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I've come across some odd things in my years as a heavy diesel mechanic in the realm of electrical. Thought it may be interesting to have a place for all the brains to come together to solve non-srt4 related electrical issues.

Here are 3 of my top WTF moments:

1. Garbage truck came back from dealer after a $70k engine rebuild (full long, block, new turbo, egr valve, radiator, PTO shaft, CAC, and aftertreatment stack) and was having starting issues. Put a jump box on it and it'd immediately fire with 20 amps of help. Repeat this over the course of 2-3 days.

We got the truck in the shop, started checking voltage loss, amp drops, from the battery to the starter solenoid. Everything seemed fine. I told the mechanic working on it to check his ground stud. Hooked his multimeter to the battery ground, hot lead to the starter ground. Full battery voltage.....wait, wut?

Started checking battery cables. The dealer had hots hooked to grounds, hots hooked to the frame, alternator to battery cable went from B+ to battery ground. We put the hot lead from the DMM onto the frame and it had 12.6V. Everything had power. The truck ran flawlessly, other than it needed a tiny boost to start.

Any ideas why this would work at all? No blown fuses, no fires, no smoking, it just ran, perfectly.

2. Driver reported lights not working on truck, more specifically, all the rear body lights (another garbage truck). After hours of messing with the the tech accientally touched the hot wire of the circuit for the marker lights to a ground buss. Lights came on. Tech proceeds (i wasn't aware of this) to put the hot back on the ground, lights came on. Didn't blow a fuse, again...just worked. So his solution was an inline fuse between the hot post of the buss bar and the ground buss bar. It blew a fuse. He touches them without the fuse, lights work. He ended up putting a 40 amp fuse (found this week during other non-related repairs) in the fuse holder and all the lights have been working. Apparently his did this last year, and it's just worked, ever since. Removed the fuse because hot -> GOOD fuse -> ground??? Lights quit working. ???????

3. Box trailer (53 ft, aluminum) - Marker light out. 1 single marker light. Break out the DMM, 50 ft test lead back to the battery, verify battery voltage to ensure I've got a good ground for testing. Pop the light cover, pull the bulb, bulb is good. Test for power on the prongs of the base. 13.1v, 0.0v. Good source, good ground. Replace bulb (maybe my eyes aren't working). No luck. tighten prongs to ensure good connection. Recheck voltage, re-install new bulb. Still out. Verify good ground to trailer circuit. Truck has a ground strap from frame to 5th wheel. Ground good. Recheck voltage and ground..I've got to be crazy. 13.1 hot, 0.0 ground. Pull light base, ground connector in tact, scuff ground screw area w/ emery cloth. Reinstall, Check continuity from ground prong to trailer. Good connection. Check voltage, 13.1. 0.0 ground...install ANOTHER new bulb (maybe I damaged it?) No luck.

Trailer left shop with 1 marker light still out and I felt like an idiot. Luckily, so did 3 other techs because they all checked it after I was so puzzled. The light is wired IN SERIES! Lights behind it work with the bulb in, nothing with the bulb out, as they should. Did I mention the voltage on the prongs was correct when the bulb was in. This was mid way down the trailer, so parasitic drain wasn't the cause. WHAT THE ACTUAL F***?

Anyone care to help me wrap my head around these?
 
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