Common Electrical Faults Diagnostic Simulator

Unwanted resistance in the Right Front circuit, on its ground side. Nothing is open — the lamp is lit, just barely — so this one is found entirely with drop tests. Source reads 5.00 V. At the Right Front load you get about 0.11 V where a good circuit gives 4.98 V, so the lamp is not getting its share. The power side drops a normal 0.02 V, which clears everything from the battery to the lamp. That leaves the ground side, and it reads about 4.87 V — nearly the whole supply being spent on the way back to ground.

The three add up: 0.11 at the load, 4.87 on the ground side, 0.02 on the power side, 5.00 V of source. They always add up. When one of them is far bigger than it should be, that is the part of the circuit that has turned into a load, and here it is the return path.

This is the fault that fools people, because the light still works. A circuit that is merely dim has nothing open in it to find with a continuity check — the only thing that shows it is a drop test taken while current is flowing. On the board itself SW1 switches 220 kΩ into Left Front.

The Left Front feed is broken between C2 and that circuit's feed test point. Source reads 5.00 V and the +5F rail reads 5.00 V, so the supply is fine. At the Left Front load you get nothing, its ground side reads nothing, and the whole 5 V shows up as a power-side drop — so the break is upstream of the lamp. Working down that side, C1 drops nothing and the +5F rail is live, which leaves the last stretch: red on the +5F rail and black on the Left Front feed reads the full supply, and that pair brackets the open.

On the board itself SW2 puts this same fault in Right Rear. Same readings, different light — which is the point of practising it here before you meet it there.

Right Rear is fed from somewhere upstream of the switch. With the switch on there is nothing to find: all four lights work, every drop test reads normally, and no amount of measuring will say otherwise — because with the switch closed the shorted circuit is getting the same 5 V as everyone else, just by a different road.

Turn the switch off. Three lights go out and Right Rear stays lit. Now the measurements separate: the +5F and +5R rails read 0 V, and so does the feed on every other circuit — but the Right Rear feed still reads about 4.98 V. Its power-side drop is 0.02 V when every healthy circuit shows the full 5 V. Something is connecting it to the supply ahead of the switch.

The lesson is that a short to power hides behind anything that is already on. It only shows up when you turn the circuit off and something refuses to go off with it — a brake light that stays lit, a fan that will not stop. On the board itself SW3 puts this fault in Left Rear.

The Left Rear circuit is shorted to ground, downstream of C6. Measuring alone will not find it: the eFuse has shut the board off, so every reading past the fuse is 0 V and all four lights are out. You have to divide the board up, and it takes two steps each time: pull one feed jumper — C2, C3, C6, C7 — then press RESET. The eFuse latches, so disconnecting the short does not bring the board back on its own; nothing changes until you ask it to try again. Pull C6, press RESET, and the fuse stays up: the other three lights come back on and the fault is in whatever you just disconnected. Pull any of the others and RESET trips it straight back off.

Pulling C8, the Left Rear return, does nothing — the 1 Ω path reaches ground on its own, so the short keeps conducting and RESET just trips the fuse again. That asymmetry is the lesson: to kill a short to ground you have to take away its power, not its ground. Pulling C1 or C10 does stop it, but only by killing a whole group, which tells you almost nothing.

On the board itself SW4 puts this fault in Right Front, where C3 is the jumper that isolates it.