Speedo sensor dumb question #2
 

Car is 79 930. Speedo sensor fits in recess on right side of transaxle. Sensor has two wires that go to dedicated posts on speedo. How, exactly, do I test sensor to determine if it is working. Multimeter dial points to_____? for checking? Which wire should read what? I presume multimeter's ground indeed needs to be grounded to car. Has anyone ever experienced internal sensor stimulator (INSIDE of transaxle "going bad" or falling off? Thanks - I'm past my ability (I paint pictures for a living). EPS

Not totally sure how to test it but,
The speed sensor is a proximity sensor, it uses as "stimulator" a toothed wheel which is attached to the flywheel or is part of the flywheel (this wheel cannot go bad) If there is a problem it is the speed sensor that went bad or is loose and is to far from the wheel and does not give a signal. It should give a square wave signal.

sensor
 

Is it not Hall effect then?

I think it is a reed switch with 8 (?) magnets on the differential.

Interesting reading;
"smokey speedometer". http://forums.pelicanparts.com/porsche-911-technical-forum/332604-smokey-speedo.html

I'm pretty sure the the 79 930 speed sensor and the 87-89 G50 speed sensor work the same, it's a reed contact with 8 magnets, the sensor with two wires is located on the side of the differential by the axle.

See the bottom post in this link for a detailed explanation on how to test:

http://forums.pelicanparts.com/porsche-911-technical-forum/356477-speedometer-stops-working.html

Post your results.

Good Luck.