TH400 blowing ATF @dipstick

[TomW]

The TH400 transmission in my 1986 Suburban C20 with 7.4 liter engine shifts fine even when towing a 5,000 lb travel trailer. The ATF color is good, and there are no leaks.

A month or two ago, I noticed ATF coating the dipstick tube. A week or two after that, I checked the oil level (level surface, engine warm but not hot, go through the gears, etc) and had to add two quarts. Now I knew I had lost some ATF during two different radiator change-outs, but was surprised the level was so low. I also verified the vent tube on top of the tranny was not blocked.

Last week, we took the travel trailer out of state, and the Suburban ran & shifted fine. Then an odd thing happened. We were set up at our campsite one morning (unhitched) when I remembered one of the trailer's propane bottles was empty. The campground had a refill station, so I loaded the empty bottle in the Suburban, cranked the cold engine up, and drove the block or so to the refill station where the engine was then shut off.

Around ten minutes later, the motor was started back up, and I drove back to the campsite. After backing into the site, I saw a path of oil drips which corresponed to my back-up path. Popping the hood, I found oil running down the ATF dipstick tube & splashed on the exhaust manifold.

I have heard of ATF boiling out under heavy load, but this was a cold engine. And, when towing, I somewhat baby this truck because it is old and I like it.

The transmission has around 60,000 miles on it since being rebuilt around 10 years ago. Like I said, it shifts fine, and the ATF oil color is not dirty.

What could cause ATF to blow out of the dipstick tube on a cold engine?

Thanks,
Tom

[87c10slammer]

maybe a bad seal on the dipstick. i would try to get one of the newer locking/expanding seal ones.

[75K30]

At first thought I would double check the vent. Alright you have done this, so we need to determine what causes fluid to vent from the dipstick tube.
Heat causes this issue, and a blocked vent, but this sounds as though you have covered this already.
You know I thought and I thought about this and the fact that you had two quarts required to top off trans might indicate that you have the wrong length dipstick.
Have you ever had to replace the dipstick, or the tube?
Could it be possible that you have too much fluid in this thing?
The only other possibility is that there is some sort of blockage in the filter, or a stuck check ball.
A good service might not be a bad idea.

[TomW]

Originally Posted by 75K30

... A good service might not be a bad idea.

The master plan is to take the truck back to the guys who rebuilt the transmission and get their opinion. Luckily, since the Suburban is not my daily commuter, I've got a little time.

I don't know too much about automatic transmissions, but I am baffled as to what the problem is. I hope the transmission shop has seen this before.

Tom

[dmueller71]

I'm having a similar problem. Where is the vent located on a turbo 400. It's been 20 years since I've had the privilege of messing with one of these things. Thanks

[75K30]

Welcome to the site;
The vent tube is located on the top of the transmission case, just as the bellhousing meets tappers down to the main body. It is not two pieces like a manual, but this is the best way for me to explain.
Anyway, it favors the pass side slightly, and some are plastic, so be careful not to break it off.

[TomW]

The problem ended up being high pressure escaping around the 3rd gear engagement device. I don't know that much about automatic transmissions, so everyone feel free to jump in and help.

The man at the automatic transmission shop told me that pressure is high in reverse, and that reverse movement requires the use of third gear.

Engaging any gear requires the use of ATF pressure. In my case, the 3rd gear engagement mechanism was worn and allowing pressure to blow back into the pan.

Tom

[gmctrucks]

Tom did they fix it and if so if you don't mind what was the charge? The reason im asking is most trans shops charge so much. Tom maybe adding a trans cooler would be better than just having the one in the radiator oh I have seen some trans coolers with elctrical fan in front of them.

[TomW]

They probably fixed it; I'm giving it a week or two to make sure.

They quoted $750 for a rebuild up front before doing anything. That was significantly up from the $400 they charged me ten years ago. Surprisingly, they said it was getting tougher & tougher to find TH400 parts.

Tom