Leaking Driveshaft Yoke

Written by: Mike Ervin

I had a problem that I thought was a rear tranny seal leaking.  I probably changed the seal three times, and it still leaked.  I had the truck up on jack stands working on the exhaust for a few days with the front of the truck higher than the rear.  This thing was leaking pretty bad.  Enough to cause a puddle on the floor.  Then I found the answer to my problem in Car Craft magazine.  This is a very easy fix to an annoying problem.  I never knew this existed.  Learn something new everyday I guess.  Below is a snippet of the article.
Failure is Cause for Weeping

Some GM transmission yokes, primarily those used on trucks, have a weep hole that relieves pressure buildup caused by the yoke moving back an forth as the vehicle passes over bumps and other road irregularities.  If the trans in your truck leaks fluid when the vehicle is parked uphill, fluid may be coming out of the weep hole due to failure of an output shaft O-ring.

If your truck has this weep hole (arrow in photo below), you will be able to see it with the driveshaft removed by flipping the yoke to the side.  It will be in the center behind the u-joint.

The O-ring in question (pn GM3764601, 700R4 shown) is installed in the cup on the transmission output shaft (arrow in photo below).  Access it by removing the transmission extension housing.


Remove and Replace

First thing.  NEVER EVER get under your truck without some good jack stands to support it.  We want to keep you alive.

To remove the extension housing, you will need to remove the driveshaft.  Be prepared to catch any tranny fluid that comes out.  Mark the location of the rear u-joint on the pinion yoke at the rear so you can install it in the same location.  This will make sure it is not installed out of balance.  You will need to pry the u-joint out of the rear yoke a little.  Be careful to not let the cups fall off the u-joint.  Next remove the speedo cable from the sleeve in the extension housing.  Now unbolt the extension housing from the transmission.  Again catch the transmission fluid.  Make sure the rubber seal on the extension housing is intact.  You should see something similar to the above photo depending on what type of transmission you have.  Make a mental note of where the metal cup is located at on the output shaft and slide the cup off.  The O-ring, (pn  GM3764601) will be in the groove inside the cup.  Replace the O-ring and slide the cup back on the shaft.  Try to get the output shaft in the center of the cup as best you can.  This will let the driveshaft yoke go all the way in without it hitting the end of the cup.  The rest is the reverse of removal.  To help from ruining another O-ring, use a little tranny fluid on the outside of the yoke before sliding it back into the transmission.  This will also help save the rear seal from damage.