Any folks here understand how a Power Probe actually works?

I noticed something WEIRD when using a PP recently and wondered if anyone might be able to explain it...

I was checking a (VR) Crank Sensor circuit - car IGN on - with a Power Probe and noticed that when I touched either of the sensor wires, a (fuel pump I think) relay would click-on in the fusebox

It would stay-on for a while but when it clicked-off - touching EITHER wire would switch it back on...

I was using a PP III but I swapped to an Amazon cheapie probe and it did the same thing so it's not a peculiarity or a fault with the PP

This is in 'sense' mode - I'm not pressing the +/- button here

I'm guessing with my limited electronics knowledge that the PP passes a small current to the tip (so it can check for resistance and thus determine what it's touching) and then measures the overall voltage - e.g. it's a multimeter in resistance AND DC voltage mode at the same time??

I'm therefore further guessing that this is being picked-up by the ECU as a 'crank signal' and it's triggering the fuel pump???

Am I way-off-beam here - I'm really guessing???

Thanks!

I don't pretend to understand how the Power Probe actually works. :silly: But Chad was kind enough to take a measurement of the 'tip' voltage of his Power Probe with a Pico:



For reference, the large pulse in the middle is with the rocker switch on BEEEEEEEEEEP power.

I'm guessing that the smaller 2-ish volt pulses are probably what is being used to check continuity. I agree that the ECU is probably seeing those pulses on the VR crank sensor circuit and thinking the engine is turning. :lol:

That's super interesting - thanks for that!!

I realised I could test my own probe (super basic - no screen) on my desk scope but I only have a desk 12V power supply and it's too noisy (AC ripple) to see detail like that and I'm not keen standing in the street with a laptop attached to my car battery - people will ask me to fix THEIR cars

The 2V pulses would DEFINATELY look like a crank signal tho - the actual sensor puts-out a 3v AC wave so...

Thanks for that!