69'442

If I don't let my car warm up for at least 5- 10 min my transmission (th400) won't shift. Once it is warmed up it shifts fine. What can cause this? Fluid level is good. Thanks

bdub217

I'm going to take a stab in the dark since the pros haven't posted yet. I believe your problem is the Torque Converter going bad. Wise to get the tranny rebuilt if indeed this is what's wrong.

ah64pilot

69 - There are a few things you can check, like the vacuum tube coming off the intake manifold that goes down the back of the block to the vacuum modulator, make sure it's clean and clear of debri. Even a little grease in the tube could restrict air flow until it warms up.

Another thing you can look at is your modulator. If the vacuum modulator has trans fluid coming out of the vacuum port it's bad. You can check the diaphragm when it's cold by putting a length of hose on it and using a mighty vac to pull a vacuum on it. If you can't get it to hold vacuum the diaphragm is bad and it needs to be replaced.

There may be other reasons for this, but these are the most common. Don't go tearing into your transmission yet as Bdub suggested.

Bdub, for one, your torque converter has nothing to do with whether it shifts or not. And secondly, if your torque converter is bad it does not mean you need a transmission rebuild. Have you ever taken a transmission out of a car? I don't understand why you keep taking "shots in the dark" when you have no first hand experience. You are going to make yourself known as an unreliable source for information...if you haven't already.

ihengineer76

Has the transmission been rebuilt? Shift kit installed or any other modifications? If there are too large of clearances between the bores and spool valves, this can cause leakage which in turn changes pressure that affects shifting. When the valve body warms up, the gaps get smaller, leakage drops and pressure goes back to near where it should be. I'm not saying this IS your problem, but just a possibility. A pickup of mine had very similar problems and needed the valve body honed and slightly oversize spools installed to get clearance back to where it belonged.


-Tom

Highwayman

How long has it been since the transmission has been treated to a fresh filter?

olds1968

possible that the seals in the tranny have gotten hard and the fluid passes through and when it gets warm the seals soften up to shape.

bdub217

The torque converter normally takes the place of a mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated from the power source. If it were to slip would it not shift correctly? And I never told him to tear his tranny apart, nor do any other crazy thing, just put the possibility out there.

ah64pilot

If it were slipping it wouldn't go anywhere. Or it would be letting go during acceleration and cruising. The OP didn't give that symptom so I would not suspect a converter problem.

Rickman48

I agree with seals leaking internally - put some 'trans medic' in it, and find some $$$$.

bdub217

okay, thanks! and seals leaking because heat creates friction so it cant "grab" when there's no heat?

69'442

Thanks everybody. 3 weeks ago I had a bad tranny leak. Turned out to be the grommet for the fill tube and the pan gasket. Changed both and the filter and new fluid. No more leaks but now this. I will check the vacuum tube and the modulator. I think it has a shift kit but not sure. How can I tell? I don't know if or when it was rebuilt. Bob

oldcutlass

Also double check the fluid level, I too would recommend the modulator check. Do not add any trans fluid conditioners yet.

matt69olds

Drop the pan and replace the filter. While the pan is off you can look and see how much crap is in the bottom. If its a bunch, start saving money for a overhaul. When you are ready to reassemble install 2 o-rings on the filter suction tube. Make sure the suction tube is in good condition. I have seen the plastic tubes cracked, especially if somebody places a floor jack under the pan and dents it. Reinstall the pan, fill to the correct level and hopefully all is good.

As far as your shift kit question, most spacer places that come with shift kits are plated with a gold tint. Trans-Go plates are black and much thicker than stock or most other aftermarket.

ah64pilot

Read the OP's comment:

My bet is you left off the vacuum hose on either the manifold or the modulator...if it was working before you changed the gasket and such.

69'442

Ok, I blew out the line to make sure it was not clogged. I then changed the modulator. It finally stopped raining and I took it for a spin. It shifts great now with no warm up. It actually shifts the best it ever has. I got lucky. Thanks for all the help. You guys are the best.

classicjoe

Just saw this thread...Glad you've got your problem resolved.... I had the same thing happend to me a few weeks ago when I removed my a/c box. As a result of me moving the trans vacume hose by the manifold, the vacume line by the modulator, which was old and hard, broke off. I replaced the hose and the trans shifts great again.