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Help with Testing New Wiring Harness without Letting the Smoke Out?

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I'm not sure what gauge wiring is used in a std harness; but I think it's around 14, which can handle 15A. Some/most circuits are protected by, IIRC, a single 10A fuse in the fuse block. Check each of the fused circuits one-at-a-time and see if any blow a fuse. The O/D circuit is protected by a 50A slow-blow fuse.

One way to test un-fused circuits--for instance, headlights and the 'cigar lighter' if you have one--would be to use a current-limiting power supply in place of the battery--which you know can supply mucho electrons--and activate one circuit at a time. Connect the PS to the hot and ground circuits in the boot, make sure all switches are open/off, set 12V and 10A. Then, power-up one circuit at a time and see if the alarm sounds. At 10A, the headlights would be dim but should show some light.

'Cheap' DC P/S (note I'm not familiar with any, this just came up with a search of 'usual suspects').
 
I swear by one of these, connect it to the battery and then probe any wire you want, you can press up to power a wire or down to ground it
It has a built in overload trip which would trip well before you let the majic smoke come out

circuit tester.png

Also ideal for checking for bad earths, if you suspect a bad earth, connect to the negative and earth direct back to the battery.
About 20 quid here in the UK. Just search for auto probe
 
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