER with less vacuum applied, since it seems to be slipping with the vac line connected but engaging with the line disconnected.
Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
im no expert on th400s like others here - i've only rebuilt one (maybe they will post), but your original symptoms dont sound like the vacuum modulator. there should be several hydraulic pressures in the valve body that accomplish automatic shifting. if the tranny works using manual shifting, maybe it's just a sticking valve or something else simple in the VB. but i really don't have the necessary tranny diagnosing experience for you, so it could be a typical symptom of something else . . . sorry no better help
