brokentee

Can anyone help? I have a 73 - 3 speed auto. It's shifting really hard. It's not making any noises but it keeps tearing off the the tires almost every time it shifts. When it's cold it shifts fine but when it warms up it shifts hard and chirps the tires. I have been shifting myself and it doesn't seem to be as bad if I let off and shift manually. The tranny was rebuilt (before I got it) and the lazy bums cut the lines and put in compression fittings instead of removing them properly so it leaks a little fluid. Is that the problem or could it be something else?

7T1vette

No, the cut lines are not your problem. Have you checked your vacuum modulator and/or the vacuum line going to it. That system makes the clutch shifts earlier and softer with higher levels of vacuum. If the vacuum isn't reaching the modulator, or the modulator is bad, the tranny shifts later and harder than it should. Verify that there is vacuum reaching the modulator; if so, change the modulator.

chris75stingray

could they have put in a shift kit?

kevinator80

I had a vacuum line come off the check valve going to the headlights that caused a condition similar to what you describe. I assume it reduced the vacuum available to the modulator and caused the hard/delayed shifts.

7T1vette

That'll do it!

Five-spot

For what it's worth, compression fittings don't leak if they are installed and tightened properly. Usually, you have to retighten them just a little after they've been under pressure for a little while.

brokentee

I've been having problems with the headlights popping up (surprise, surprise). I have a whole knew vacuum hose kit but haven't installed it yet. Could this have something to do with it?

7T1vette

Yes, if there is a large vacuum leak path. Then the manifold vacuum would be lower, which would be 'read' by the vacuum modulator as your accelerator pedal being applied a lot more [that it really is]. Thus, harder and later shifts. Plug the leak(s) and your problem will go away. It still could be a bad modulator. You'll have to fix one of the problems first to see if the other remains, or not.

dobroman2001

My 76 TH-350 was shifting hard (just like you stated) with the exception of it shifted hard all the time. I found that the spring located at the 1-2 accumulator was broken. (trans guy told me about it!!) Changed that spring and the problem stopped...

Here's a link (see page 41)

http://th350.netfirms.com/browser.htm

It was sorta of a pain to change because there is a snap ring that hold the cover on... but I messed with it till I got the cover off. I replaced the "o" ring while I was there..

Not sure if this is your issue but just something to think about...

Tab,

brokentee

Hey, thanks folks. I'll check all these areas. I have to finish installing the rear end then its on to other issues. I'll check back.

Quick question: Where is the modulator? Is it the unit under the car where the vacuum hose leads to next to the tranny?

Thanks

7T1vette

The vacuum modulator is a "can" which mounts to the side of the tranny [near the oil pan splitline] and has the vacuum line running to it. You can remove it by loosening the retaining screw/clip, but you need a pan under that to catch oil running out. It's an easy fix....but finding the correct replacement is not as easy.