Rear AC blowing 15A fuse; Fuse panel light lit

[StrongJava]

Fuse #8 in our fuse panel blows whenever I try to put in a new 15A fuse. The associated fuse light is on permanently. AFAIK, that circuit includes a handful of lights, the main AC and the propane gas detector. I removed bulbs, disconnected the propane gas detector and the AC/Furnace controller, but the fuse panel light remained on.

When I disconnect one of the 12V leads in the rear AC, the fuse panel light goes out.

My understanding is that would indicated the short is in the AC.

What can I troubleshoot next?

I hope this doesn't mean I need to buy a new RV...I mean, AC. No, I mean RV.

 

[Roller4tan]

I'd turn off the 120v breaker to the ac unit and check the wiring the junction box and control box in that ac unit.

 

[StrongJava]

Thanks. That is what I'm in the process of trying to do, but ended up with a little surprise:

I disconnected the 12V leads at the rear AC, and wanted to test the voltage coming to them. I put a new fuse in, but to test I realized I needed reconnect the propane gas alarm. When I reconnected the propane gas alarm, the fuse blew.

So now, I'm confused on the real cause of the circuit problem.
---

Additionally, I removed the 1A breaker from the propane alarm and put another 15A fuse in the panel only to still have it immediately blow.

Now I'm wondering if I had a problem with the panel?

Thoughts?

 

[Roller4tan]

disconnect the battery and remove the fuse panel to check the wiring. (my fuse panel is separate from the 120v breaker panel). If yours is integrated together, you'll need to take the steps necessary to insure no power to the panel. There must be a short somewhere as you've said.

 

[StrongJava]

Mine is separate. And of course the RV park where we are blew a transformer so we're relying on 12V for now. But, when that is restored, I'll do that.

What I've found since my last post is that I thought maybe it was a light switch (one of the decorative two bulb fixtures on the back wall) but with that disconnected I reconnected the propane gas alarm, and then check the light on the fuse panel; no light. Then I reconnected the 12V wires in the AC, and the light at the panel came back.

In this configuration, without a fuse installed, and the fuse panel light illuminated, the multimeter indicates an open circuit (OL) when the the black probe is on the left and the red probe is on the right; when I reverse them, -12.79 M Ohms is displayed. I wasn't expecting negative resistance.

Does that sound like the fuse panel itself?

Tim

 

[cookie]

Using your ohm meter across the left and right fuse terminals won't tell you much. One of those, we don't know which, provides12 volts and the other is a pathway to the circuit.
You will need to determine which side, left or right, provides the12 volts by setting your meter to dc volts then one probe to ground and the other to the terminal.
With everything on that circuit turned off, If that is possible, and having your meter set to ohms, there should be no pathway to ground on the non12 bolt side. 0 resistance.

Peace
Dave

 

[StrongJava]

The right side of the terminal shows 13.5xV and the left side between 11.97. I pulled a couple of other fuses (plus all the empty spots) and tested others with the same results.

Check of resistance showed 0L for every terminal tested.

 

[danemayer]

The LED at each fuse location allows a small amount of current to pass through when the fuse is blown/removed, as long as there's a ground path further down the circuit. So what you've determined is that there's power to the fuse location - which you already knew. Now you know the voltage, which is probably the output voltage of the Power Converter.

When checking continuity across 2 points in the fuse box, as Dave said, the reading isn't telling you much.

To get a continuity reading that's more meaningful, turn off the Power Converter's circuit breaker, along with the battery cutoff switch. That removes all power from the fuse box. The read ohms from the non-powered side of the fuse box, usually the LED side, to ground. If you have 0 ohms, something is shorted to ground - which is causing the fuse to blow.

If you can disconnect/turn off the downstream devices on that circuit, at some point the ohms reading will no longer be 0 and you will have located the part of the circuit that's shorted.

But it sounds like you've already narrowed that down to the 12V AC wiring.

