JohnA
Forum Member
The Forester has a curious new issue. It's a deep thunk in the drivetrain that occurs when the car is stopped - out of gear, clutch out, engine off - or running. You can feel it though the seats, like there's some tension being released from part of the driveline - there are times am certain it's coming from the rear - other times less sure. It's definitely not the air con compressor - I can hear that, too, but it's not felt through the seats like this is.
The Forester has an ADS drivetrain - the front diff is helical WRX STi diff, the fluid centre coupling (could be the culprit?), is uprated from 4kg stock to 20kg and is of the kind used in WRX STi rally cars. The rear diff is an R160 LSD, I think from an older model WRX STi - it's geared lower than stock at 4.44, instead of 4.11.
The Torsen is mechanical with spider gears and can't imagine it storing tension, the gearbox itself could not be thunking and functioning - nor can I feel the thunk through the stick. This may be relevant information given the Toresen and centre coupling are in the housing.
The viscous coupling is a closed unit and my understanding is that it should only solidify when the wheels are slipping - this thunk happens after I've been simply pootling around town between traffic lights. I've wondered about the electric handbrake letting go after stopping but not sure that makes sense, mechanically.
Can't imagine a rear LSD thunking without any vehicle motion, either, though it (or something back there) does make a slight scraping nose when I turn tight left. ADS had to machine the rear diff housing to fit the R160.
It happens one time only, until the car moves again - somehow movement is storing energy/tension in the driveline.
The thunk comes and goes. Sometimes it happens often. Sometimes it goes away completely. Dirt road and off road performance remains pretty amazing. There's no other noise from the driveline. Obviously the car needs to be looked at, and soon, but before the issue is exposed to the assertions of a mechanic with business survival front of mind, has anyone else experienced such a thing?
Thanks for any input!
The Forester has an ADS drivetrain - the front diff is helical WRX STi diff, the fluid centre coupling (could be the culprit?), is uprated from 4kg stock to 20kg and is of the kind used in WRX STi rally cars. The rear diff is an R160 LSD, I think from an older model WRX STi - it's geared lower than stock at 4.44, instead of 4.11.
The Torsen is mechanical with spider gears and can't imagine it storing tension, the gearbox itself could not be thunking and functioning - nor can I feel the thunk through the stick. This may be relevant information given the Toresen and centre coupling are in the housing.
The viscous coupling is a closed unit and my understanding is that it should only solidify when the wheels are slipping - this thunk happens after I've been simply pootling around town between traffic lights. I've wondered about the electric handbrake letting go after stopping but not sure that makes sense, mechanically.
Can't imagine a rear LSD thunking without any vehicle motion, either, though it (or something back there) does make a slight scraping nose when I turn tight left. ADS had to machine the rear diff housing to fit the R160.
It happens one time only, until the car moves again - somehow movement is storing energy/tension in the driveline.
The thunk comes and goes. Sometimes it happens often. Sometimes it goes away completely. Dirt road and off road performance remains pretty amazing. There's no other noise from the driveline. Obviously the car needs to be looked at, and soon, but before the issue is exposed to the assertions of a mechanic with business survival front of mind, has anyone else experienced such a thing?
Thanks for any input!
