Update: I didn't ground the ground clip on the probe correctly.
Hi, I'm seeing something that confuses me and would really like to see if anyone can explain it. My knowledge of electronic is very basic. I recently bought a picoscope and I've used it to look at the signal on this device (see pics) which is the idle stabilizer valve from an 80's Bosch Motronic fuel injection system.
It has 3 pins - the middle one gets battery voltage, and then the computer grounds it via one of the other 2. If it grounds one side, the valve opens, allowing air to pass through. Ground the other side, it closes. I've also attached the circuit diagram of the driver and it looks straightforward enough.
Knowing all of the above, I expected to see a signal of 0 to battery voltage (around 14v with the engine running) with the probe tip on one of the 2 ground pins, and the probe's ground clip on chassis ground. I used 2 channels to watch both grounds at the same time.
But to my surprise, instead of 0 - 14v, what I got was -7 to +7 volts! Everything else was as I expected. Now, I don't think the cars electrical system has a negative supply. I double checked that I was using DC coupling on my scope, and I definitely was.
I'd like to understand why I see -7 to +7 instead of 0 to 14. I thought it might have to do with the inductance of the coil in the device, and/or back EMF when it starts turning. But that's only a vague idea for me - I don't really understand how it would happen.
Can anyone shed any light on it for me? Thanks!