inop speedometer

Haven't run my restored SC on the road yet, but with the engine running and the car on jackstands with the transmisson in gear, I get no speedo movement. Assuming that the leads from the speedometer sensor on the gearbox are connected and that the sensor is properly in place on the side of the gearbox, what are the typical points of failure here? should I change out the sensor? Is there a way to test it?

thanks,
Stephan

Yes,

Place an ohm meter on the wire from the sender to the speedo (can't recall the wire color. I have to look at service manual). This wire is connected to the back of the speedo. With one wheel off the ground, rotate the wheel. The ohm meter should read zero Ohms then infinite (or Over load) four times per revolution.

Tony
'78 911SC

A lot of people put the sensor on the transmission on upside down (because the way it *seems* that is should go on is upside down).
Maybe try it the other way!

That's my vote too. Jim T hit the nail on the head. When I reinstalled my enigne/tranny, I initially had put the sensor on upside down, with the same results. It looks like it should go on upside down.

------------------
'83 SC

[javaschlava]

Dear speedo diagnosticators,

Similar problem on my 83 SC found out that ohm meter trick works well.

Place an ohm meter on both ends of the sensor wires and it should go open and closed 0.0.

I found that on mine the sensor was good but when I took out the speedo I mixed up the two wires from the sensor and had them both into the 31b terminal on the speedo!

Silly me. They are both brown w/red tracer and one goes to 31b the other to the neg terminal.

Thanks for your input it helped.

Mark

[JP911]

The speedo on my 76 (same setup as an SC) used to stick after the car would sit, but would magically come back to life after a few miles of driving.

[Early_S_Man]

Just for the record, and any newbies reading this thread for the first time ... the correct number of magnets on the carrier plate is EIGHT, not four ... and you get EIGHT contact closures [Ohms reading of 0.0] and opens per revolution of the axle or output flange!!!

[khamul02]

While we are on this subject. Mine has for the last month worked for 30 miles and then stopped until I parked. After it is parked at work or whatever for an hour or so it will work again for about 30 miles. Could this upside down thing be the cause or do you think a wire has come loose?

[Walt Fricke]

Quick test:

Sensor - as described, 8 events per differential revolution. The sensor is just a reed switch that shorts the speedo's signal input to ground. Sensor failure seems by far the most common problem (leaving aside mis-installation). The reed switch must have a limited number of cycles alloted to it before it fails. But that's why the ohmmeter trick works fine, as it is just an on/off switch. You could use the continuity tester, for that matter, and listen for its beeping.

Of course, there could be a break in either of the two wires between sensor and instrument. There is a two pin connector back by the shift coupler that connects the sensor to the wiring headed up front. I don't know why Porsche didn't just ground one of these wires back there, but they didn't - the ground is up front.

Speedo - pull it from the dash, pull the sensor wire off, and attach another wire with a bare end or its equivalent. With ignition in run position, tap the end of this added wire to ground rapidly. The speedo needle should jump. If you are good, you might even be able to hold a "speed" on the needle. If the needle stays put, there is likely a problem inside the speedo itself. This procedure is from the Porsche shop manual, no less!

There is only one trick I know about to these speedos. The internal circuitry is not directly connected to the speedo case. I think this was to accomodate positive ground cars, but I'm guessing. In any event, you have to be sure there is a good ground to the insulated ground terminal on the speedo, not just to the case (though jumpering over from the case is fine). Absent this instrument ground you will blow a diode inside the speedo. Repair involves replacing the diode (not hard once you have opened the case), and being sure you have the ground done properly.

Walt Fricke

[khamul02]

Thanks Walt, I'll run through these suggestions this weekend.