I was trying to measure pin 1 on my MAF sensor on my truck. It is supposed to read 2.2 to 2.7 K hertz. I put my setting to hertz. I put the red probe on pin 1 and the black probe on the negative terminal of the battery. It read 0. I removed the negative probe. It read 2.5 K hertz. I pushed the throttle up and down. The reading went up and down from 2.2 to 5 K hertz, showing my my sensor was working perfectly. What I don't get is why I had to use just the red probe? I was told you need the ground to the negative terminal. But this turned out not to be the case. Does anybody know why?

 

It could be the loading of the DMM could drop pk-pk input to comparator/frfeq cntr
and it stops counting.....guess on my part.

What is model # of DMM ?

Regards, Dana.

 

Are you sure you were probing the output? I had a look for images with Google and in spite of there being dozens and dozens, not a one of them that I found had a pic with wires with labels.

DMM input impedance is really high, so a tiny current is sufficient for it to read something on AC (frequency input would be AC coupled). There is probably enough capacitance to the truck body or just "something" to allow enough signal to be sensed with the black lead floating. It may be that the red wire isn't actually the output, but there is enough signal there to measure - unless you make a "good" connection to ground with the black lead and then the signal becomes millivolts riding on some high voltage that cause the meter to reject the millivolt signal.

In some of the work I used to do, I'd sometimes just hold the clip tip of an oscilloscope probe near something to pick up enough signal to give me some info about what was going on - no direct electrical connection of any sort to the circuit.

 

It is a Uni-T UT210D. Oh, yes I was probing output. The MAF sensor goes into the air intake duct right after the air filter. It connects to the car's ecu by a 5 pin connector. I backprobed pin 1, which is the air flow signal pin. And it worked great. Right at 2.5 K hertz at idle, responds to throttle very well. With the black lead floating. But the moment I ground the black probe to the battery negative terminal or to pin 2 it turns to 0. I thought the part was bad, was gonna return it. Until I realized what was going on. Here is the blog I used:

https://troubleshootmyvehicle.com/gm/3.5L/how-to-test-the-maf-sensor-1

 

This is a guess based on how multimeters work:

It's not uncommon for some DMMs to require a "zero crossing" to measure frequency, ie. the signal must alternate between positive and negative voltage. Your MAF signal is all positive, above the 0V chassis ground. Disconnecting the black lead, while leaving it near the car chassis, can capacitively couple only the varying part of the signal (often called the "AC component") into the meter, as noted by the post above. This effectively removes the DC offset and provides an "AC" signal to the DMM that alternates about 0V.

On some DMMs, it is possible to engage the Hz function while the meter is set to AC volts, which may work better in this case because AC voltage ranges are usually routed through a coupling capacitor inside the meter.