Check above the A/C air return. There's probably a gray box with control circuits/relays that turn on the A/C and furnace. Take care you don't get shocked while standing on a step ladder.

 

[StrongJava]

I must of read your mind. Although I think we know I'm not that smart!

<strike-out>I just finished two more rounds of test. The first with the converter off but battery connected. Results were the same albeit about 1V lower for each side.

The second test was with both the converter off and the battery disconnected. Resistance remained at OL.</strike-out>

I forgot that I had something on the circuit still disconnected. When reconnected, and tested the resistance again, there is HIGH resistance on non-12V side of that circuit.

I believe, that confirms there is a short in one of the loads, correct?

 

[danemayer]

I forgot that I had something on the circuit still disconnected. When reconnected, and tested the resistance again, there is HIGH resistance on non-12V side of that circuit.

I believe, that confirms there is a short in one of the loads, correct?

If you put one lead on the non-12V side of the fuse position and the other lead on a ground, and read 0 Ohms, that would indicated that the other end of your circuit is grounded/shorted. Then when you disconnect that thing again, and re-take the reading, it should go to infinite resistance.

Those two readings would confirm the short, and which device has the short.

 

[StrongJava]

Update: I got a new AC thermostat/control group and installed them today. When wiring things up, I left off the two wire pair (12V) that goes to the rear wall of the coach. I also noticed when I hooked everything up but didn't have the fuse in yet, that the fuse panel fault light came on. I should add that before I hooked everything up and was trying to find which wire was the 12V supply at the AC, I put the voltmeter on it (without the fuse in) and there was ~9 1/2 to 10 volts coming across the wire. WTH?

I proceeded and the the fuse did not pop when I put it in. Tested AC and it ran, new thermostat works. Turned off the AC breaker and pulled the fuse, then added the wires for the rear wall back into the circuit. Fuse in/breaker on. Light on left rear wall works, but no apparent power now to the LP detector. Went to turn on the light on the right side of the rear wall and the fuse blew. WTH?

Pulled the bulbs from that light and tried another fuse. It worked. AC is on. Left rear wall light is on. LP Detector 1A fuse pulled and no bulbs in the other light.

This circuit used to work and I'm pretty confused now. I decided to trace what I could. The 12V wires in the AC are orange/white and white. The lamp on the right side is also orange/white and white, and appears to provide white/purple and white to the other lamp. But, the LP Detector is white/yellow and white. So, this tells me there is a junction/box somewhere hidden, no?

Any ideas on a good next step? I could replace the detector and try to find a new switch for the light, but I'm not sure any more, especially with as much voltage getting to the AC on the 12V line from (I assume) the fuse panel with the fuse removed?!? It seems I need to ensure correct voltages on all segments first?

Your thoughts?

 

[wdk450]

Update: I got a new AC thermostat/control group and installed them today. When wiring things up, I left off the two wire pair (12V) that goes to the rear wall of the coach. I also noticed when I hooked everything up but didn't have the fuse in yet, that the fuse panel fault light came on. I should add that before I hooked everything up and was trying to find which wire was the 12V supply at the AC, I put the voltmeter on it (without the fuse in) and there was ~9 1/2 to 10 volts coming across the wire. WTH?

I proceeded and the the fuse did not pop when I put it in. Tested AC and it ran, new thermostat works. Turned off the AC breaker and pulled the fuse, then added the wires for the rear wall back into the circuit. Fuse in/breaker on. Light on left rear wall works, but no apparent power now to the LP detector. Went to turn on the light on the right side of the rear wall and the fuse blew. WTH?

Pulled the bulbs from that light and tried another fuse. It worked. AC is on. Left rear wall light is on. LP Detector 1A fuse pulled and no bulbs in the other light.

This circuit used to work and I'm pretty confused now. I decided to trace what I could. The 12V wires in the AC are orange/white and white. The lamp on the right side is also orange/white and white, and appears to provide white/purple and white to the other lamp. But, the LP Detector is white/yellow and white. So, this tells me there is a junction/box somewhere hidden, no?

