I wanted to ask a general electronics testing question. I was testing a two wire magnetic speedometer transmitter sender [a magnet with two wires, one positive and one negative], located in the rear axle of a vehicle. 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 (didnt have an analog available). 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+ and fluctuating. If there was no problem would I get the same output reading regardless of the ground I chose?