944 Turbo Clutch Release Problem
02-25-2016, 06:58 AM
#1
Burning Brakes
Thread Starter
944 Turbo Clutch Release Problem
This is a long post:
MY 1990, 944 Turbo S Clutch Problems
Over the last few months I have experienced problems with the clutch, as there were problems shifting into all gears, which required me to start the car in first gear and drive using the gear box as a “Crash Box” without any syncromesh. This method allow me to get home in all instances of failure.
The Saga:
The master cylinder was adjusted as per WSM with the required clearance on the pushrod. The gear change would work for a while and then the same fault would re occur.
The master cylinder was checked and cleaned, the slave cylinder checked, cleaned and re installed, the system bleed and the same problem re occurred after a while..
The master cylinder was replaced with a new unit and system adjusted and initially the clutch worked and then the problem reappeared. Tried re adjusting the master cylinder pushrod length, but problem would always re appear.
The slave cylinder was replaced with a new unit, adjusted and initially worked and failed again.
During all this time I also conducted a full research on the web and Rennlist. A lot of information was gained.
I measured diameter of the flexible rubber hose in the clutch fluid line when relaxed and under full pressure and it increased in diameter by 5mm. I calculated the fluid volume, assuming that the diameter increased over it length and this was approximately 9 mls,. This indicated that the slave cylinder was being starved of fluid and possibly reducing the effective length of movement of the slave cylinder push rod. A new hose fitting was fitted and system re bleed. Again clutch worked for a while and then the symptions re occurred.
WSM states that a new clutch has a set up measurement of 34mm measured via the inspection hole, from the outside side of the cup of the throw out lever to the forward side of the inspection hole with a wear limit of 18mm from the outside of the cup to the forward end of the inspection hole, The movement of the throw out lever required to activate the clutch is 18 mm, so hence this clearance distance.
My cup distance was measured at rest and was 26mm, which is equivalent to half the wear on the clutch plate. The maximum movement of the throw out lever was measured at 16mm, insufficient to disengage the clutch completely from the flywheel surface and the pressure plate. Even with all new parts. 16mm will not give enough movement to completely disengage the clutch, hence the drag and non ability to engage any gear.
I pressure bleed the system, pressurized the system and vacuum bleed the system with final bleed by the old method of actuating the clutch by a second person. The symptions kept occurring. I even developed a 3rd leg system by a wooden piece that held the clutch in the fully depressed position, whilst I did the final bleed.
I could put the car in ramps, jack up the rear and place it on 4 jack stands in around 10 minutes to get at the slave cylinder.
I reviewed the design using basic engineering and I then decided to turn up a thimble piece to fit over the push rod end of the slave cylinder to effectively increase the length to take up all the wear in the clutch actuating system. My first thimble was made to give 4mm extra length to the pushrod, as I assumed that the pawl surface would have worn some over 192,000kms and the cup would have also worn.
I installed this thimble piece by removing starting motor and removed the slave cylinder by just undoing the two mounting bolts and swinging the slave out of the mounting hole, without undoing the brake fluid piping, thereby saving bleed the clutch system again and again.
The 4 mm worked at first and then the old failure returned, so I assumed that I need a longer thimble piece, so I turned up a thimble, which increased the overall length of the slave cylinder pushrod by 6mm. I installed the new 6mm thimble and I now have a fully operational clutch. I have tested this with a couple of long runs in traffic, which required the clutch to be operated regularly and all appears to be OK. I’m due to undertake a super sprint early in March, so this will really test the operation of the clutch under full torque and temperature etc.
The thimble Piece dimensions:
The push rod diameter is 9mm the thimble piece is turned up to an outside diameter of 11mm. I ground up a 9mm drill with a semi-spherical end to ensure a good inner surface abutting the pushrod end and I turned and filed up the outside end of the thimble into a semi-circular end. The overall length of the thimble piece is 18mm, the inside length is 12mm overall.
The slave cylinder has an internal spring which holds the push rod into the cup of the throw out lever and the pressure plate acts on the throw out lever in the opposite direction, so the thimble piece is held in position by constant pressure.
On reassembly I ensures to put moly grease on the semi-spherical end of the thimble to reduce wear.
I will now turn up another thimble piece giving an effective increase to the slave cylinder pushrod by 8mm and keep it in my racing tool box.
I spoke with some automotive techos at our last super sprint and one told me they weld the end of the pushrod and to gain the extra length. I believe that the method above is much simpler method.
Tails 1990 928 S4 Auto, 1990 944 (952) Turbo
MY 1990, 944 Turbo S Clutch Problems
Over the last few months I have experienced problems with the clutch, as there were problems shifting into all gears, which required me to start the car in first gear and drive using the gear box as a “Crash Box” without any syncromesh. This method allow me to get home in all instances of failure.
The Saga:
The master cylinder was adjusted as per WSM with the required clearance on the pushrod. The gear change would work for a while and then the same fault would re occur.
The master cylinder was checked and cleaned, the slave cylinder checked, cleaned and re installed, the system bleed and the same problem re occurred after a while..
The master cylinder was replaced with a new unit and system adjusted and initially the clutch worked and then the problem reappeared. Tried re adjusting the master cylinder pushrod length, but problem would always re appear.
The slave cylinder was replaced with a new unit, adjusted and initially worked and failed again.
During all this time I also conducted a full research on the web and Rennlist. A lot of information was gained.
I measured diameter of the flexible rubber hose in the clutch fluid line when relaxed and under full pressure and it increased in diameter by 5mm. I calculated the fluid volume, assuming that the diameter increased over it length and this was approximately 9 mls,. This indicated that the slave cylinder was being starved of fluid and possibly reducing the effective length of movement of the slave cylinder push rod. A new hose fitting was fitted and system re bleed. Again clutch worked for a while and then the symptions re occurred.
