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Old 07-04-2008, 02:37 PM   #1
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TH400 blowing ATF @dipstick

My apologies up front for a non-aluminum post.

In anticipation of a trip to the transmission shop, I posted what follows in two separate truck forums last week, and apparently stumped everyone. . I thought it would be nice to have a clue to what the shop said when I showed up there yesterday. Unfortunately, they were closed for a long holiday weekend.

Anyway, here's my question:

Quote:
The TH400 transmission in my 1984 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
Anyone have any insights?

Thanks,
Tom
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Old 07-04-2008, 03:58 PM   #2
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tom

i recieved a locking trans dipstick in the mail from GM for my '88 ck1500. they must have felt at the time for a need to keep the oil in the tranny due to the filler tube design.

must have been a common problem for them to recognize it and send the new dip sticks out in the mail.

what kind are you running now? locking or non locking?

john
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Old 07-04-2008, 04:04 PM   #3
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Tom, when your transmission is in reverse, the internal pressures are higher than when it is in any other gear. The natural increase in pressure from being in reverse is being coupled with another factor, such as one of the internal pump seals leaking fluid past it.
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Old 07-04-2008, 04:38 PM   #4
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OEM dipstick

Quote:
Originally Posted by john hd View Post
... what kind are you running now? locking or non locking?
Thanks John.

I'm running the original, non-locking dipstick. Up until this year, there has never been a problem. Something has changed, and I don't know what it is.

Tom
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Old 07-04-2008, 04:42 PM   #5
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The pressure block or what?

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Originally Posted by overlander63 View Post
Tom, when your transmission is in reverse, the internal pressures are higher than when it is in any other gear. The natural increase in pressure from being in reverse is being coupled with another factor, such as one of the internal pump seals leaking fluid past it.
Okay, so why is ATF blowing out all of the sudden?

I don't know automatics, so I can't even put together a list of suspect components.

Tom
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Old 07-04-2008, 05:56 PM   #6
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Quote:
Originally Posted by TomW View Post
My apologies up front for a non-aluminum post.

In anticipation of a trip to the transmission shop, I posted what follows in two separate truck forums last week, and apparently stumped everyone. . I thought it would be nice to have a clue to what the shop said when I showed up there yesterday. Unfortunately, they were closed for a long holiday weekend.

Anyway, here's my question:



Anyone have any insights?

Thanks,
Tom
Even though your vent may not appear to be blocked... this problem screams of a blocked vent.
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Old 01-11-2009, 05:22 PM   #7
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bkahler, in another post, asked about the correct ATF to add to his TH400. After replying to the post, I saw This Old Thread pop up at the bottom of Brad's thread, and feel inclined to update it with both the final outcome, and some confusing information about ATF.

Terry's crystal ball reading in post #3 was what was happening to my Burb's tranny. While my transmission was not worn out, by the time the tranny was disassembled to service the failed part, it would have been stupid to NOT assemble it with new parts. The rebuild was done by a shop run by a man I feel knows his business. He has done work for me before, and he does all the city's transmission work in addition to the local Mercury dealership's. Additionally, he took me out on the shop floor & showed me EXACTLY what was going on with all the parts in question. He is so good that his shop does not even test-drive repaired vehicles. I know this because I have checked odometer readings in the past.

After this rebuild, I asked about the proper ATF to add. While I do not recall the exact conversation (look how old this thread is), he basically said, "Put ATF in it; Nowadays, the difference between stuff sold for Ford or GM is nothing more than marketing."

I now how have over 2000 towing miles on the rebuild, and feel inclined to trust Don's observation. But I have not done any followup research on the accuracy.

Tom
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Old 01-11-2009, 05:54 PM   #8
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I have had this issue on a THM 350, the solutions were to replace the vent, replace the dip stick and tube with a locking one and install a trans cooler. Change your tranny oil and filter every 25K.
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