TH400 Behaving Badly

[Tire Burner]

I have an old 86 Truck that was Abused by the Previous owner. He Half-@#%$ Everything!! It has a TH400 Transmission. The Transmission was fine until i went 185 Miles non stop with it. The Guy I bought it from didn't have the Right Dipstick/Filler Tube for it, so he used one for a Ford (Can you Imagine?!!). Anyhow, i tried to find one at the local parts store, but they couldn't get one for me before i had to make the trip, so i figured i would just keep my Eye on the Fluid level (It was Leaking Pretty Badly). I Forgot. When i got to where i was going, 3rd Gear was gone and it Whined pretty badly. To make matters worse, i was 185 Miles from home on the Weekend with no Transmissions Shops open. I found someone that would do it, but it was one of those Places that make you Nervous. Looked like he Lived there and there were old Stripped cars all around, But it was all i had. Sure enough, he Rebuilt the Tranny and it shifted nice. He only charged me $450. I got it home no Problem, but lately it's developed a Problem. It shifts fine when it's Cold, but get it hot or Step on it hard and it forgets that it has 3rd Gear. Stop, Let it sit a While and it acts like it never had a Problem. I've checked the Vacuum Line and it's fine. Did this guy Ream me?  Perhaps it's the Modulator?

[VileZambonie]

What do you mean it forgets it has 3rd gear? Do you mean it won't upshift? Make sure you have no vacuum leaks going to the modulator diaphragm. If ok pop the hose off of the modulator and turn the little set screw counter clockwise and see if it helps.

[Tire Burner]

I Just came back from trying that. I took 4 passes on a Long road. The First three times it shifted Fine. on the 4th pass though is when it started acting up. What are the Symptoms of an Overheating Transmission? I have a Separate Transmission Cooler, but i'm running a 2200-2600 Stall Torque Converter.

[VileZambonie]

You have an auxiliary cooler in addition to your factory cooler correct? Rmember the purpose of the oem cooler is to regulate the AT fluid not to keep it cold. If you are towing or frying tire then add and additional cooler but don't bypass the oem one.

Overheating of the trans causes aeration of the fluid and venting.

Also make sure your detent solenoid or switch isn't causing the problem. Pull the wire off of the side of trans and see if it still does it.

[Tire Burner]

I think that MIGHT just be my Problem. I haven't looked yet. I don't really know how to begin, but i might replace that Cooler. Most likely it was half-@#%$ like everything else on that truck. I'll see if i can reuse what's there right now, but based on the other things that were done to the truck, i doubt it. I bet you're right. I bet he Disconnected the OEM Cooler.

[Tire Burner]

You're Da Man!! As you figured, the Transmission Cooler was isolated. You know, it just makes sense that a transmission cooler would be Supplemental. The Radiator is BIGGER than the transmission cooler to begin with. I got a Couple of Fittings, Some Hose Clamps and Cut the Cooler Hose. I then Routed it through the Radiator. I was nervous, but i went out and stepped on it. It shifted Wonderfully. I went back and forth and back and forth. I think i made 6 Passes Romping on the thing the whole while. Not even a Hiccup. My Problem was most obviously Heat Related. Thank you for Helping me. This was bothering me since i have to make that same 185 Mile Trip again this coming weekend.