I had a "hidden" fuse break happen to me in high school; been there/done that!
This sounds like a common problem, so it's time to go to market with my Fuse Tester. A fuse clip, momentary switch, and AC plug all connected in series. Install the suspect fuse into the clip. Plug the tester into a wall outlet and press the switch. If you see a blue flash the fuse *was* good! :)
Engineering trivia: AG (as in 3AG size) stands for "Automotive Glass"
Over my career (nearly 50 years), I've seen this dozens of times. Normal temperature cycling of fuses often causes a microscopic break in the tiny filament in the fuse. The take away lesson is a very general one: "Don't assume anything ... especially true with a visual inspection!". -- Bill Whitlock, analog circuit designer (and former drag-race car builder)
I can't remember where I heard this, but I've tried to impart it to my daughter: "A smart person learns from his mistakes, a wise person learns from the mistakes of others." You've given us a chance at wisdom.
Had a problem once when you put on the brakes, the headlights came on, and vice-versa. I scoured the wiring diagram looking for any possible path and could not see how it could be possible, short of a physical cross connect. I started checking for shorts in the bulb sockets and that's when I found an 1157 dual filament bulb that had a broken filament that was touching the adjacent filament and would readily provide enough current across the brake circuit to the running light/headlight circuit and itself would still illuminate. You don't normally suspect a working bulb so it missed initial inspection.
I remember fuses like that. A data aquisition system I used to use had its inputs protected with fuses that had possibly lower current ratings. Like yours, you couldn't really see the wire, but checking had its own issue - One problem quickly appeared, back in the days of the simson 260, you ran a good chance of blowing the fuse with the ohmmeter's battery. If you used a vtvm, that was ok, it had high enough impedance, that the fuse would survive, but there were some that still swore by their old friends.
We wound adding a fuse test fixture to the front of the thing -- it was just a holder with a 1k resistor in series, to keep the current down.
We had a product with a 1/10A fuse. There was no way to see the element without a magnifying glass, so a quick continuity test was the only way. When I ran the service department the first thing I always did was check the fuses and power traces for continuity, then pull the fuses to make certain that the customer hadn't replaced a 1/10A with a 20A.
It seems that the customer would always install a larger fuse until the wiring harness burned. As soon as the Magic Smoke was let out, though, they would send the unit in to have the smoke refilled.
Using wireless chips and accessories, engineers can now extract data from the unlikeliest of places -- pumps, motors, bridges, conveyors, refineries, cooling towers, parking garages, down-hole drills and just about anything else that can benefit from monitoring.
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