ZR51 Performance

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S=Symptom and C=Probable Cause

S) Sudden development of clash in any gear

C) These failures occurred when a fracture in the ring caused the phosphorous-bronze lining to delaminate. The closer the fracture was to the phosphorous-bronze lining seam, the sooner the collective component failure occurred. When this occurs, the binding ring instantly looses its conical binding retention strength. Discontinue using the afflicted gear until the unit can be serviced.

S) Roughness, vibration or body boom at any speed.

C) Possibly caused by a worn transmission extension housing bushing. The front u-joint and slip-spline drive shaft yolk are held in alignment by this bushing. This can be caused by excessive looseness at the yolk slip-spline among other things. This looseness could be induced by either the splines of the yolk being worn (most unlikely unless high h.p. application), or the tail house bushing being worn (more
likely). If the outer diameter surface of the yolk slip-spline is rough then the problem is directed towards a worn tail housing bushing. This usually means that the yolk slip-spline must also be replaced along with the seal. GM only offers the yolk slip-spline as an assembly package complete with driveshaft and universal joints.
As far as leaks go, an occasional drop or two from the seal is OK. The slight internal pressure during operation causes the seal to constrict ever so slightly around the slip spline OD surface, enough to retain virtually all of the oil while in use.

As an example, a new bushing and a moderately used yolk will yield a vertical play between .010 - .015 in. at the universal joint. That would be about a 3-1/2 inch moment arm distance from the tail house bushing centerline axis. A new slip spline yolk and a new bushing ~ .008 - .012 in play.
Consider servicing the bushing if the play exceeds .025 in. or the previously mentioned symptoms are prevalent.

A few other bearing isolation tests are:
1) If vehicle is stationary and sound still present, then
the rear main bearing can be ruled out.
2) If it is directly proportional to engine RPMs and goes away
with depressed clutch, then the throw-out bearing may be worn.
3) If the wine becomes louder progressively through the 1-2-3 gears,
then the transmission alignment is off slightly causing the input bearing
to whine. Check front mating surface of transmission for high spots
then check the bell housing mating surface for any distortions throughout.
It only takes a 2mm burr on the front matting surface to cause such a
I usually find these burr or bump conditions to be caused by improper
procedures of the tech when they try to pry the transmission at the mating
surface when attempting to line up and insert the 4 drive beam attachment
bolts. An alignment leverage bar (Phillips screwdriver) should be used is the bolt hole still vacant when fitting the first bolt on each end. The use of floor jacks to manipulate the angles of the transmission and differential will make the job much easier.

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Send email to billb@zfdoc.com or call Bill Boudreau at (602) 319-6575.
Last modified: July 19, 2003