Erratic idle - locked gears - bad crank position sensor?
 

Recently drove my 74 911 to my fabricators shop for some metal work - car was running perfectly. Picked it up and immediately noticed that the idle was off. Sits at 2k, moves up and sticks/revs higher as the throttle is depressed. In addition, the 915 won't go into any of the gears while the car is on, but gears go in fine when the car is off. In searching around this combo sounds like symptoms of a crank position sensor that has jumped ship. My car has a 3.6 964 swap with a Patrick Motorsports cabin harness that has a built-in diagnostic port. I have tried three different scanners and cannot get the any readings. My next move is to pull and test the crank sensor with a multimeter. Anyone know the signal reference this sensor needs to put out? I would appreciate any useful advice.

Thanks in advance!!

Was any welding done on car?

Yes but the battery was disconnected.

Not going into gear while running sounds like the clutch is out if adjustment. High idle could be sticking throttle linkage. What work was done on the car? Anything around the pedal cluster or trans tunnel?

Rear RS fender flares were welded on. Clutch adjustment was fine when the car was driven over and everything shifts fine when the car is off. Only gate issues are when the car is on.

Anyone with a similar experience?

Have you shared your observations with the shop? Clearly something happened while your car was there.

I'd start by troubleshooting the symptoms you are seeing. For the high idle, is the linkage on the engine returning to the idle position? That's easy to check, and can rule out a linkage/cable issue.

Re: the trans not going into gear while the engine is running, check the clutch adjustment as suggested as that's also easy. Once confirmed good, try again. You can hold the clutch in for a few seconds to let things spin down (from the high idle). If nothing else is wrong, it should go into gear easily.

Thanks for your input. While that all makes sense, it seems odd to me that both the throttle cable and the clutch adjustment would simultaneously go out of adjustment while the vehicle sat for two weeks. I just adjusted the clutch cable less than 500 miles ago. I did share my observations with the fabricator, he said the vehicle did not move while at his shop. As far as the idle, it sits at 2k when I start the car and if I throttle it, the idle moves higher and higher. The crank angle sensor came to mind since a failed unit can cause erratic idle and locked gears from what I read. I plan to pull it and test it this weekend when I have spare time.

Crank sensor will do nothing to keep the transmission from going into gear unless it broke off and got wedged in the clutch. The fact that the idle goes higher and higher if given throttle sounds like sticking linkage. I wonder if they jacked the car up (maybe to take tires and wheels off) and they jacked from the center on the engine/transmission and bent/ crushed something.

john

@Ken.e - don't assume they simultaneously went out of adjustment. Unfortunately we have no idea what the shop did to your car. This could easily be two problems that happened to come to your attention when you picked up the car.

Valid points. The fab guy said he used the supplied jack pads but to stownsen914's point, we don't know what could have happened while the car was there. I will go over the throttle linkage and clutch adjustment this weekend. Thanks guys http://forums.pelicanparts.com/suppo...s/beerchug.gif

Update - disconnected the throttle cable to see how the motor would run per advice from a friend. Engine idled perfectly, connected the cable back on and everything seems fine now. There must've been a hang up somewhere. I then checked the clutch cable, and it did need a minor adjustment to meet 1mm spacing. I adjusted it and still cannot access any gears. I loosened it a bit more and still no access. Will continue to do research but a bit stumped here. I would appreciate any useful advice.

Progress is good ...

Re: gear selection still being off - the symptom is a bit odd, but I'd do a visual check of the linkage for slop at connection points or bushings, restricted movement, or anything just looking off. And check the engine mounts - if one is failing, droopy, or loose, it could make for unhappy shifting.

Check the clutch release lever on the bottom of the trans. When a jack is used under the engine, if it isn't carefully located, the lifting block or pad will bend that arm. Then the clutch cable does not make a straight line to the arm. BTDT. Look for bending or damage to the throttle linkage under there too.

Update - I went over the linkage over the weekend, everything appears fine. I also made some adjustments to my Wevo shifter and coupler which did not help much. Adjusted the clutch cable, that did nothing. In racking my brain, I thought that perhaps the trans is low on gear oil. There is some oil under the trans which leads me to believe there may be a slow leak. I filled the trans when I did the 3.6 swap back in 2019. Thinking 3.5 years - slow leak - likely needs oil. I have been meaning to switch from Swepco to Kendall anyway so will do that this coming weekend. @PeteKz - good call but the guy that did the work on my car knew to use the jack pads. My car is lowered so not sure that you could even get a jack under there.

can you get the transmission to change gears while the engine is not running?

If you can while off, but can't while running i'd look at clutch not releasing for some reason.
If you can't change gears with the engine off either, i'd say you might want to remove the inspection plate to have a look at things while a helper attempts to change the cears (engine off of course sind all the trans fluid will be drained.)

I am able to shift the transmission normally while the car is off. Not going into gears while the car is on. I did a test earlier per a friend's recommendation; put the trans into all gears with the car OFF then turned the ignition key and the car jolted in every gear. This tells me the clutch is doing its job and the issue must be in the clutch cable somewhere.

Over the years I have had the issue of not being able to shift into gear after starting the engine . The car would grind in any gear I tried. On level ground I would shut off engine, press and hold clutch down, shift into 1st. While clutch is pressed down start engine and car would roll. The clutch and clutch plate are stuck together. The way I would fix the issue of the stuck clutch I would put the car into first and get the car moving and slam on the brakes. Granted, the engine will stall but it will also free the clutch. This has happened to me about 5 times over the past 19 years. Why it happens ???? Also, my clutch is all adjusted properly and shifting is normal when unstuck.

targa80 makes a good point. After sitting for some time, the clutch disk can fuse itself to the pressure plate and/or flywheel. Condensation followed by rust perhaps? Never happened to me in a Porsche, but it did happen once in a non-Porsche that sat for a few months outside. To see if this is your issue, with engine off, push the clutch in while in gear and see if the car can be pushed. If it doesn't move, your clutch isn't releasing properly and could be fused.

Hey guys - good call but tried this and the car moves fine while in gear with the clutch pedal in. I am going to recheck clutch linkage over the weekend.