Fuse keeps blowing when I plugin to 120V on 79 International

[roadtripryan]

I have a 78 ambassador international that is new to me. One of the 12v circuits works until I plug in to 120V, then the fuse blows. All other 12V circuit work fine.

I tried turning off all breakers, and plugging in to see if it was a particular circuit that was causing the problem, but it still blew as soon as I plugged in to 120.

Any ideas on what would cause one 12v circuit to have the fuse blow every time I plugin to 120? Not sure how to troubleshoot it. I assume a short somewhere where the 120 line must be touching this particular 12V circuit, but how the heck would I find it?!?!?

Thanks,
Ryan

[HowieE]

There should not be a connection between the 2 systems. The first thing I would do is remove the fuse that is failing, check the voltage on the input side of that fuse. If you have a battery on board it should be 12 volts DC. If you do not have a battery it should be zero. If that checks I would plug into 120 volts and again read the voltage on the input side of that removed fuse. If the trailer is wired correctly you should get the same results as before. If you get something else, AC or DC, let us know.

[roadtripryan]

Interesting. Both the circuit that keeps blowing the fuse, and the circuit next to it read the same, 13.65 when on 120V plugged in, and about 12.65 when on 12V only.

Maybe my theory is way off base? Any ideas on what plugging into 120V might change in the 12V circuitry?

[drboyd]

Something tells me your Univolt is the culprit. That's about the only thing the 12V and the 120V system have in common.

Are you still using the original Univolt? If you are, it's time to change to a decent charge controller.

[HowieE]

Those DC voltages seam to be inline. With the fuse pulled and plugged into shore power test the input side of the fuse for AC voltage. There should not be any but that will test the converter for leakage.

If that OK I think you have a high resistance near short downstream in that circuit. While not plugged into shore power turn on every thing in the trailer and then remove the fuse in question. What goes out will define what is on the circuit in question.

You will have to look at each fixture on that circuit looking for corrosion path between the hot lead and ground or any other path to ground.

[NavyCorpsman]

Find out what is on the downstream end of that circuit. If you are getting 12 - 14VDC on the supply side of the fuse, you more than likely have a dead short somewhere. Since it is new to you, look for non airstream installed things. A screw or rivet in the wrong place may be the problem. You can buy a signal tracer at Lowes or harbor freight that can help pinpoint the problem. Happy Hunting.

[roadtripryan]

Thanks for the info. Your posts helped me track it down. I found the bathroom fan switch was on, but no fan even on just battery, and the fuse was getting hot.

With the bathroom fan switch in the off position, the fuse works and does not blow even on shore power. I assume this tells me the fan is bad.

Does anyone know if this is a standard 6 1/4" ventline? And if I replace it, I assume I DON'T want to put an weight on the roof? Just lean over from the ladder? Anything difficult about the replacement?

Phew! I thought this might be a very difficult problem to hunt down.

[A W Warn]

You should be able to purchase a new motor. Look on page 14.
http://ventline.com/i/u/6148923/f/RV...alog_11-11.pdf