Speed / Reference Sensor Positions
 

Ok, this would be real simple if I had the factory manuals, but x-mas is over and I'm broke. I just want to verify the measurements for the reference and speed sensor positions. My search yielded some questionable info from 'google' as follows:
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:This is word for word from a Chilton manual: : :"Porsche specifies that the distance between the engine speed sensor and the :flywheel must be 0.8 mm. If the distance approaches 2mm, a longer cranking time :will result, and the engine may not start at lower temperatures. The engine will :run normally during warm-up, but may idle higher than normal at operating :temperature. If the distance approaches 3mm, the engine will hesitate during :acceleration, and will miss. Due to heat expansion, the engine may stop running :for short periods." : :I imagine these symptoms could also be caused by a faulty sensor that is producing :a weak signal, but that is purely speculation on my part. :Dave C. : :> What exactly happens if the gap on the sensors isn't correct? I broke the :> little housing they were in while doing my clutch and saw the slotted hole, :> but I just tightened the new housing after inserting the sleeve that the front :> sensor goes in (and which was the reason it broke), figuring this would let :> things line up correctly. Should I worry about the gap? :> : ---------------------------------------------------------------------- :Subject: Re: More Info On Starting Problems :Date: Mon, 22 Mar 1999 23:35:44 -0500 :X-Message-Number: 65 : : Reference mark sensor must be 5mm +/- 0.1 mm :out from the flywheel, secured with Loctite. The :speed sensor is to be 0.8 mm from the ring gear :teeth. Don't get the speed sensor and reference :mark sensor mixed up. The speed sensor is the :rear one with the D stamped in the housing by it, :the front one is the reference mark sensor with :the B stamped in the housing. See pages 28-14 and :28-15 in your manual. : :Hope that helps,
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I constructed the following sketch for simple reference, so can someone please verify the 0.8mm and 5.0 mm measurements?
Thanks,
Scott

Should you worry about the gap? How cold is it in MD? If the sensors aren't hitting any thing and the car starts and runs fine in all temp ranges I'd say your ok, but I can tell you from experience the sensor positions are critical in cold weather. If you can't start it in the cold reposition them and try again. It's a pain in the ass to do and once the car runs for a few seconds you don't know if you got it right till it get the engine metal gets cold again. Not much room back there hah?

ch944

I'm in the process of putting together my clutch (amoung other things), so at the moment I've got all kinds of room and I want to do what's right. I know the position is too close since the speed sensor face was worn thru the case. The bottom bolt from the bracket was missing as well - I'd really like to slap someone.
Again, can someone verify the dimensions.
Thanks,
Scott
84' 944na

The stock system uses cranks sensors that work off of reluctance and the distance is critical. The gap distance between the sensor and what it reads changes the voltage the ECU sees. Find the correct mm distance and set it.

Kevin
PCM
www.perfectchoicemotorsports.com

0.8mm is the gap needed.

ch944

Wow, it's been a long time since i've heard the word 'reluctance'. So these are magnetic type pick-up coils. Now, can anyone explain what the speed sensor (the rear sensor) does as all those ring gear teeth are wizzing by?

Its counting them, so the ECU knows where the engine is at all times. Consider the tooth as a pulse. The ECU reads the pulses by knowing how many pulses are in 360 degrees. ECU logic is counters and timers.

Kevin
PCM
www.perfectchoicemotorsports.com

My 86 944NA daily driver starts and runs great in spring summer and fall but when winter come starts a bit hard and runs rough and rich for a few minutes and then runs fine when warmed up. I have changed and tested everything except the speed sensors. I have found that the DME isn't sending the proper signal to the idle control valve when cold. It always tells it to be closed and it needs to open up on a cold start in cold waether for sure. any thoughts?

Thanks for the explanation Kevin. It's probably a good idea to put this on the check list when doing a clutch job, since any drift in the dimensions could have a profound effect on performance.
Thanks again All