The Stovebolt Site Forums The Shop Area General Truck Talk speedometer leaking gear oil

Hi,
My '71 GMC 3/4T 4wd started leaking gear oil from the transfer case up through the speedometer cable housing, and dripping onto the clutch pedal. It was during a ~200 mile round trip where the gear train was obviously hot. Any solutions? I thought there might be some kind of seal at the connection to the transfer case, but when I took it apart, I couldn't find anything. Anyone else have this problem? Any solutions? Thanks,
Ray

Ray,

Could there be a vent for the transmission/transfer case that is plugged! It sounds like gear lube is being forced uphill under pressure.

Stuart

Thanks Stewart! That makes a lot of sense, I'll check it out.
Ray

You sure it is gear oil?

That one is a first. Does your tranny fluid go down.? If it is a automatic It doesn"t seem possible.

It is actually the transfer case - when I opened the plug (cold), gear lube started oozing out. Yes, I'm sure it is gear lube, not only because of the distinctive smell, but when I reached up under the dash, the speedo cable housing had drips of it on it.
Ray

Good idea to check the vent as stated above. There is an small oil seal in the fitting for the speedo gear. It is held in by a small round snap ring. A new seal (if still available) will prevent gear oil pumping up the speedo cable, provided that the fitting is not badly worn where the speedo gear shaft rides.

I unscrewed the cable housing from the fitting on the transfer case, then got a wrench on the fitting and unscrewed it from the transfer case. The speedo gear would then just lift out of the transfer case. I cleaned the fitting and the gear in solvent but didn't see any oil seal or snap ring in the assembly. I put it all back together again - now looking for a vent but don't quite know where to look. Getting dark and threatening rain and I'm just laying on some cardboard in the driveway. May have to try again tomorrow. Truly a shade tree mechanic here. Thanks everyone,
Ray

It sounds like the transfer case is over-full It should not run out when the fill plug is removed. It should be 1/2 - 1 inch below the level of the plug when full. An over-full case may do what you described. I have never experienced it myself. Drain some gear lube out, and see if that helps any.

addition: Your speedo. housing has seals inside of it that should keep this from happening. These seals may be original? If so - the housing either needs to be rebuilt(replace worn out seals) or replaced. Try draining some gear oil first, though. That may solve the problem.

There aren't usually any seals on a speedometer shaft, just some little grooves that sling the oil off the shaft. Your gear oil may actually be over full from water getting in the case somehow, the oil floats on top. You probably should change the oil.

The vent usually looks like a little mushroom and is on top of the case somewhere. Its usually a threaded fitting that kind of looks like the little thingy on top of a pressure cooker that regulates the steam pressure. It may have a hose barb and go up to the frame somewhere.

The seal and round snap ring I was refering to is in the fitting and seals the plastic driven gear. I'll look and see if I have any in the trans overhaul kits I have. They are usually red and odd looking.

The truck is up on ramps to make it easier to crawl under. Because of the angle, the gear lube level is tilted toward the rear where the access hole is. I'll need to get it off the ramps and on the level to check the gear oil level accurately. If it is over full, I'll drain some out.
When I took the speedo fitting out of the transfer case, I could just pick up the gear (about 4" long, all steel with a square female connection where it joins the speedo cable). There were various grooves and rings on it, but no evidence of any seals or anything else. I slipped it back into place, screwed in the fitting and re-attached the speedo cable. Haven't checked it again to see if the speedo is working correctly. I'll look again for the vent, no luck yet.
Ray