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Scanner Danner had one like this, it was over 24V on the load side of the blown fuse.
How is this possible?
Actually quite easy to understand when you look at the circuits.
It's common knowledge that interrupting the current flow to an inductor (coil, relay, solenoid, injector etc) results in a back EMF voltage spike.
This can be suppressed with a resistor across the inductor, or a diode (plain or zener) returning the current to positive supply.
Now due to circuit design, the diodes may connect to a different fused feed than the solenoids themselves.
In the case of many solenoids, they are driven PWM (duty cycle) so are on and off many times a second.
So that back EMF would normally be returning to battery but with that fuse blown it will actually charge a capacitor up inside the ECM/PCM to a voltage higher than battery voltage if there's no load on it.
The ECM/PCM is actually powering itself via a ground side switched load that isn't energised, and another protection diode.
If a incandescent test lamp was used instead of the LED, I bet it would not have lit. LED's will turn on with extremely low current. Probably just normal leakage from what ever that fuse was for. LED testers are horrible for checking modern auto electrical systems IMO.
I've always been partial to actually continuity-testing the fuses with a beeping multimeter, with the key off. I know this only works if the capacitors are drained and the fuses aren't battery-circuit, but at times it soothes my obsessiveness.
Maybe I ought to just read power probe volts, but load-test with a lamp when necessary?
Nice meter. I have the same one.
Love your content and I’m hoping you can help. I’m diagnosing my ‘12 Range Rover Autobiography for a lean condition on both banks. No CEL, just LTFT between 12 and 25 on both banks and STFT compensation most of the time. Both level out to stoich at 1500 rpm. No vacuum leak (smoke tested and propane introduced) air/fuel sensors okay, fuel pressure good. Question is, I have two MAF’s bank 1 is reporting -30ish volts (bank 2 is normal) unplugged bank 1 MAF and my scan tool (Autel 906bt) and I get -32 volts (bank 2 is zero unplugged). I have 12v, 5v ref, ground, temp ref, and temperature ground (checked with MAF in place and unplugged) This is also exactly the situation with my MAPT. My IAT is reading 107F (cold start/ambient temperature 78) all of which leads me to believe my lean condition is caused by a failing MAF. So is my scan tool giving me fake news? Should I disregard the scan tool data and move along with the diag?
Sorry just read Simon's pinned reply. That makes sense. So that circuit and all other circuits on that leg were loaded at the time? Do you think CD lights would have the same effect if they were partially active? Would they maybe cause a charge and discharge effect on the voltmeter? Or are they normally isolated until triggered. What were the trouble codes triggered?
Excellent, Excellent case study. Love to know more!
All the best.
Actually saw this a few years ago and put it out of my mind. What I could not understand was where the extra potential difference came from (3v approx). Was it inductors or loaded capacitors or was it a raised earth PD from a module on that circuit? I think there would be a simple answer but, I scratched my head so much my hair fell out! Excellent study of what can be a real confidence killer to electrical troubleshooting techs.
All the best and do give me a follow up or reply to satisfy my curiosity!?
What were the codes?
Will you be able to record the training and posted on your channel?