| Re: [GMCnet] Transmission overheating [message #96866] |
Sun, 22 August 2010 19:52  |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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I started a new thread because this is different than "bilge blower in engine
compartment." The most common cause for transmission overheating is the torque
converter, and the result is that the transmission oil level rises. Remember
that there is about 9 quarts of oil in the transmission, about half of which is
in the torque converter. The oil pan itself holds probably less than half.
When the oil gets hot it all expands and the "extra" volume ends up in the oil
pan. All THM 400 or THM 425's are notorious for their small oil pan and limited
capacity to absorb oil expansion - that's why all the experts warn against
overfilling the transmission. When the oil level gets too high it rises into
the rotating parts and then the problem rapidly gets worse, since the oil now
starts to absorb air and becomes a foam. Oil level then rapidly goes way up and
"burps" out the breather. Unfortunately, in the GMC case the oil can then get
sprayed on the exhaust system and the result is a dramatic fire. I suspect,
without any proof, that in the example below there was nothing wrong with the
transmission or any other part of the vehicle. Just an example of high fluid
level coupled with overheating from a climbing a grade without downshifting.
Whether or not that is true, it is still something to watch out for. And a
good reason to buy the kit that hooks a hose to the vent,(securely attached!)
keeping the effluent out of harms way.
Gary Casey
'73 23, without the vent kit, yet.
Turned out the rebuilt transmission self destructed, blew the vent hose off and
had sprayed transmission fluid onto the hot exhaust and caused the fire.
Fortunately when I got up from using the extinguisher I was faced with a big
American LaFrance fire engine and 5 firemnen. They checked the coach out and
even crawled under it to make sure the fire was out. When I went back to get
into the coach there was a policeman standing there and wouldn't leave until the
wrecker came about 3 hours later.
The transmission was a total loss. Good Tennessee civil servants.
So I recommend never put a bilge blower in the engine compartment.
--
Gene Dotson
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