Any ideas on a good next step? I could replace the detector and try to find a new switch for the light, but I'm not sure any more, especially with as much voltage getting to the AC on the 12V line from (I assume) the fuse panel with the fuse removed?!? It seems I need to ensure correct voltages on all segments first?

Your thoughts?

Since DVM's are a high input impedance device, they can measure voltage when no appreciable current is available on the circuit (the measuring current for voltage readings is measured in microamperes). You are most likely measuring the small current voltage passed through the LED on the fusepanel when the fuse is not present. An old school analog needle meter uses a lot more current for measurement, and I bet that sort of meter will measure 0 volts under the same connections.

I believe your propane detector should have its own dedicated wires to the power source, bypassing the fuse panel because it is a gas safety device. The detector has its own fuse on its circuit board.

 

[StrongJava]

Thanks Bill.

I guess the only part that doesn't add up, and maybe I'll need to isolate loads in the circuit then, is why the fault light is lit on the fuse panel. I suspect I had wires connected on the light with the potentially bad switch.

That leaves me then with changing the switch in the light. Any idea on a source of those? Maybe some small electronics store...

Since DVM's are a high input impedance device, they can measure voltage when no appreciable current is available on the circuit (the measuring current for voltage readings is measured in microamperes). You are most likely measuring the small current voltage passed through the LED on the fusepanel when the fuse is not present. An old school analog needle meter uses a lot more current for measurement, and I bet that sort of meter will measure 0 volts under the same connections.

I believe your propane detector should have its own dedicated wires to the power source, bypassing the fuse panel because it is a gas safety device. The detector has its own fuse on its circuit board.

 

[Bob&Patty]

Tim, you might have a pinched wire at a J-box or wire harness under a slide. Happened to me....grounded B+ wire was only grounded with the slide OUT. Just a though.

 

[StrongJava]

I'll keep walking wires and doing tests.

To confirm though, if I pull a fuse at the panel and the LED goes bright, that means there is a completed circuit (i.e. a load) or a short in the wiring, correct? So, ideally if I have ensured everything is off, then none of the fuse panel lights should be illuminated if I pull the associated fuse.

Right now, if I pull each fuse, the following light: #3 (off-door side) and #8 (the rear AC).

Tim, you might have a pinched wire at a J-box or wire harness under a slide. Happened to me....grounded B+ wire was only grounded with the slide OUT. Just a though.

 

[wdk450]

I'll keep walking wires and doing tests.

To confirm though, if I pull a fuse at the panel and the LED goes bright, that means there is a completed circuit (i.e. a load) or a short in the wiring, correct? So, ideally if I have ensured everything is off, then none of the fuse panel lights should be illuminated if I pull the associated fuse.

Right now, if I pull each fuse, the following light: #3 (off-door side) and #8 (the rear AC).

The LED fuse indicators light when there is +12 volts DC on the supply side of the fuseholder, and ground (or near ground) on the load side of the fuseholder. A complete load circuit can cause this near ground condition at the fuse holder. UNDER NORMAL CONDITIONS THE INSERTED FUSE SHORTS CURRENT AROUND THE LED AND IT DOESN'T LIGHT. A BLOWN FUSE OPENS THIS LED BYPASS CURRENT PATH, THE LED GETS CURRENT, AND LIGHTS UP. What the LED does without a fuse present is just a confusion factor. If necessary, get a DC current meter and measure the load current across the fuseholder (with the load circuit turned ON). If you have a yet undiscovered wiring short, you can get small, resettable circuit breaker devices that fit into the fuseholder blade slots, which will save you on fuses until you find the final culprit.

I had an intermittent short in the 12 volt line to my refrigerator that I fought for over 6 months until I found a supply wire stretched tight around a sheet metal plate at the rear of my refrigerator, making an intermittent short with road vibrations.