Thumping noise when turning left or right
Thread Starter
Burning Brakes
Joined: Mar 2018
Posts: 1,114
Likes: 337
From: New Brunswick, Canada
Thumping noise when turning left or right
Hoping someone can help me out.
My 92 C4 has a consistent thumping noise coming from rear wheels when turning left or right, even in a very slight turn.
The thump frequency is tied to speed. Faster speed, faster thumping. If going in a straight line, no thumping is heard.
Backstory:
Car is new to me and was stored for about 15 years. I have replaced all fluids except transmission and front differential oil. Those are next on the list. Diff locks have been flushed with UDT999.
Car is now back on a lift with all 4 wheels hanging free. When I rotate a front wheel, the other front wheel will counter-rotate. When I rotate a rear wheel the other one always spins in the same direction. Is this normal?
When I rotate the traction control **** clockwise, I get one fairly quiet knock from the diff lock slave but when I rotate the **** counter-clockwise, I get a series of 3 or 4 knocks that start loud and then get progressively quieter. Regardless of **** position, the yellow light stays on. This sounds messed up to me, but this is my first Porsche so I turn to the Rennlist community for guidance. It seems like maybe the transverse diff lock is stuck in the fully locked position??
I have checked for play in wheel bearings and CV joints and all looks good. I can slide the half axles in and out a small amount and they slide smoothly. CV joint boots look ok.
My 92 C4 has a consistent thumping noise coming from rear wheels when turning left or right, even in a very slight turn.
The thump frequency is tied to speed. Faster speed, faster thumping. If going in a straight line, no thumping is heard.
Backstory:
Car is new to me and was stored for about 15 years. I have replaced all fluids except transmission and front differential oil. Those are next on the list. Diff locks have been flushed with UDT999.
Car is now back on a lift with all 4 wheels hanging free. When I rotate a front wheel, the other front wheel will counter-rotate. When I rotate a rear wheel the other one always spins in the same direction. Is this normal?
When I rotate the traction control **** clockwise, I get one fairly quiet knock from the diff lock slave but when I rotate the **** counter-clockwise, I get a series of 3 or 4 knocks that start loud and then get progressively quieter. Regardless of **** position, the yellow light stays on. This sounds messed up to me, but this is my first Porsche so I turn to the Rennlist community for guidance. It seems like maybe the transverse diff lock is stuck in the fully locked position??
I have checked for play in wheel bearings and CV joints and all looks good. I can slide the half axles in and out a small amount and they slide smoothly. CV joint boots look ok.
Intermediate
Joined: Feb 2006
Posts: 48
Likes: 17
From: Surrey . UK
Sounds like a rear wheel bearing . ..i had one crack on me and had to replace it as it sounded a little like your description, it was very hard to diagnose, and I had the gearbox out, the diff rebuilt with new bearings , new CV's joints etc etc still had the noise .
I originally replaced both rear bearings with non Porsche ones to save a few quid and couldn’t believe it was a bearing ....but it was in the end as it had cracked in the ball outer casing so I had to then replace both rears again with Porsche rear bearing this time , much better quality ....your pays for what you get ...go cheap buy twice is the saying ;-)
Cost me lot in the end to save not a lot the 1st time around 😉
I originally replaced both rear bearings with non Porsche ones to save a few quid and couldn’t believe it was a bearing ....but it was in the end as it had cracked in the ball outer casing so I had to then replace both rears again with Porsche rear bearing this time , much better quality ....your pays for what you get ...go cheap buy twice is the saying ;-)
Cost me lot in the end to save not a lot the 1st time around 😉
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Joined: May 2001
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From: Auckland, New Zealand.
Thread Starter
Burning Brakes
Joined: Mar 2018
Posts: 1,114
Likes: 337
From: New Brunswick, Canada
Rennlist Member
Joined: Aug 2014
Posts: 4,861
Likes: 725
From: Massachusetts
My rear wheels were moaning a bit under load at the track last weekend. I jacked up the right rear wheel (the loudest of the two wheels), but couldn’t feel any play. I took the car to the shop today and my Indy took a quick drive around the block and confirmed that both bearings are totally shot. Don’t wait until there’s play if you’e hearing the wheels moan or you feel vibration under load.
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Thread Starter
Burning Brakes
Joined: Mar 2018
Posts: 1,114
Likes: 337
From: New Brunswick, Canada
Thanks Foxman,
I will try to describe the sound better. I will record it the next time the car is off the lift but for now a text description will have to do.
1) Imagine someone is sitting in the engine bay with a large rubber hammer and is pounding on the drivetrain with about 2 or 3 thumps per revolution of the wheels. The thumping noise is really quite loud.
2) The thumping occurs at the same loudness regardless of how slight or how sharp a turn I am making.
3)It feels as if pressure is building up in the drivetrain, and then is suddenly released.
4) The thumping is there regardless of speed or loading. The only time the noise is not there is when going in a perfectly straight line.
I won't rule out bad wheel bearings, but would they really exhibit all of the above symptoms?
This is very frustrating for me, because I am almost 3 years into a restoration and sorting of my first Porsche and was actually on my way to get it safety inspected when the noise first appeared. I was looking forward to taking my first road trips right after the safety inspection but instead it is back on the lift and I am looking at what is likely some expensive repairs.
