Can someone explain SC speedo sensor wiring to me?

[Jim2]

I'm doing some custom replacement wiring and think I've overlooked how the speedo is hooked up. I'm using 911 SC speedometer and trans speed sensor.


On the back of my SC speedometer there are four pins:

+
A
31b
-

To run my own wires I thought hook up would be simply running two wires from the trans speed sensor to the A and 31b terminals (and presumed I put 12v to + and ground the - terminal). So far there is no signs of life.

I'm starting to wonder if Porsche did something altogether different with the A and 31b terminals and the speed sensor, perhaps injecting 12v and grounding. Any advice out there from someone who's rewired between the sensor and speedo?

[cmonref]

According to Pelican's electrical diagrams, for the '82 SC (Part 5, Page 2):

+ is 12v
A goes to the "Mileage Counter"
The sensor's two wires are connected to 31b and - (the minus connection)

- is also ground among the instruments, and connected to chassis ground

[porwolf]

Here are my 1979SC speedometer connections:



The '79 speed sensor is an ungrounded 2-pole (speed sensor wires 1 and 2) magnetic pick-up switch that responds to a rotating magnet inside the transmission. That magnetic switch makes 8 contact closures per revolution.

[Tremelune]

Man, these things break often. I have one with broken gears and I'm hoping to find a way to replace the odo bit with something digital.

[Jim2]

Cmonref and Porwolf, thanks for the info it's exactly what I needed! Now I'm just hoping I didn't cook something inside my speedo from having it hooked up wrong. I'll be on it tomorrow and test it out.

Thanks guys!

[speedking]

Originally Posted by porwolf

Here are my 1979SC speedometer connections:

Your pic fixed my problem - thanks!

My 79 SC doesn't have cruise control, so it has the empty spade connector, and before I saw the pic, I was unsure which connector should be empty. Next time a gauge comes out, I will take a pic before removing wires.

[ritter4242]

Hi! I'm doing an 81 transplant into a 78, and before I try and weld the plate in for the mileage counter, is it actually needed for anything more than turning that light on? Can I just not use the counter? (part number 911.641.225.02)

This is literally the last thing on my transplant before I kick it over (short of putting everything back where I got it from to get the this damn counter)

[ritter4242]

Hi! I'm doing an 81 transplant into a 78, and before I try and weld the plate in for the mileage counter, is it actually needed for anything more than turning that light on? Can I just not use the counter? (part number 911.641.225.02)

This is literally the last thing on my transplant before I kick it over (short of putting everything back where I got it from to get the this damn counter)

[ritter4242]

So I went ahead and assumed the part can be ignored for the engine swap, but I very got an extra wire behind the speedo that I can't identify, it's brown and black striped.

Well it won't let me upload a picture at the moment (I'm on my cell phone) but it comes out of the same sleeve as the other ground wires. Unfortunately I did not take the speedoout of this car.