Mobile Amateur Radio Power: Check the Fuseholders

The Yaesu FT-857 I have in the car has been not turning on lately, which I feared had something to do with being cooked inside a closed van for a week on the top level of a Camden parking garage during the hottest part of the summer.

But, no, as it turned out, that had nothing to do with it: when I got the radio on the workbench, it powered up just fine. Back in the car, it’s dead.

Which implies a power problem. The radio power comes from 10-AWG zip cord, through a pair of 40 A fuses, directly from the battery. The zip cord terminates in Anderson Powerpoles (of course) under the driver seat, mated to the end of the cable that came with the radio. That cable uses craptastic Molex connectors (equally of course) that are instantly suspect when any problems arise, plus a pair of smaller in-line 3AG glass fuses.

Voltage at the Molex connectors: anything from 4.8 V to 11.9 V, depending on imponderable factors. Voltage at the Powerpoles: ditto. So maybe it’s not the Molex connectors, after all.

The 40 A fuses are the kind the high-power automotive sound system folks use, complete with gratuitous goldish-plated everything. These I got surplus at a minute fraction of sticker price and mounted on the air filter housing, thusly:

Engine compartment fuses for radio power
Engine compartment fuses for radio power

I plugged a 12 V bulb in place of the radio, then went a-measuring. Voltage downstream of the hot fuse: 0 V. Tah-dah, it’s a bad fuse!

Nope, the fuse element is intact.

The zip cord terminates in ferrules penetrated by 1/8-inch setscrews. Applying a wrench, I find that the setscrews are somewhat loose, although nothing catastrophic. Tighten all four screws and the radio turns on just fine.

Case closed!

Until the next day, when the radio doesn’t turn on. Reinstall the lamp, re-measure, once again find 0 V downstream of the hot fuse.

Pull the fuse out again and it comes apart in my hand.

Defective 40A fuse
Defective 40A fuse

Huh. That would explain everything.

I suspect the fuse was marginally defective from the factory and finally failed after that prolonged heat wave. Living in the engine compartment isn’t easy under the best of circumstances, so I’ll give this one a pass.

Being that sort of bear, I plucked a spare fuse from the ziplock baggie of fuses & bulbs that’s tucked into the van’s jack compartment, popped it in place, and the radio works fine again.

Problem solved, for sure!

Side note: those fuseholder screws go through the air filter housing, into nuts with Loctite, and I ruined the threads to absolutely prevent the nuts from coming off. You really don’t want a nut loose inside the engine air intake, downstream of the air filter and upstream of the throttle…

6 thoughts on “Mobile Amateur Radio Power: Check the Fuseholders

  1. Bad contacts can heat the contacts of the fuse and de-solder itself. IMHO, glass fuses are not suitable for automobile applications. Vibration will eventually get them.

    1. Nothing else in the fuseholders looked even faintly wrong. Much to my surprise, the end seals hadn’t leaked any crud inside, either.

      The end of that fusible link looks like it just never bonded with the solder (?) nugget inside the end cap.

      At least the situation was nothing like that!

  2. I used the same kind of fuseholders for mine (I also got ’em cheap). I screwed ’em straight into the plastic cover over the wheel well with self-tapping sheet metal screws. I figured I’d do something more permanent “later”. After 299k miles, I sold the car, so I guess that hack was permanent enough. I found some of the AGU form factor fuses with LEDs in ’em to show if they blew, with fancy lightning-bolt style elements at Radio Snack. Worked fine.

    1. the plastic cover over the wheel well

      None o’ that here; solid metal as far as the eye can see. I didn’t want to poke holes through the coating, lest rust eat the panel alive, and this was the only substantial bit of plastic to be found.

      Not that it was the brightest decision I’ve ever made, but … the van will go to its grave with those nuts in place!

  3. I have seen that style of fuse fail before in *exactly* the same way. They are very low quality and belong on overpriced, poorly-engineered aftermarket audio systems, and that’s about it.

    I go to for all of my vehicle wiring needs – their rubberized insulated fuse holders take standard automotive fuses (or maxi fuses for the big suckers) and I have never seen a failure on the road.

    1. They are very low quality

      That’s sort of the impression I got: flashy, but crappy.

      Thanks for the link. The next time one fails (and it won’t be long, I’m sure), I’ll be on the case.

      Of course, it might fail hot, which would be exciting…

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