Speedometer Testing

[Woznaldo]

I'm trying to test my non-operational speedometer to sus out what and where the fault lies. I know it's a very simple system so there can't be too much wrong?

So far i've just tested the resistance between the red and white wire and have got either 200 or 400 ohms (wasn't my multimeter so couldn't be sure about scale).

The question I really need to the answer to is: If I jack up the front of the car, start the engine, put it in gear, should I be able to read a pulse between red and white wires? If so, how?

The last I need to know is whether the system is a variable reluctance (VR) or hall effect?

No doubt Scoff will know all of the above of the top of his head.:coffee:

[Woznaldo]

The 'setting up variable reluctance sensors' pdf has some useful info:

ihttp://www.link-electro-usa.com/tuning.html

[Scoff]

Yeah, its VR, hall is digital.

You can't test for a pulse with a multimeter, you need an oscilliscope to see it's waveform. frequency and amplitude will rise with rpm (speed).

Hard to test the clocks too, without rigging up a toothed wheel in a lathe or drill and mounting a known working sensor above it.

[michael tierney]

i think other people suggested wd 40 on the stepper motor in another thread a while ago!

[Woznaldo]

I thought I might be able to put a LED on the red and white wires and run the car in gear (on jacks) and get a pulsing flash with the LED flashing brighter and faster as wheel speed increased?

I'm guessing that the voltage probably won't get above 2 volts?

[RICHIE]

I thought I might be able to put a LED on the red and white wires and run the car in gear (on jacks) and get a pulsing flash with the LED flashing brighter and faster as wheel speed increased?

I'm guessing that the voltage probably won't get above 2 volts?

i could post you a known working gear box pickup mate as i dont need it anymore as i have the stack dash

[Scoff]

I thought I might be able to put a LED on the red and white wires and run the car in gear (on jacks) and get a pulsing flash with the LED flashing brighter and faster as wheel speed increased?

I'm guessing that the voltage probably won't get above 2 volts?


With the high impedance input of the tachometer the voltage will get very high, vr sensors will make more than 50v without a problem. not much current ofcourse, so attaching an LED will sink most of that. Not sure if you'd make enough current to light the LED at all. there's also something like 12 teeth on the diff that the sensor reads, so you won't see the flashing, it'll be too quick.

Better just to measure the resistance, if it looks ok then it's probably working. More likely the clocks have an issue.