88IROC350TPI

I've had the TH350 in my car for 5-6 months now and everythings been smooth. I always noticed the car will shift automatically prematurely. For instance lets say I'm sitting at a stop light about to make a left turn... The cars in D and I take off by the time I fully complete the turn the car will already be in 3rd. It pretty much instantly shifts out of 1st when you move foward, goes to 2nd for an equally short amoutn of time, then goes to 3rd. No missed gears, no trouble going into gears. I always thought this was no big deal, doesn't really bother me but the other day I had the carb off and forgot to hook the vacuum line from the modulator back up. Took it for a spin down the road and the car shifted exactly the same as it does with the modulator hooked up. I'm no trans expert but I always thought the modulator controlled the shift points and without it hooked up the car wouldnt shift automatically? With it non-functioning does it have some sort of "default" shift points? Could this explain why the car shifts rediculously early?

Thanks,
Chris

ME Leigh

That sounds crazy. I'm no tranny expert, not even close but you might want to try an adjustabl modulator. It worked pretty well for me. I can adjust the shift point quite abit. I can make it shift anywhere in the 3500-6000 range.

When i disconnect the vacuum line, the tranny will not shift at all. Maybe you have other internal problems.

88IROC350TPI

see I just figured oldschool transmission designed for a non-performance car + 9.5" converter = early shifting but when I got back from the cruise around town and noticed the line was disconnected and it made no difference in the way the car shifted, I got concerned. It never stays in a gear, never slips, and the fluid is full and red.

Apeiron

Without the vacuum line connected, the modulator thinks you're always at WOT, and it should shift late, if at all.

RB83L69

If it stack-shifts with the vacuum line disconnected, then you have a defective modulator; a stuck governor; or line pressure problems. Very low line pressure will cause this. The trans will not live long if you abuse it in this condition.

I'd try a new vacuum modulator first. Get yourself a red stripe one. Small, gold in color with a red stripe. Not a pink stripe, not a green stripe, not a big black can; small, gold, red stripe. There's only those 4 variations that exist for THMs, so just look up every possible Glide/350/400 modulator you can find listed, and open the box, and buy the one that has the red stripe.

When you put it in, drive it around, and see if it's better. If that fixes it, there's an adjustment screw in the place the vacuum tube hooks up. Adjust it in ¼ turn increments until it shifts like you want it.

88IROC350TPI

I will definately try to find a red-striped modulator but the parts stores around my way are filled with morons so I doubt I'll find one. I do recall when my friend rebuilt this transmission he replaced the modulator with one he had laying around because I had bent the original while in-transit. I beat on this car constantly and drive it quite a bit with no ill effects yet. I check the fluid like every week and its always new looking.

Someone I work with who knows a bit more about transmissions than I do said it could be the weights and springs in the governor? He said they could just be designed for a car with a higher gear than my 3.92's?