2001 Legacy - 4EAT Auto driveline thump?
 

Hi,

Sorry my first post is a tech question - New to the site and to owning a Subaru. Its a 2001 2.5GX estate Auto which has 97k on it and is mint inside and out.

I bought the car just over a week ago and initially everything was fine. I had the car inspected by an engineer as it was a distance away and no faults were identified however on the drive back and once the gearbox had fully warmed up I noticed that there is a thump / knock coming from the driveline.

From cold the box is smooth and no noises are apparent. I know that these boxes change the gearchange once at operating temperature to be more positive in engagement and that seems to be when the problem starts. On changing gear or near to a change there is a noticeable thump that come from under the car which you feel through the seat. The thump is more apparent on part / light throttle and at its worst at around 50 - 55mph when cruising when light throttle changes are made. Around town at 20 -30mph it isnt so noticeable and may have been missed when the engineer test drove it as he would have driven it at these speeds around the area it was bought and 5 - 10 miles may not have been enough to get it hot enough.

I originally thought it may have been an overdrive problem but the box is a 4 speed rather than 3 with OD and the thump happens at other gear shift points.

There is too much fluid in the transmission which I need to drain off asap - would this cause the symptoms? Ill drain it off today to check.

Would old fluid / non genuine gearbox filter cause problems?

I think Ive ruled out torque bind in the diffs as no scrabbling of tyres / noise on acceleration through a tight circle. It does have an odd tyre but Im changing all four in a couple of days.

It has been suggested it might be the rear diff bushes? Most of the posts on this talk about under hard acceleration / enthusiastic gear changes which you dont really get on an Auto but could the fact that the thumps start when the shifts are more positive point toward this?

I dont have a huge experience of Auto boxes and what I do have is on less complicated set ups (Hilux Surf 3 Spd OD agricultural!) and no experience of Subaru drivelines at all.

Help and pointers would be really appreciated as dont want to let the problem become terminal if its an easy fix early on and is spoiling what is otherwise a great car :confused:

Over-filling causes serious overheating of the transmission. The TCU would normally change the shift maps and lock the torque convertor in an attempt to lose heat.
Either get the excess fluid sucked out. Or, if you've got an external ATF filter change that, so you lose a filter full of fluid. Both less messy than opening the drain plug.
You would normally get a flashing power/auto light to indicate temperatures over 90/100 degrees.
On turning the ignition on, you would normally get 16 flashes of the power/auto light, indicating a stored transmission fault code.
HTH

Ok ill see whether I get any flashes when I go out later.

Ill double check the fluid level using the correct method when I get back. I tried to succk some out the other day but failed so definitely liking the idea of the less messy method to remove some. I can also get a look at the state of the fluid as it may need changing anyway.

Thanks

No flashing to suggest stored gearbox fault. Ran out of time to check fluid.