GMC tranny problem

Yes, we retrofitted all the 4L60 & 4L80 dipsticks to the locking type. This was going on at the same time as the vent tube modification referred to in Tommy's post below.

Trouble was, these two modifications did nothing to get to the core problem of overheating. It just covered GM's backside somewhat against lawsuits resulting from the hot oil fires.

In the 1991 model year, GM finally somewhat bullet-proofed that tranny against failure by redesigning & spring-loading the 2/4 clutch release.

Towing still gave the transmission fits, but if the owner installed the optional GM external cooler and stayed out of overdrive, chances of the transmission's survival increased quite a bit.

Allan

There was TSB (technical service bulletin) out on this problem from GM sveral years ago that concerned some modification to the vent I think. Ask your dealer about it.

Tommy

The TSB you refer to installed the rubber "dump" hose on the older models ('88 thru '90 as I recall).

8" of rubber hose with a tie strap. They were trying to get that hot oil to dump away from the exhaust pipe to lessen the event of fire.

Allan

Do you have the 4L60 (square pan) or the heavy 4L80?

Are you absolutely sure it isn't getting hot?

That blowing of the oil out during towing is usually a precursor of a great big failure coming on; also, the heat will cause a converter to turn blue on that model.

The only other thing that can cause this is a plugged vent. Wayyyyy , way up on top of the case just above the pump. It should have a “dump” hose attached to it and dropping down on the passenger side on a ’91 model.

Allan