I wanted to ask a general electronics testing question. I was testing an Analog Variable Reluctance Speed Sensor, two wire [a magnet and coil with two wires: one positive and one negative], located in the rear axle of a vehicle. It is a passive type of sensor, which does not require any external power source; no power goes to the sensor. The power/signal is generated from a shutter wheel in the transfer case in the rear axle. I wanted to see if there was a constant uninterrupted output signal going to the speedometer. I did the test twice, using a digital volt meter (but I do not have an oscilliliscope to look at the signal; or even an analog meter). I set the meter on auto. On both tests I put the positive lead from the meter to the positive wire on the sender.

On the first test I placed the negative ground lead from the meter to the negative wire coming out of the sender. The reading was 000.3 to 000.7 MV.

On the second test I placed the negative ground lead from the meter to a ground point on the vehicles chassis. The reading was much higher, a 2.0+ [I assume it was MV but might have been V] and fluctuating.

Is it normal to get two different voltage readings from the way I did the test?


thanks,
atoz

 

Let's see if I'm remembering my electrical stuff correctly, someone chime in if I'm wrong...

The first test you did was a voltage drop test. By placing the ground probe on the load's ground side, you are testing how much voltage the load is using, which was .003 to .007

The second test, by grounding the probe to the chassis, was showing you how much voltage was available in that circuit up to the point where you placed the positive probe.

 

But what was you conclusion? Is it normal to get two different voltage readings from the way I did the test?