Rear end diagnosis....

Recently my 2001 awd Astro developed some sounds from the rear axle/differential area. The best way I can describe it is that it sounds like I put on 6 ply off road tires and pumped them up to 90psi. Before I start digging around back there I wanted to get some peoples thoughts on what it could be (gears, journal, bearings...). I did change the oil in both differentials about a month ago, I used 80-90w Valvoline and no additives. Should I be using any additives, could this be related???

Thanks

Is the noise present when accelerating, decelerating, or cruising at a steady speed?

Old Master,

I knew you'd come thru for me ;)!

It actually comes and goes, which is very strange. The noise has been basically non existent for a week, and now is back again. It is different under different conditions it seems. Sometimes it starts at low speed (15-20) then goes away after 25. Sometimes it hits at 40 and stays there. at 65 I don't even notice it??? It seems more prevalent if I have 2 or more passengers in the van. It also seems to sound/feel like it might be coming from the passenger side of the rear axle, so I'm wondering if it's the outer bearing.

One final note....when I changed the differential fluid a month ago the new gasket did not have the extra holes in it, so I'm wondering about that too?

Thanks!

Noises are tough to diagnose...but you might be on to something with the wheel bearing. In the early stages, wheel bearing noises can be intermittent, but once the wear pattern is in the bearing or the cup, it usually progresses very rapidly. Sometimes you can detect it by raising the rear wheels and spinning them by hand and listening/feeling at the rear axle housing.

Holes in the gasket???? The gaskets should match perfectly.

Ok, I will jack up the rear and see if I can feel anything....

I meant the 2 extra large holes used for "drainage" inside the pumpkin (on the sides of the housing). This was a Fel-Pro gasket, but those holes were covered?? I debated cutting them in, but left it alone.

Check for axial play in the axles:

http://mysite.verizon.net/strey/files/AxialPlay.jpg

This can indicate side gear thrust washer wear. Radial play will indicate bearing wear. Either will generate noise.

I'd cut the drainback holes clear, but that's just me.

could be a u joint

I'm re-hashing this topic since I've tried some things but still haven't fixed it. Blue Bowtie: I don't have a dial indicator, but I can tell you there is a fair amount of axial play that I can feel with both wheels off the ground. I am feeling no radial play. Since I started this post, I have replaced both outside rear wheel bearings, but the noise is still present. I can add this diagnostic info though. We've had some cold mornings in the 30's, and the noise is almost non existent at those temps - I'm guessing because of oil viscosity. As the day gets warmer or on longer drive times, the noise increases. Also, I towed around a small trailer a few days (about 1000 lbs) and while towing odly enough the noise disappears! As soon as I remove the trailer, I hear the noise. I did check u-joints, but this noise is definitely rear axel related.

I was thinking possibly the inner axel bearings and thrust washers (as bowtie mentioned)? I see a shim kit is also needed. What are the specs on that and how would I know where to shim the axels to if I replace the bearings/thrust washers?

Thanks!

Since this application involves an offset 10-bolt axle under a vehicle weighing 4,500 pounds, it's not uncommon to have carrier bearings getting a little loose after about 100K, and that can translate into axial play.

It may be time to check ring/pinion lash. You can try to check with feelers, but an indicator and red lead (or some other gear marking compound) are going to be best tools. Repair can be a somewhat labor intensive process, and requires setting up the carrier position just as you would for new gears, except you will have to pay more attention to the coast/unloaded side of the teeth to indicate proper contact.

Wonder if I might ask a similar question within this thread.. my '95 GMC Safari - AWD has suddenly developed a vibration around 50-55 mph that is unreal! If you had a can of soda in the front seat, when you opened it - it would completely fizz out the vibration is so bad. Once it starts, it won't go away at any speed. Yet, when you stop and start over it may not happen again. This morning it happened for about 20 miles. In the afternoon, it ran perfectly.

Took it to a mechanic and put it on the lift to check out U-joints - they seem fine. We both suspect rear bearings, but not sure. My question is: is this a difficult job? Guy that does my work has not replaced any of these. I see the axles have to be removed to get to the outer bearings - is it tough to do? Are the bearings pressed on? Any hints or ideas would be appreciated. This baby has 210,000 on it but the engine is strong and tranny is great.