My current theory is that the flushing and cycling of the lateral diff lock slave has caused it to stick in the locked position. I think this would explain why the traction control light will not turn off. The thump is the forced slipping of the diff lock itself, but I really don't know enough about the drive train to say anything for sure. Currently the slave cylinder bolts and hydraulic lines are soaking in penetrating oil in case I need to remove it.
Does anyone know how the system knows when to turn on the traction control light?
I will try to describe the sound better. I will record it the next time the car is off the lift but for now a text description will have to do.
1) Imagine someone is sitting in the engine bay with a large rubber hammer and is pounding on the drivetrain with about 2 or 3 thumps per revolution of the wheels. The thumping noise is really quite loud.
2) The thumping occurs at the same loudness regardless of how slight or how sharp a turn I am making.
3)It feels as if pressure is building up in the drivetrain, and then is suddenly released.
4) The thumping is there regardless of speed or loading. The only time the noise is not there is when going in a perfectly straight line.
I won't rule out bad wheel bearings, but would they really exhibit all of the above symptoms?
This is very frustrating for me, because I am almost 3 years into a restoration and sorting of my first Porsche and was actually on my way to get it safety inspected when the noise first appeared. I was looking forward to taking my first road trips right after the safety inspection but instead it is back on the lift and I am looking at what is likely some expensive repairs.
My current theory is that the flushing and cycling of the lateral diff lock slave has caused it to stick in the locked position. I think this would explain why the traction control light will not turn off. The thump is the forced slipping of the diff lock itself, but I really don't know enough about the drive train to say anything for sure. Currently the slave cylinder bolts and hydraulic lines are soaking in penetrating oil in case I need to remove it.
Does anyone know how the system knows when to turn on the traction control light?
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Thread Starter
Burning Brakes
Joined: Mar 2018
Posts: 1,114
Likes: 337
From: New Brunswick, Canada
That's a good suggestion Ricardo, but both accelerometers were recently cleaned and tested. Also I checked for codes with UDT999 and found none.
But on that note, I am wondering if on the last bleed cycle of the lateral diff lock actuator if I forgot to hit the "reduce pressure" button. I doubt I forgot, but if I did, surely the pressure would reduce on its own since the accumulator loses all of its pressure in a matter of a day or two. Unless there is a one way check valve there somewhere that I don't know about.
But on that note, I am wondering if on the last bleed cycle of the lateral diff lock actuator if I forgot to hit the "reduce pressure" button. I doubt I forgot, but if I did, surely the pressure would reduce on its own since the accumulator loses all of its pressure in a matter of a day or two. Unless there is a one way check valve there somewhere that I don't know about.
Thread Starter
Burning Brakes
Joined: Mar 2018
Posts: 1,114
Likes: 337
From: New Brunswick, Canada
Update: Rear wheels are locked on my C4
I got someone to hold one rear wheel to keep it from turning, while I tried to turn the opposite rear wheel.
No dice. The two rear wheels are solidly locked together.
We tried this with ignition key on, key off, traction control on, traction control off. No matter what we tried, the rear end appears to be fully locked.
As mentioned earlier, this car had been in storage for around 15 years so of course I changed all fluids and this included flushing/bleeding the locks. Which is the more likely cause of the locked wheels?:
1) Lateral slave cylinder is stuck in the full lock position.
2) Clutch pack plates are stuck to each other. ( I know this happens to motorcycles that have been stored too long)
3) Something else?
I have searched the net and not found a similar situation. Hoping someone has some advice.
No dice. The two rear wheels are solidly locked together.
We tried this with ignition key on, key off, traction control on, traction control off. No matter what we tried, the rear end appears to be fully locked.
As mentioned earlier, this car had been in storage for around 15 years so of course I changed all fluids and this included flushing/bleeding the locks. Which is the more likely cause of the locked wheels?:
1) Lateral slave cylinder is stuck in the full lock position.
2) Clutch pack plates are stuck to each other. ( I know this happens to motorcycles that have been stored too long)
3) Something else?
I have searched the net and not found a similar situation. Hoping someone has some advice.
Thread Starter
Burning Brakes
Joined: Mar 2018
Posts: 1,114
Likes: 337
From: New Brunswick, Canada
Checked frunk. No Jimmy to be found, but noise is coming from the rear so I drained transmission oil. Oil was nice and clean. No sign of Jimmy.
But since Hoffa went missing in 1975 and my car was built in 1992, I have pretty much ruled out any Hoffa connection.
But I do have a simple question:
Does the C4 have a pre-load on the rear LSD clutch packs like the C2?
I have read that the C2 LSD has a breakaway torque of 15-37 foot-pounds but I cannot find a breakaway torque spec for the C4. In fact I cannot find any mention of breakaway torque for the C4.
If the C4 has a preload, then that would explain why both rear wheels appear to be locked together.
But since Hoffa went missing in 1975 and my car was built in 1992, I have pretty much ruled out any Hoffa connection.
But I do have a simple question:
Does the C4 have a pre-load on the rear LSD clutch packs like the C2?
I have read that the C2 LSD has a breakaway torque of 15-37 foot-pounds but I cannot find a breakaway torque spec for the C4. In fact I cannot find any mention of breakaway torque for the C4.
If the C4 has a preload, then that would explain why both rear wheels appear to be locked together.