# Testing an ABS sensor with a multimeter

1. Apr 25, 2017

### Guineafowl

As I understand it, there are two main types of ABS sensor - variable reluctance, two-wire and Hall effect, three-wire.

The sensor I wish to test is the former. My meter has a frequency and duty cycle button, but which range should I use - AC or DC? I plan to test at the sensor plug while spinning the wheel.

Another way to phrase it would be: Does a two-wire ABS sensor produce AC or pulsed DC?

2. Apr 25, 2017

### Averagesupernova

AC

3. Apr 25, 2017

### Guineafowl

Many thanks. So to confirm, I set the meter to AC volts, then frequency, then spin the wheel and watch.

4. Apr 25, 2017

### Guineafowl

.. and to follow up, I assume a three-wire, Hall effect sensor would need to be plugged in and back-probed to test. Would the output wire of this give a speed-dependent DC voltage?

5. Apr 25, 2017

### Averagesupernova

When testing for something it is generally the practice to expect some kind of result. Do you know the what to expect? Will measuring frequency give you some insight to something? A sensor of this type is typically going to generate a voltage or if defective it simply won't.

6. Apr 25, 2017

### Guineafowl

I'm not entirely sure, that's the problem. A defective ABS sensor (VR type) needs to a) give a signal and b) give a plausible, speed-dependent signal. So I really need to know if a VR sensor signals wheel speed by increasing in voltage or frequency. I suspect it's both.

7. Apr 25, 2017

### Averagesupernova

A 3 wire hall sensor is basically a switch to ground. One wire is ground, another wire receives a positive voltage, 5 volts for instance, and the third wire is a switch that opens and closes to ground as the metal moves past the sensor.

8. Apr 25, 2017

### Guineafowl

So these sensors should output a duty-cycle variant, pulsed DC to signal speed, correct?

9. Apr 25, 2017

### Averagesupernova

It is strictly frequency dependent assuming the voltage is high enough for the receiving circuits to process.

10. Apr 25, 2017

### Averagesupernova

No, just a square wave. No change in duty cycle as speed changes.

11. Apr 25, 2017

### Guineafowl

So DC frequency setting to watch these sensors, then.

12. Apr 25, 2017

### Averagesupernova

Both AC and DC. Just try some things. You won't hurt anything measuring these voltages. Just don't get your meter over on amps. There will be an AC component as well as DC with output of a hall sensor. However, you won't likely get a reading unless the signal wire is hooked the the circuits on the vehicle. The short answer is that this type of sensor needs a pull up resistor to generate a voltage. After all, a switch that opens and closes can't generate a voltage on its own.

13. Apr 25, 2017

### Averagesupernova

BTW, I don't think you can define a 'DC frequency'.

14. Apr 25, 2017

### Bandit127

Agreed. I think a 0 to 5V square wave is AC with a DC offset. Not DC that switches between 2 different levels.

15. Apr 25, 2017

### Guineafowl

I see - I think my terminology is a little off, then. My meter will set to DC frequency, but this must be what is meant.