TH400 Modulator valve

[tolly4503]

Is it possible that a bad valve could be my problem, My tranny acts like its stuck in 3rd gear, when I start out after sitting, but once it gets going its fine. It acts like its stuck in a high gear. Im at work now but if the rains holds off Im goin to pull the vacuum line and check for oil. I almost forgot, the PO put a Edelbrock carb on and didnt hook up the kickdown switch, would that cause my problems also.
Thanks

[eightbanger]

Not sure I completely understand the question, I had a bad mod valve on my th400 the fluid it was puking did not effect the trans shifting at all, so I would say that's not your prob. Are you saying that when you are driving you get to 3rd and it won't shift into 4th? or are you saying that when you are in 3rd and you floor it the trans doesn't kickdown?

[tolly4503]

When I try to go from a stop it seems like its stuck in 3rd gear, but once it gets moving a little bit it goes back to shifting normal

[eightbanger]

So when the trans fluid warms up a bit it shifts normaly? obvious question, but have you checked the fluid level? other than that you may need a rebuild.
Sorry I can't be of more help, maybe someone else will chime in.

[tolly4503]

the fluid level is good, and looks in good condition, doesnt smell. I'm goin to drop the pan tonight, look for anything in there check the modulator and the governor. Really hoping its salvagable cant afford to have my truck down or the rebuild.

[eightbanger]

Modulators are a cheap and easy fix....you could try that and go from there, let us know what you find.

[Hottrucks]

also try a manual pull down??...I just put a mod and filter in mine together they cost $30

[ChevLoRay]

My truck has a non-stock carb and the P.O. removed and then didn't reinstall the kickdown switch. TH400 kickdown is electric, not mechanical by linkage.

I had my transmission valve body modified for better shifting and the shop checked out my transmission to be sure that it would downshift if the switch was operational. It would. But, they also installed the smallest modulator that they could find. It is black, and works well. I don't have the downshift that I'd like to have, but when the vacuum level drops low enough, it will downshift. It isn't a fix, but it is a possibility.

Lokar makes a kickdown switch that you should check out. It's what goes into my truck, when it is ready for that step.

[raycow]

Please state the problem more specifically. Are you saying the transmission starts from a standstill in 3rd gear? Or, are you saying it starts in 1st, shifts normally until it is in 3rd, but then doesn't want to downshift after that until you come to a stop?

Ray

[ChevLoRay]

If his does what mine did, it will downshift from 3rd to 2nd, but not kickdown into low (1st) unless you come to a complete stop. The small (black) modulator that is on my transmission helps that process, and can allow you to kick it into 1st w/o coming to a complete stop, if you are going slow enough.

To fix it, you need a kickdown switch, however you want to place it and actuate it. Some people mount the beneath the gas pedal so flooring the gas pedal will also switch the gearbox into a lower gear.

[dwcsr]

Kick down is just that only affects down shifting while in motion. Your issue sounds more like junk in the valve body, keeping it from shifting to first at a stop. do check the vacuum line for oil at the modulator. It may not hurt to change the filter and modulator. snap a piture of the pan before you clean it and post it here. If the junk in the pan is brown its clutches, sliver that a magnet won't pick up its aluminum and silver that a magnet will pick up its metal. gold is brass. The colors will tell you whats failing.

[swampfox]

If I remember right the modulator affects the up shift. My vacuum line came unhooked and the trans would not up shift out of 1st from a stop. I replaced it later on with an adjustable one, that would give lot firmer shift when I got on it.

[bigsnookmaster]

Originally Posted by swampfox

If I remember right the modulator affects the up shift. My vacuum line came unhooked and the trans would not up shift out of 1st from a stop. I replaced it later on with an adjustable one, that would give lot firmer shift when I got on it.

X-2 It sounds like an internal problem that is keeping it from downshifting after stopping. I don't know much about trannys, but if it's not slipping, it might be junk in the trans , or something simple that does'nt require a rebuild.You might try to google the question. Good luck!

[tolly4503]

Right now the truck barely moves except in reverse, so I dropped the pan, changed modulator, and checked the govenor. governor wasn't stuck, there was metal and gold flake all over the pan. The fluid was a little dark but nothing horrible, and didn't smell burnt. I'm thinking something in the forward gears is trashed, but I changed the filter and added Trans-X just to see of it helped any. Drive is still dead, 2nd gear creeps a lil but does nothing if you give it too much gas. I cant put it manually in 1st because who ever did the exhaust ran it right under the linkage, and it hits. Thanks for all the help

[ChevLoRay]

Your problem ain't the modulator.....

[tolly4503]

Ya already tried that, but for ten bucks it was worth it, I'm just gonna look for another tranny, I got one, but can't remeber if my truck is the short or long tail shaft, so Ill measure after work.