Bang noise when I step on it on snow.

[uniberp1]

1999 Forest N/a auto 205,000 miles.

I had it up on stands to diagnose a howling, which is pretty obviously the differential, and when I let off the gas there was a kind of a bang, that seemed to shook the engine.

Seems maybe the rear engagement stuck, then cut loose?

Accelerating on snow, if I spin the tires, I get the same single bang when the tires cut loose. I don't notice any torque bind, but I think this might be related.

If it's a fairly simple fix, like bolting in a solenoid or something, it might be worth fixing. I've done the standard "replace 3 quarts of ATF" every 10k or so. The fluid is clear.

The front diff seems quiet enough. 

AND... What years and combinations of driveshaft would fit my 1999. Looks like same driveshaft up to 2002, but same carrier up to 2008.

Thanks for any opinions.

Edited March 4, 2019 by uniberp1

[1 Lucky Texan]

rear diff mount bushings may be worn.

[forester2002s]

Could also be failed engine mounts.

[FerGloyale]

U-joints

 

[carfreak85]

I have this exact same issue with my wife's '99 Forester.  In snow, I can stomp the pedal and the car will act as if it is 2WD until I ease up on the throttle.  When I do, the AWD system kicks in with that signature BANG!, like someone is hitting the bottom of the car with a baseball bat.

This doesn't seem to be as widespread an issue as the head gaskets or the clocks that cut out.  Based on my research there are a couple issues.  First, the early Foresters ('98-99) have a TCU with a very aggressive AWD engagement.  There is a TSB that describes this and suggests installing an alternate TCU part number with a more mild AWD engagement algorithm.

The second issue is that the RWD transfer clutch pack can wear, or the Duty-C solenoid can fault.

I have yet to do anything about it besides the research above, but this really bothers me as we depend on this car as our all-around ski/dog/haul vehicle.

@GeneralDisorder, do you have any insights on this particular issue?

[CNY_Dave]

Could a grooved AWD clutch basket cause this?

[uniberp1]

On 3/4/2019 at 1:29 PM, CNY_Dave said:

Could a grooved AWD clutch basket cause this?

From the symptoms, absolutely. Easing the throttle allows the clutch plates to slip out of the grooves and engage.

I'd really like to get under it and take out the tailhousing but it's subzero now.

  

[GeneralDisorder]

Earlier cars have slow, poorly programmed TCU's and tend to have harsh AWD engagement. The transfer drums being grooved certainly doesn't help any. Replaced the drums on a Subaru reman with about 70-80k on it a few months back and now the transmission has developed another fatal problem - serious binding in reverse - and not the binding you associate with turning - this thing feels like it's got it's parking brake on in reverse even trying to backup straight. Not a fan of Subaru's transmission re manufacturing at the moment. This transmission was the last of 5 installed by the dealer after the first 4 they put in didn't work at all. It's making a lot of metal from the front diff, the transfer drums were grooved and obviously not addressed in the rebuild and now it's failing and going to get replaced with a low mileage used trans. 

Usually what we see is the rear diff bushings take a beating over this harsh engagement and of course due to age and being rubber..... Last one we did on an early 00's Outback the rear cradle was so rusty we just welded steel flat plate to the back of the bushing pockets and bolted the diff up solid to it. Report from the customer was that the noise is gone and he's happy with it that way.

GD

Edited January 18, 2022 by GeneralDisorder