I recently purchased a 92 legacy wagon ls. It's FWD with 4eat. Wasn't running when purchased but after replacing fuel pump and filter, it runs great. Car drives and shifts fine, but has rhythmic thumping sound that increases with speed. Almost sounds like tire but i replaced one tire and it still has noise. Took the spare tire and one at a time put it on each corner to see if noise went away, but didn't. So I don't think it's tires. Next I thought it could possibly be bearing, so I bought new bearings and hubs, but still have noise. The axles were replaced before I bought the car and the boots looked new. Despite feeling that it probably wasn't axles, I thought it was still possible, so I bought more axles and swapped them. Noise still persists. I also had rotors turned so should not be that either. I'm almost to the point of replacing the transmission but don't want to just throw more parts at it...like I have been. It almost seems like there's a spline or tooth missing....don't know for sure. It's a very regular and rhythmic thumping that starts around 10-15mph. I've tried manually starting in 1st, pushing to second, etc. Happens in every gear... acceleration or deceleration. Goes away if I move to neutral...even if I rev motor.

Edited November 1, 20169 yr by Bag

Seems like a huge mystery to everyone else too huh? ????

Which bearings did you replace?

It goes away completely when you shift to neutral? Does it change if you just coast with it in gear?

have you checked front diff fluid level or pulled the diff's drain plug to inspect the magnet for 'pieces'?

 

kinda wondering if you think it might be happening about 4 times for every tire revolution - or 1 time.

 

any sense of either of those?

Haven't dropped pan yet, but think it is more like 1 per tire rotation.

Seems oddly specific question. What were your thoughts on possible failures.

pan?

 

example, pinion turns ~4 times against ring gear in diff. (like 9 teeth on pinion against 37 on ring = 4.11 final drive)

 

I admit to being ignorant of specifics of the FWD transmission but, on my car, the front differential has a separate drain plug with a magnet - maybe inspect it for big pieces?

 

4 vs 1 time per rev :example, pinion turns ~4 times against ring gear in diff. (like 9 teeth on pinion against 37 on ring = 4.11 final drive)

 

maybe output bearing on side of the diff? that might be 1 time per tire revolution.

 

if you could get the car up to AWD Tuning in Flower Mound, I bet those guys could diagnose it. I just really am throwing out possibilities - fairtax has the experience here.

 

If you rule out bearings and axles - diff/trans is about the only thing left. If it helps, axles can be swapped side-to-side.

Edited November 4, 20169 yr by 1 Lucky Texan

Thanks. I figured you were suggesting ring gear as possible. The diff and trans are one unit on FWD right? Meaning if I buy another trans it's just whole assembly and not something where I buy trans and it end up being a diff.

Don't overlook the driveshaft. I mean what Subaru refers to as the center driveshaft. It could be out of balance, as lost a weight, or the front or rear universal joint could be defective. The joint at the front of the shaft ( transmission rear ) is usually the culprit. IT's difficult to see and diagnose.

I agree with ferret,  Inspect drive line carefully. 

Thanks. I figured you were suggesting ring gear as possible. The diff and trans are one unit on FWD right? Meaning if I buy another trans it's just whole assembly and not something where I buy trans and it end up being a diff.

 

 

If it's like my 4EAT but just without the tail section then yeah. technically, it's 2 units put together but no one 'rebuilds' the diffs (NOTE, that's not say some kinda seal or bearing at the axle output can't be repaired - but it's still tricky I think getting the bearing preload back to factory setting) - so, you might check LKQ and car-part.com for a used trans. KInda suspect the FWD version might be hard to find but, i haven't looked.

I agree with ferret,  Inspect drive line carefully. 

 

 

He says it's a FWD trans so, no rear diff or driveshaft.

I appreciate the info. I pretty much figured it's going to need a trans. Or at least a component inside of it. It actually shifts very well. May just have to pull it and see if I can identify the culprit.

destroyed rear bushings on the lower control arms will thump - but not usually in-sync with rolling.

So I went ahead and pulled the trans and diff. Suspicion has been confirmed. Missing teeth on ring gear.

 

https://plus.google.com/104985126743244456824/posts/enAZrmUSYb5

Edited December 3, 20169 yr by Bag

dang, too bad