latin944

After some 20 minute driving, I cannot shift gears anymore. New oil 75W90 in the gearbox. Same problem.

Even with the car not moving, not possible to change gears, only turning of the motor allows to change gears (but difficult).

Every morning, wtin motor and gearbox cold, all works well.

Checked master/slave cylinder functioning and the movement of the pin on the clutch through the hole, all withing specs. (Yes I had to get air out of the clutch cylinders).
Clutch pedal gets at least 2-3 inches off the floor before the clutch makes contact to start moving the car.

What could possible cause the gearbox to lock up? just heat? (yes it is 30°C / 100 °F outside here, plain summer.)

I suspect the clutch, as one hears gearbox noise when trying to shift with the car stopped, so the clutch is not liberating 100% the axle. And the car shifts gears with the motor turned off. But with gearbox in 1st gear, and clutch pedal on the floor, the car does not move the slightest. And it would have to move if the clutch is not decoupling 100%


(944 turbo 1987, 100.000 miles, easy going daily driver)

KevinGross

If this is happening in all gears, I'd start by checking the clutch. How much travel are you seeing for the slave cylinder / clutch release fork when you measure through the inspection hole?

If you shut the engine off when the car is hot, and you stop motion of the input shaft / torque tube shaft, can you engage gears?

latin944

Yes it is happening in all gears.
Travel is 15-17mm measured in the inspection hole. And clutch pedal comes of the ground for some 2-3 inches before the clutch starts engaging. Hot and cold situation.
And with cold car/motor, no problem at all.
With the motor stopped, yes one can engage gears. With Motor turning, shifting is not possible.

PD twice today, I noted that, starting from cold motor etc, everything works. One drives 20 minutes, everything ok. Parking the car for 5-10 minutes, and shifting is not possible (with motor turning)
How can a littlebit of motor heat affect the clutch? That is what makes no sense to me.
On the contrary, worn plate or worn pressure plate means that you can't get enough pressure to drive. I can'r decouple enough it seems.

I have a new clutch on the shelf. But would like to be sure of the problem before beginning on this large job.

will968

When this happens, can you have someone press the clutch while you look in the inspection hole to see if the fork is moving enough? There could still be air in the system, it's hard to bleed properly, or the slave is bad (guessing on slave vs master, but it would get most heat first).

vandal968

Did it have the trouble before the tranny oil change? I had an Rx-7 that did the same thing many years ago when the morons at the Jiffy Lube used a GL5 oil of the correct viscosity instead of GL4. The correct oil fixed it after about 20 miles.

cheers,
c

400hp944

This is a long shot, but a similar situation happened with my race car a long time ago. You mentioned that it's very hot where you are, but it doesn't happen when the car is cold. There is a slim chance that the gas tank vent hose is pinched somewhere and not allowing the tank to vent. By not allowing the tank to vent, the tank will expand and push against the shift linkage making it difficult if not impossible to engage gears. When it happens again, open your gas cap and see if you get a big woosh of back pressure vapor. Worth a check anyways...

Van

If this happens in all gears, and only when the engine is running, then it's not a problem with the transmission, but rather a problem around the clutch. I'd start by guessing it's the pilot bearing... even when to step on the pedal to disengage the clutch, something is still making the driveshaft spin...

Ken Wells

Loose flywheel bolts will also cause these problems

Autobreza

When I changed the transaxle fluid to Redline 75-90 I experienced increased (and unacceptable) shift effort. Changing to Redline MTL (specifically labeled for transaxles) completely solved my shifting issues.