Speed sensor, what type?

[ng_randolph]

There is a speed sensor tacked to the back of my speedometer (used by the 30k mile counter that turns on the Ox Sensor warning light). Does anyone know what type of sensor this is? I have seen hall effect transistors used for this, I know inductive pickups are used and some cars have just a reed switch.

I am going to use this signal for my navigation system, but it does not seem to be working. My last test was to connect the 30k mile counter with the cover off and going for a several mile drive while my brother kept a keen eye on the stepper motor in the mile counter. It did not move. The speed sensor should give a few thousand pulses pr. mile (the counter box says 3200 / mile, right in line with other cars I have seen) and the electronic circuit in the mile counter divides this by 32. Thus I would expect to see the stepper motor move about 100 times / mile. Not so.

Prior to that I measured the voltage coming out of sensor with the counter box connected. Steady 0V as I drive slowly. I tried measuring resistance across the sensor with the counter box disconnected. Again, no change as I drive along (measures open circuit).

I have taken out the cluster and will make some more measurements, but if anyone knows what type of sensor I am dealing with, it would be most helpful.

 

[///Mike]

What year car do you have? I've never seen a counter on the back of the speedo head that was used for the O2 sensor light or anything else. At least until '86 the O2 sensor counter was under the center console, driven by the speedometer cable.

///Mike

 

[ng_randolph]

'85, US spec.

The counter is indeed behind the center console, but it is not driven by the speedometer cable. It is driven by an electrical signal picked up at the back of the speedometer. See

The white plug at the back of the speedometer with one BLK and one YLW/RED wire coming out is the electrical pickup. The YLW/RED wire goes to the counter.

 

[///Mike]

Ooops

Come to think of it, I don't recall a speedo cable going to the O2 sensor counter on the '86 I was into recently. Must be some other car I'm thinking about. They say the memory is the second thing to go. ;-)

Hope you figure this one out. Would be good info to have.

///Mike

 

[ng_randolph]

Reed switch

Turning the speedometer input drive by hand, I could hear the distinct sound of a reed switch opening and closing. Not a very good one, it turns out. I opened up the speedometer and took out the little PCB with the reed switch on it.

With the ohmmeter hooked up, I moved the reed switch up to the speedometer magnet and heard it close. The on resistance was consistently in the hundreds of kOhm range as I kept repeating the test. Not very good when it is working against a 5.1k pull-up resistor in the mile counter box.

I'll either be lazy and replace the reed switch with a new one, or I'll put some more effort into it and use a hall effect sensor.

As to the speedometer cable driving the counter, that is how the '87 VW Golf was arranged. There was a mechanical 30k mile counter mounted on the fire wall (engine side), speedometer cable from the transmission to he counter, then a stub cable from the counter to the speedometer.