Speedometer sensor?

Can anyone offer insight into exactly how they work? I think I've got an under-dash wiring error I need to sort out. Troubleshooting the speedo and oil pressure sensors is part of the job.
Warren mentions in one post "dry magnetic reed swicth contacts." Can someone corroborate this? If so I should be able ot put an ohmmeter between the wire from the sensor and a ground and watch the resistance switch between infinity to 0 as the wheels rotate. Fair test?

Been re-reading old posts and may have found the answer already: two wires from sensor, one to connect to speedo input, one for ground reference. I think they may be connected together to the same terminal.

Next step: Oil pressure gauge, may be the same problem.


Thanks for the help

[Formerly Steve Wilkinson]

I had endless problems getting the speedo to work in my '83 SC after I diassembled and restored it. This probably isn't your problem at all, but what finally turned out to be mine was that I'd replaced the sensor on the transmission case backward. I knew perfectly well that this was possible, so I carefully replaced the sensor with the "potted" side--which certainly seemed to me to be the "working" side--facing inward. That was the wrong way: the metal side should be against the transmission case.

Anyway, I also kept testing the sensors I was buying and replacing--since I assumed the sensor(s) was bad--with a multimeter, since somebody told me there was simply a coil in there and that if the sensor was good, the circuit should be complete. Well, I kept getting indications of no continuity, so I assumed the tiny coil was burned out or something. What I finally learned was that you have to put a magnet against the outside of the sensor to activate the reed switch, which then makes a complete circuit. Do that and if you get an indication of continuity, the sensor is good.

The two wires from the sensor to the connector inside the center tunnel of the car can go either way; there's no "proper" order of connection.

Test the wires from the connector in the tunnel to where they connect to the speedo and make sure you've got continuity there.

Test the power wire to the speedo to make sure you're getting 12 volts there.

We can go into this deeper, if you wish, but try those things first and let me know if this solves it. And be sure to make the connections on the back of the speedo correctly. You'll blow a diode if you don't.

Stephan

[Formerly Steve Wilkinson]

I had endless problems getting the speedo to work in my '83 SC after I diassembled and restored it. This probably isn't your problem at all, but what finally turned out to be mine was that I'd replaced the sensor on the transmission case backward. I knew perfectly well that this was possible, so I carefully replaced the sensor with the "potted" side--which certainly seemed to me to be the "working" side--facing inward. That was the wrong way: the metal side should be against the transmission case.

The way the sensor works is that magnets on the ring gear (I think it is) pass by the sensor, and every time one does so, it closes the sensor's reed switch. The rate at which they do so determines the speed reading.

Anyway, I also kept testing the sensors I was buying and replacing--since I assumed the sensor(s) was bad--with a multimeter, since somebody told me there was simply a coil in there and that if the sensor was good, the circuit should be complete. Well, I kept getting indications of no continuity, so I assumed the tiny coil was burned out or something. What I finally learned was that you have to put a magnet against the outside of the sensor to activate the reed switch, which then makes a complete circuit. Do that and if you get an indication of continuity, the sensor is good.

The two wires from the sensor to the connector inside the center tunnel of the car can go either way; there's no "proper" order of connection.

Test the wires from the connector in the tunnel to where they connect to the speedo and make sure you've got continuity there.

Test the power wire to the speedo to make sure you're getting 12 volts there.

We can go into this deeper, if you wish, but try those things first and let me know if this solves it. And be sure to make the connections on the back of the speedo correctly. You'll blow a diode if you don't.

Stephan

Fixed the Speedo. Had both wires from the sensor connected together. Splitting one of them off to ground cured it.

Tracked down the oil pressure gauge problem as well. That turned out to be a combination of connecting the sensor properly at the gauge and fixing the broken wire on the sending unit.

And how did that get broken, you might well ask... Turned out a shop rag that had been stowed under the oil filter so it would be handy for wiping the dipstick caught on the wire on its way to being SUCKED THROUGH THE FAN.
Looked like a whole tribe of mice had had a rag shredding party. I pulled off a couple of shroud pieces and picked rag pieces off the cylinder fins. What a mess.