Speedometer speed sensor

[Brian 162]

Over the winter I replaced the odometer gear on my speedometer following Thrashers thread.
When I hooked everything back up the speedometer wouldn't work.
What I did was put both sensor wires on the same terminal by accident. Previous owner replaced some of the wires with non stock wiring.
I found an open circuit at the sensor connection in the tunnel.
Did I pooch the sensor?

[DRACO A5OG]

I doubt it. Remove sensor at the gear box and pass a magnet over the sensor, if speedo bounces, you are good to go Just make sure you put it back the same way you found it.

[Walt Fricke]

The sensor is only a reed switch, opened and closed by 8 magnets rotating inside the differential.

The speedometer sends a constant current down the speedo wire to the switch (which just opens and closes like any switch would do). When closed, the switch sends this current to ground. For some reason (positive ground cars VDO might have been thinking of), Porsche runs the ground wire all the way back to the speedometer, where it is grounded like everything else that needs a ground there.

The current is so low that grounding accidentally shouldn't harm anything.

To test the speedometer, pull the speedometer out, locate the signal wire, and at the speedometer connect a wire to its terminal with an exposed end, then, with the key in run position, tap the exposed wire end to a ground. You should see the needle move - you can see how far up you can get it.

Then put the speedo back in its place and do the same test at the two pin connector back by the shift coupler. If the needle moves, the wire is OK. Use an ohmmeter to check to see that the other wire of the pair is grounded.

You could check the sensor by jacking up the back of the car, and measuring the resistance, or the continuity, through the two sensor wires. Rotate the right rear wheel and see if you see alternating no resistance with infinite resistance. Analog meters are good for this, but the continuity beep setting should do also.

I assume the open circuit you found was on the sensor wires leading back to the tranny. If the right rear wheel was in one of 8 positions, the sensor would read an open. In some other 8, closed. If nothing happens when rotating, one of its wires is shot, or somehow the switch has failed, or all 8 magnets have fallen off the disk in which they are glued. Unlikely.

The speedometer in my 77 had a quirk - a ground wire ran to the main speedo case. Another wire ran from that ground to the electrically isolated speedo proper, giving it its ground. Guess: allowed for positive ground systems. Consequence: If you left off this ground jumper, you fried a diode inside the speedometer when you powered up, as the ground path then ran through that diode with too much current. My electronics skills are limited, but I can identify a fried component sometimes, can test a diode to see if it is working grossly, and can solder in a replacement, so I brought back to life two failed speedos.

But by 1980 I think this quirk was gone. You'd know from your explorations.

[Brian 162]

Thanks all I'll try that test.

[craig245t]

I also replaced the odometer gear on my '82 SC speedo. The speedo was lazy/erratic before but will not work at all now. The odometer still does not work.

The connections i noted before removing the speedo puzzled me but i put it back the way i found it.

The two wires from the gearbox sensor were connected to the same terminal on the rear of the speedo (31B)....it made no sense to me but my knowledge is limited.

The connections were......

-ve = 1 terminal containing 2 brown earth wires
31b = 2 brown/red wires from gearbox sensor
A = not connected
2 = blank connector
+ = 12v live feed (ignition controlled)

I've seen pics from users on UK impact bumpers forum suggesting this is correct but any suggestions welcome.

I've attached a pic of the connections before removal......

[craig245t]

I've just fixed my speedo thanks to reading this thread!!

I was assuming that the wiring in my post above was correct and that the speedo was faulty. I had started replacing components on the circuit board too thinking the fault was there.

After reading the thread it got me thinking that the 2 wires from the gearbox sensor shouldn't be connected to the same terminal on the speedo - it makes no sense (to me).

Walt Frickes post provided the answer.

I moved one sensor wire from the gearbox to the -ve terminal on the speedo, then piggybacked the 2 brown earth wires onto this terminal too. The speedo and odometer now work!! Happy days.

Brian - I would try this, it worked for me.

Thanks to all for the thread.

[Brian 162]

I tried Walt's test and nothing happened at the speedometer.
I checked for continuity at the sensor. At first I had continuity, after removing the harness I lost continuity.
I found 2 splices in the sensor wire in the tunnel.
Tomorrow I'm going to look under the car feed the wires through the hole remove all the tape and see what I got.

[Walt Fricke]

Nothing like a previous owner butchering something to lead to hard to diagnose problems.

[DRACO A5OG]

so true ^^^

[Brian 162]

I removed the spliced wires at the sensor repaired and re-installed. No difference.
I borrowed a speedometer from my mechanic and it worked. So at least I know the wiring and sensor are fine.
There's one more thing I'm going to try before sending the speedo for repair.
There was a metal tab that fits on the front of the assembly and for the life of me I don't know what it does. It only fits one way on the upper left screw or lower left screw.
I had it mounted on the upper left screw so I'm going to mount it on the lower left screw.
Maybe it's a ground?

[Walt Fricke]

" I borrowed a speedometer from my mechanic and it worked."

In the best tradition of troubleshooting.