I let a valet park my car. He left the lights on, killed the battery, and then when he offered to jump start me he reversed the cables. I realized what he had done because of the sparks and the smoke. After we got it all straightened out we did get my car started and aside from a burnt wire smell all that was wrong was a dead radio and cigarette lighter.

I located and pulled the fuse (same fuse for both devices) and sure enough it was fried. With keys out of the ignition, I put in a new 20 Amp fuse and as it made contact sparks flew and the fuse blew.

Since the ignition was off, but the fuse contacts were energized, I assume some wires must have melted together to bypass the ignition switch with a high amperage circuit.

How do I troubleshoot this?

Thanks in advance for any help.

-Steve

 

Using the removed fuse contacts connect a multimeter across the two contacts w/ resistance measurement set to lowest range.

Now start disconnecting components (radio, lighter, etc) out of that circuit (wiring diagram necessary). When you see circuit go to open, you have found the faulty component.

If not a component, then possible wiring melted and grounded somewhere, however this seems unlikely since fuse opened and provided protection. There is a fault location tool that will do this. Check youtube videos by briansmoble1. He describes use of this tool to locate a wiring short.

good luck

 

this is just one of the reasons i don't Valet my cars.

that's bad news man, a lot of electrical components do not like being wired backwards. a fuse will blow but sometimes not before damage is done (since fuses take a while). the problem usually doesn't affect the wiring since as much as it affects components internal to the electrical devices (such as capacitors, diodes, and especially IC's. )

i agree you will need to start disconnecting components from the circuit - such as unplugging everything connected then using the DMM to measure resistances to ground. disconnect the battery for any resistance measurements. note that you want to remove the fuse and measure resistance on the load side of the fuse (the side that is not energized when the fuse blows). that can be verified by making VDC measurements when the battery is still connected (touch the (+) DMM lead on the metal tabs on the outside of the fuse while it is inserted).

if you're not electrically savvy with a DMM you will either learn or fork out hundreds to someone who is. electrical diagnostic rates are usually $60-$100/hr.