915 wont select reverse when hot

[dcottam]

My car will not select reverse when everything is warm.

When cold everything works fine, but when hot it is impossible to select reverse. The gears grind as if the clutch is not working, but driving forwards everything seems OK - I am able to go through all 5 gears. After cooling down for 90 minutes I can select reverse again no problem.
When the car is hot I cannot select reverse when the engine is off either.

Any ideas?

Thanks to all

[burgermeister]

I would guess clutch.

You're basically performing the test for clutch disengagement - press clutch, wait 5 seconds, try to put it in reverse. If it grinds, the clutch is not disengaging.

Check the adjustment of all the parts first, especially at the actuating lever on the transmission.

[Joe Bob]

Last time you changed the transmission fluid or checked the level?

[dcottam]

Transmission fluid was changed 2 weeks ago

[Joe Bob]

Did the reverse problem start before the fluid change?

[dcottam]

The car is new to me. I had all the fluids changed as a precautionary measure. The problem only manifests after driving for at least an hour, so didn't show up in PPI or while with the shop that did the fluid changes. The last owner of the car passed away so impossible to tell if it was a known issue or not

[MichaelSJackson]

Dave,

If it's only reverse, then I would guess coupler adjustment; not clutch. Quite a few owners have problems finding reverse, because of improper coupler adjustment. However, I've never heard of one aggravated by heat.

Not finding reverse is different than grinding when attempting to slide into reverse. (The grinding mentioned by burgermeister. He's correct, there should be no spinning after holding the clutch depressed for 5 seconds.)

[walt]

I always run the shifter between 3rd and 4th before shifting into reverse or I get a grind or won't go in at all. Don't know why but it's worked for 20 years. Never heard of a problem that is heat related.

[JSV798]

When I had the oil changed on my 1978SC a couple of years back I found I could no longer engage 5th. Had to adjust coupler rotation slightly and the all was well. So, if an oil change can affect gear selection then I am prepared to believe hot oil might do something similar.

[dcottam]

It's grinding - sorry my description was not good. When hot it grinds when trying to select reverse as if the clutch is not working (but the clutch works fine on all forward gears). I have tried holding the clutch in for 5 or more seconds, going from 1st to reverse, going from 5th to reverse etc but the result is the same - grinding.

This has only happened to me when the car is warmed up. When cold reverse has been fine. I suppose that could be coincidence and it is not to do with the car being cold and /or warm

Thanks to all for the responses

[Eagledriver]

It has to be the clutch. The grinding means that the input shaft is spinning. The only way that can happen is if the engine spins it through the clutch. The forward gears all have syncros so a slight clutch drag can be overcome. Just tighten up the clutch cable or whatever adjuster is on your car. Should be a very small change.

[alexandervdr]

in my experience, how the 915 shift is very sensitive to the coupler setting. Turning the coupler one spline left or right can make a huge difference. Heat does effect mechanical settings. Assuming your shifter and coupler bearings are ok, adjusting the coupler is the easiest and cheapest way to find out if your problem relates to it. It does not take more than 30 minutes if you have done it before.

[AFB24911S]

I have a similar problem, mine just doesn't want to shift into reverse away. My owner's manual says: "The clutch pedal must be depressed several seconds before shifting into reverse; only then move the gearshift to the left, overcoming the spring resistance, and to the front." (My manual covers the 1977 911S, Carrera, and Turbo, and has icons for both the 911 Turbo transmission and the 915 in my car.)

I've found if I wait about 15-20 seconds with the clutch depressed it only grinds a very little bit or not at all going into reverse. It's even easier if I move forward slowly in first and then wait.

I think Eagledriver's theory is correct, our clutches are not fully disengaging. With a properly adjusted car, I would think, you can still get a grind if you try to shift too quickly because the input shaft is still rotating forward due to the momentum of the flywheel. But once the input shaft is still (after you wait a while with the clutch in) if the clutch is right it ought to shift into reverse.

[Walt Fricke]

Guys - adjusting the shift mechanism cannot cause or cure grinding when trying to engage a gear. So it has to be the clutch, or perhaps the pilot bearing. It could be that the pressure plate warps a bit when warm, or that the clutch disk warps or expands a bit. So adjust the clutch. A little more disengagement might cure this without risk of the clutch slipping.

Just what year is the car?

The temporary fix when you need reverse is to stop the engine, put it in reverse, and start the engine. Chances are the brake will hold the car just fine as you ease the clutch out and go backward. To switch to forward just put the clutch in and shift as you normally would.

But adjusting the clutch isn't difficult.

[proporsche]

when you take off does your shifter vibrates??

Ivan

[dcottam]

Originally Posted by proporsche

when you take off does your shifter vibrates??

Ivan

No vibration - certainly nothing noticeable

Originally Posted by Walt Fricke

Just what year is the car?

It is 1978 - but it is a hotrod and the gearbox is not original to the car.

Originally Posted by AFB24911S

"The clutch pedal must be depressed several seconds before shifting into reverse; only then move the gearshift to the left, overcoming the spring resistance, and to the front." (My manual covers the 1977 911S, Carrera, and Turbo, and has icons for both the 911 Turbo transmission and the 915 in my car.)

I've found if I wait about 15-20 seconds with the clutch depressed it only grinds a very little bit or not at all going into reverse. It's even easier if I move forward slowly in first and then wait.

Reverse is opposite 5th in my car - I thought that was the case for the 915 box. Is that not correct?

I have tried waiting for 5 to 10 seconds with the clutch depressed and the car in first, but not 15-20.

THanks again to all.

[bkreigsr]

As Walt says, crawl under the car and take up the excess slack in the clutch cable.
When adjusted correctly, deflection should be between 3/8 and 1/2 inch when pressed with your finger.
Bill K

[coldstart]

I had this issue the first year I had my car. I had to push it back into parking spots. Fairly embarrassing.

It turned out to be clutch cable adjustment (I installed a new cable) and the shifter coupler bushings (new bushings).

Good luck.