Sweeney
Administrator
Hey everyone, I had a service call that demonstrated a failure of a fuse that was very different. I've heard it from others, and read it in posts but in 20 years of messing around with cars and electronics, I've never seen it...until this week. I’m sharing it here in case it helps anyone troubleshooting a problem where an accessory doesn't work, yet there is voltage at the connectors - and you KNOW for a fact the device is 'good'
I got a call about an awning that wouldn’t extend or retract. When I checked, the power at the plug—12.8 volts, solid enough to run the motor was present. The symptoms pointed to a bad motor, so I tested it with my diagnostic tool (we call it the “coddi” around here) and confirmed it was shot. I swapped in a new motor, thinking I’d fixed it. But nope, the awning still wouldn’t move. Something was off. The 'new' motor did test fine when I sent voltage to it from a known good source. I started worrying about a broken wire hidden in the wall or under the RV. That is a worst case scenario as wiring is typically the first thing done when they build and RV - so everything is "on top" of it.
The customer mentioned they’d checked the fuse and saw a “faint glow” on the blown fuse indicator on their fuse panel. I didn’t see it myself, but it got me thinking. I tested the fuse with my multimeter—and it tested fine too. Voltage on both sides of the fuse, and, continuity. But something didn’t sit right. The awning motor needs a good amount of current (amps) to work, and even though the voltage looked fine, it wasn’t acting fine.
Here’s the deal: a fuse can sometimes show voltage on a multimeter but not carry enough current to power something like a motor. A multimeter barely draws any current, so it’ll happily show you voltage even if the fuse is weak. It’s like checking water pressure in a hose with a tiny pinhole—you’ll see pressure, but you won’t get enough flow to water your lawn. Voltage is the pressure; amps are the flow. You need both.
Look at this fuse. You'll see that the internals that should be horseshoe shaped look like they burned up, then fell over -- making an ever so slight contact....
I pulled the fuse, plugged in a new one and immediately everything started working just like it should
So, here’s my takeaway: don’t trust a voltage reading alone. If your device isn’t working but your multimeter says the voltage is good, check the fuse’s ability to carry current. A weak fuse can trick you by passing just enough juice to show voltage but not enough to run your gear.
Hope this helps someone out there chasing their electrical gremlins (or “Purple Monkeys” as we call ‘em)!
Pro-Tip: DOn't use cheap fuses off Amazon, look for Bussman or "littlefuse" -- available at most good auto parts stores. The cheap ones out of china are notoriously bad for not blowing reliably. I"ve seen 5 amp fuses not blow on a 20 amp load -- its not worth burning down your camper to save a few bucks on a safety item. $6 for 5 fuses seems like a lot but it really isn't. Thats a buck a piece for something to save $20000 or more worth of camper.
I got a call about an awning that wouldn’t extend or retract. When I checked, the power at the plug—12.8 volts, solid enough to run the motor was present. The symptoms pointed to a bad motor, so I tested it with my diagnostic tool (we call it the “coddi” around here) and confirmed it was shot. I swapped in a new motor, thinking I’d fixed it. But nope, the awning still wouldn’t move. Something was off. The 'new' motor did test fine when I sent voltage to it from a known good source. I started worrying about a broken wire hidden in the wall or under the RV. That is a worst case scenario as wiring is typically the first thing done when they build and RV - so everything is "on top" of it.
The customer mentioned they’d checked the fuse and saw a “faint glow” on the blown fuse indicator on their fuse panel. I didn’t see it myself, but it got me thinking. I tested the fuse with my multimeter—and it tested fine too. Voltage on both sides of the fuse, and, continuity. But something didn’t sit right. The awning motor needs a good amount of current (amps) to work, and even though the voltage looked fine, it wasn’t acting fine.
Here’s the deal: a fuse can sometimes show voltage on a multimeter but not carry enough current to power something like a motor. A multimeter barely draws any current, so it’ll happily show you voltage even if the fuse is weak. It’s like checking water pressure in a hose with a tiny pinhole—you’ll see pressure, but you won’t get enough flow to water your lawn. Voltage is the pressure; amps are the flow. You need both.
Look at this fuse. You'll see that the internals that should be horseshoe shaped look like they burned up, then fell over -- making an ever so slight contact....
I pulled the fuse, plugged in a new one and immediately everything started working just like it should
So, here’s my takeaway: don’t trust a voltage reading alone. If your device isn’t working but your multimeter says the voltage is good, check the fuse’s ability to carry current. A weak fuse can trick you by passing just enough juice to show voltage but not enough to run your gear.
Hope this helps someone out there chasing their electrical gremlins (or “Purple Monkeys” as we call ‘em)!
Pro-Tip: DOn't use cheap fuses off Amazon, look for Bussman or "littlefuse" -- available at most good auto parts stores. The cheap ones out of china are notoriously bad for not blowing reliably. I"ve seen 5 amp fuses not blow on a 20 amp load -- its not worth burning down your camper to save a few bucks on a safety item. $6 for 5 fuses seems like a lot but it really isn't. Thats a buck a piece for something to save $20000 or more worth of camper.
