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.
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