WSM states that a new clutch has a set up measurement of 34mm measured via the inspection hole, from the outside side of the cup of the throw out lever to the forward side of the inspection hole with a wear limit of 18mm from the outside of the cup to the forward end of the inspection hole, The movement of the throw out lever required to activate the clutch is 18 mm, so hence this clearance distance.
My cup distance was measured at rest and was 26mm, which is equivalent to half the wear on the clutch plate. The maximum movement of the throw out lever was measured at 16mm, insufficient to disengage the clutch completely from the flywheel surface and the pressure plate. Even with all new parts. 16mm will not give enough movement to completely disengage the clutch, hence the drag and non ability to engage any gear.
I pressure bleed the system, pressurized the system and vacuum bleed the system with final bleed by the old method of actuating the clutch by a second person. The symptions kept occurring. I even developed a 3rd leg system by a wooden piece that held the clutch in the fully depressed position, whilst I did the final bleed.
I could put the car in ramps, jack up the rear and place it on 4 jack stands in around 10 minutes to get at the slave cylinder.
I reviewed the design using basic engineering and I then decided to turn up a thimble piece to fit over the push rod end of the slave cylinder to effectively increase the length to take up all the wear in the clutch actuating system. My first thimble was made to give 4mm extra length to the pushrod, as I assumed that the pawl surface would have worn some over 192,000kms and the cup would have also worn.
I installed this thimble piece by removing starting motor and removed the slave cylinder by just undoing the two mounting bolts and swinging the slave out of the mounting hole, without undoing the brake fluid piping, thereby saving bleed the clutch system again and again.
The 4 mm worked at first and then the old failure returned, so I assumed that I need a longer thimble piece, so I turned up a thimble, which increased the overall length of the slave cylinder pushrod by 6mm. I installed the new 6mm thimble and I now have a fully operational clutch. I have tested this with a couple of long runs in traffic, which required the clutch to be operated regularly and all appears to be OK. I’m due to undertake a super sprint early in March, so this will really test the operation of the clutch under full torque and temperature etc.
The thimble Piece dimensions:
The push rod diameter is 9mm the thimble piece is turned up to an outside diameter of 11mm. I ground up a 9mm drill with a semi-spherical end to ensure a good inner surface abutting the pushrod end and I turned and filed up the outside end of the thimble into a semi-circular end. The overall length of the thimble piece is 18mm, the inside length is 12mm overall.
The slave cylinder has an internal spring which holds the push rod into the cup of the throw out lever and the pressure plate acts on the throw out lever in the opposite direction, so the thimble piece is held in position by constant pressure.
On reassembly I ensures to put moly grease on the semi-spherical end of the thimble to reduce wear.
I will now turn up another thimble piece giving an effective increase to the slave cylinder pushrod by 8mm and keep it in my racing tool box.
I spoke with some automotive techos at our last super sprint and one told me they weld the end of the pushrod and to gain the extra length. I believe that the method above is much simpler method.
Tails 1990 928 S4 Auto, 1990 944 (952) Turbo
02-25-2016, 10:46 AM
#2
Rennlist Member
I bet one of the little damper springs on the clutch disk has broken and is wedged in between the pressure plate fingers, preventing it from moving fully and releasing properly.
If you ever step on the clutch pedal and it only moves part way and gets rock hard (like there is some sort of interference) that's a broken throwout bearing guide tube.
Good luck!
If you ever step on the clutch pedal and it only moves part way and gets rock hard (like there is some sort of interference) that's a broken throwout bearing guide tube.
Good luck!
02-25-2016, 03:47 PM
#3
Burning Brakes
Thread Starter
Hi Van,
Thanks for the comment. I have a full clutch pedal movement, the one thing that I have not checked is the full movement of the slave cylinder piston rod, however due to the wear (perceived) on the pawls and within the cup of the yoke piece, it would have taken up a percentage of the slave cylinder travel before disengagement movement of the throw out bearing of the clutch.
Thanks for the comment. I have a full clutch pedal movement, the one thing that I have not checked is the full movement of the slave cylinder piston rod, however due to the wear (perceived) on the pawls and within the cup of the yoke piece, it would have taken up a percentage of the slave cylinder travel before disengagement movement of the throw out bearing of the clutch.
02-25-2016, 04:05 PM
#4
Rennlist Member
When the problem comes back, is it intermittent, or does it stay until you installed the 4mm booster piece?
It could also be a failing release fork (what I believe you're calling the yoke), although usually those just snap and the clutch pedal stays at the floor.
I've seen those springs on the disk fail - specifically on the Turbo S (because it has a set of smaller than normal springs).
It could also be a failing release fork (what I believe you're calling the yoke), although usually those just snap and the clutch pedal stays at the floor.
I've seen those springs on the disk fail - specifically on the Turbo S (because it has a set of smaller than normal springs).
02-25-2016, 04:27 PM
#5
Burning Brakes
Thread Starter
The problem was intermittent even when I installed the 4mm thimble piece. Now with the 6mm all appears to be operating satisfactory. With the 4 mm thimble piece I carried out a full day racing in our last Super Sprint with only one instant of being unable to engage gears, this was on the dummy grid. The drive to and from the track was 40 kms both way and I undertook 6 sessions of 4 laps with only the one incident.
So far with the 6mm thimble all is well with 18mm movement of the vote at the cup end. Fingers crossed!
So far with the 6mm thimble all is well with 18mm movement of the vote at the cup end. Fingers crossed!
