Diff diagnosis

[Mac Townsend]

91 legacy AWD w/240,000 miles. I'd like to get another 20-30 out of it
if I can.

I'm getting a howling from the back of the car. Loudness is roadspeed
dependent not engine speed dependent. It seems to have been getting
louder in recent weeks...so SOMEthing needs to be done pretty soon.

1) How can i diagnose whether it is the rear diff or possibly a wheel
bearing (one side or the other). I've tried jacking up the car, but that
then unloads the bearing and I can detect no noise by manually turning
the wheels.

2) Assuming it to be diff, are these things solid enough that a junkyard
unit would likely be usable (assuming I can check vehicle mileage and
it's in the 100-150 thou range). What year units would interchange with
91 Legacy?

TIA (Thanks In Advance)

 

[David]

91 legacy AWD w/240,000 miles. I'd like to get another 20-30 out of it
if I can.

I'm getting a howling from the back of the car. Loudness is roadspeed
dependent not engine speed dependent. It seems to have been getting
louder in recent weeks...so SOMEthing needs to be done pretty soon.

1) How can i diagnose whether it is the rear diff or possibly a wheel
bearing (one side or the other). I've tried jacking up the car, but that
then unloads the bearing and I can detect no noise by manually turning
the wheels.

You have an open rear diff, right?

Have you tried jacking up all 4 wheels, starting the car, placing in gear so wheels
spin? Try to hold the left wheel so only right spins. Then do the same so left
spins. If it's a problem with the diff or axle joints I'd expect you to be able to hear
it when you're doing this. I'd expect axle noise to get worse, since the angle of deflection
is greater with the wheels in the air. But bearing noise is likely to be quieter.

Also, while the car is jacked up you might want to put it in neutral, not running, and turn
*both* back wheels by hand (might need an assistant).

2) Assuming it to be diff, are these things solid enough that a junkyard
unit would likely be usable

I think so. Yours lasted 240k (or more if still OK). If it it's an open diff, and it operates
OK in the junkyard (doesn't make clunking noises when you turn the various shafts), I'd have
high hopes for it. I'd try to avoid driveline parts from a WRX or similar car that
might have seen a lot of hard launches.

David

 

[TG]

This is a piece of cake, put the car on a lift and use an automotive
stethoscope. Make one...just a plastic tube will work. Most muffler shops
will let you hoist it up for a minute. TG

 

[hippo]

I'm assuming you've checked your diff oil level. Could be that simple.
Agree with both previous posts too. Cheers