Technical GM TH400 issues

I need an automatic trans EXPERT to weigh in here. Trans is out of a mid 70's Cadillac. This trans sat for probably 30 years, before I fired it up. Installed fresh ATF, new front & rear seal, new filter.
ISSUES:
1) Whines in Park & Neutral. Sometimes loudly, sometimes not.
2) Fluid level changes drastically - no leaks.
3) Has worked as it should but lost all gears more than once.

Filled with ATF, checked w/ trans warmed up. Right at full level. Drove it half a mile, and it lost all gears. Checked fluid - not on the stick. Towed it home. Started it up to use power steering to push into the shop, and it went into gear. Added 2 1/2 qts to get it back to full. Checked the next day (warmed up), and its way over full. Let it run for 30 minutes, and it's 1 1/2 pints low. Add again. Seemed to be working but whining. Until today. Backed it out of the shop, then nothing. Shifted through the gears a couple times and it works. Drove around the shop, and it started shuddering under acceleration. Now, no gears again. WTF? Maybe torque converter?

 

dirt dauber in the vent tube?

 

How many quarts of fluid have you put in it?

 

Have you dropped the pan to see if the filter is clogged? If it is, you probably have more troubles.

 

Whoops, I see in your description that you installed a new filter. Some whining in park and neutral can be normal. Loud whining typically means the pump is starved for fluid. Was there any debris in the pan? I've never had a torque convertor cause a shudder, that is normally a slipping clutch.

A 70's 400 had a plastic tube between the filter and the pump pickup. I believe it is sealed by an O ring. If that was missing it might cause the pump to suck air and whine. If the pump can't pump fluid, there is no pressure to apply the clutches and you lose all gears.

 

Sounds like the pump is not picking the fluid up at times. Is the transmission behind the original engine/flex plate?

 

My transmission sat for about 30 years also. Pulled it for a rebuild just to be safe. The transmission guy couldn’t believe it worked ok. All the seals were dry and cracked. Just bite the bullet and rebuild it

 

This is the most likely problem, the o-ring jar racer referred to is on the tube where it goes up in the case. The best filter to put in it is a Hi flo which is a brass screen , if it has a deep pan the pick up tbe is longer than one for a shallow pan. So it could be any number of things.

 

If the filter you installed wasn’t the metal mesh type, replace it with one.The cloth type was very restrictive and the crap made these days are even worse.Bad news is the pump is on its way out.

 

Ha! I have no idea. I don't do an inventory, just add until it's full. But then it wasn't. And then it was over. And then it wasn't.

I went outside & fired it up, and it went right into gear again and I drove it on my lift. Wasn't whining. Any chance it can be the TC?

 

It sure sounds like the whining is pump cavitation due to getting air into the pump intake (filter or tube). The cause of this is still unknown.

the odds of it being the converter itself are pretty low, I think. But...since you keep suspecting it, do you not feel confident in how you installed the TC? If you put it in wrong, usually the pump gear tangs break off, and it makes noise, and doesn't work at all.

 

What is your exact procedure for checking the level?

Hot? Cold? Running? Not running?

 

I would recommend you watch a few Precision Transmission vids on YouTube to get familiar with the inner workings. Not a trouble shooting thing per se, but still good for understanding the basics and some specific things to look for as mentioned above.

 

If the filter setup is for a shallow pan and you have a deep pan, this kind of thing will happen.

 

Mouse nest in the oil pan

 

If your level was at full and then goes way up when parked for a while, the converter anti-drainback system is probably not working. The converter level is higher than the pan and at rest it will drain back into the trans without a valve to keep it full while parked. I'm not familiar with GM trans as my work has always been with Ford trans systems, so you will need to find the information for that.

For example, on some Ford transmissions the drainback valve is held in place with a tab on the filter and drops out when you change the filter. Don't notice it and forget to put it back during a filter change and misery ensues...

 

If you installed a paper filter, I have had those that will not flow right out of the box. I think they get damp or moisture in them when sitting on the shelf and they will whine and cause all kinds of problems. Also the feed tube for the filter should get two O-rings. They come stock with one, but it’s best to add another one. The Muddobber, or even a spiderweb, in the vent as Squirrel suggested is a real thing and it will definitely cause what you’re talking about!

 

I've seen an anti-drainback valve in the pump, it's a wound flat spring steel thingy that fits into a hole in the stator support. I'd have to go looking to see if they had them on the later transmissions, I know they did on the early ones.

 

Pull the Modulator out and pull the shaft out and clean it. Probably tighter than old dicks hat band.

 

That doesn't cost anything except a few minutes and it is prudent to check the vacuum line from the manifold to the modulator to make sure that there are no leaks (usually on the short pieces of hose) on it too.

 

yeah, but it's not a vacuum modulator problem...at all....

 

One thing to check since it is on a lift is to remove the TC to flex plate bolts, push the TC back as far as it will go, and measure the clearance between the TC lugs and the flex plate. Should be about 1/8". Much more than that could be pulling the TC out of the pump tangs. Most likely not the problem, but worth checking. 30-year-old dry seals more likely. Nice thing is T-400s are easy to rebuild and don't require anything more than a few home-made tools to do it.

 

Thanks for most of the replies. Between a couple of answers on here, and talking to a local expert, it's sounding like a pump problem. The cure is one I wanted to avoid: pulling it out & spending a bunch of money. I have used several automatics that sat for years, with no problems, but this car fights me constantly.

 

Just to make sure, you are checking the fluid level with the engine running, right?

 

Everyone mistakes the whining noise for a pump gear noise when in fact, it is the pressure regulator valve buzzing in the pump because the pump is cavitating.

 

It was pretty common back in the day, on THM400,s when the factory style dacron element filter would get plugged up, it would whine and would not want to move at times.

Before you condemn the trans, I would replace the filter and the o-ring on the pickup tube.
And try it again.
May save you the time to remove and reinstall and the cost to rebuild.

Bill

 

He says in the first post that he installed a new filter already.

 

Yes, running in Park, warmed up.

@justpassinthru It has a new filter, O-ring, gasket, fluid.

@Kevin Ardinger Either way, it can't be fixed with the trans in the car, which is what I was hoping for.

 

Overhauling a THM400 is not difficult.

 

@gimpyshotrods It's one of the few things I haven't done. Local trans shop estimate: $2200-$3000 with me doing R&R.