TH400 troubles

[Gyoas759]

I'm trying to figure out what my trans is up to. 1989 3/4 ton GMC suburban with a TH400. It only has 47,000 miles so while it's acting up, I have trouble believing that it could be going bad so soon. (I double checked carfax and all to be sure. It's only 47k)

Bought the truck a few months ago and didn't drive it more than a couple miles at a time picking up a few parts and such. Mostly was sitting while I did some body work/rust repair. Today I finally brought it home from my shop. There was a high pitched whine that seemed to be coming from the trans. Made the trip home (10 miles) fine and I made a mental note to top off the fluid. This afternoon, I went out to Napa to get some trans fluid and just about a mile away, the trans disengaged and I had to leave it sit. I checked the fluid. It was all but empty. Walked to Napa. put 3 quarts in it (Full level again) and it went back in gear. 3 miles later, it disengages. I slow down and it will engage again. Then as I speed up, it seems to slip. The whole trans now seems pretty weak. Sometimes it doesn't want to grab right away from a stop. Sometimes it seems okay. It's particularly bad going up hills. I've also noticed that it seems okay when cold but gets goofy once the engine is warmed up. I left it with Aamco to take a look at it. Hoping it's something simple.

Can a th400 go bad that quickly? Everything I've read about them says they're all but bullet proof. Or is there something that maybe I missed? I'm more of a body work kinda guy. Anyone have any ideas on what it might be? Or should I be shopping for a replacement trans?

 

[nvrenuf]

The 400 is a great trans but it depends on fluid to function, once it's run too low all bets are off. The bigger questions -

Where did the fluid go in the first place?

Where is the fluid level now? (low again?)

 

[Gyoas759]

From what I saw when I was underneath it, the fluid was seeping out of the gasket seal on the bottom pan of the trans. When I got the truck, it was about a quart low. I checked twice after re filling the fluid and it was still full. Looked underneath the truck too and didn't see any obvious leakage. Just the bottom of the pan is damp with red trans fluid.

 

[4X4HIGH]

The filter may have fell off and also the modulator could be bad and sucking the fluid into the intake and burning it. Bets to drop the pan and have a look there first. If there is metal shavings in the pan then it's time for a full rebuild.

 

[diesel4me]

Good advice,the fluid level could have burnt the clutches,but the whining could be the front pump starving for fluid due to a clogged filter also..some debris in the pan is normal if was never changed yet,so dont condem it if you see some stuff in the opan,if it looiks like brass or steel shavings that is clutch disc lining,but I have seen TH400's with a ton of that in the pan that still worked fine..

I'd replace the filter and fliud,check the modulator for a ruptured diaphram (ATF will be in the vacuum hose,and some will pour out if you remove the modulator and tip it upside down)..the TH350 uses the same modulator,if you have one lying around,you can substitute that one for test purposes..

...also,if the downshift switch on the gas pedal sticks it can affect the shifting and gear engaugement..quick way to test for that is to unplug the wire going into the tranny case on the drivers side..