M33 AC blowing 15 amp fuse - RESOLVED

[GK2018]

I have a Mallard M33 and it blew the 15 amp fuse for the main AC unit, I shut down the AC and DC power replaced the 15 amp fuse turned it back on and boom fuse blown again. It leads me to believe there is a short somewhere and so I was wondering if anyone has or knows where to get a wiring diagram from or any suggestions as to where to look to resolve the problem? I havent noticed any loose wires on the T-stat or the breaker/fuse box and looks to be in good order so it must be elsewhere i'm assuming. Everything else works like a charm just not the main AC unit. Thanks!

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[GK2018]

Re: M33 AC blowing 15 amp fuse

Update: it only blows the fuse when I flip the breaker to turn on the converter and it still blows the fuse even if the 12V battery is unhooked and it is only running on shore power.

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[danemayer]

Re: M33 AC blowing 15 amp fuse

The A/C units run off 120V AC (shore power or generator). But there's some 12V interaction. The thermostat uses either 5V or 12V DC to signal the A/C unit to start/stop. On many trailers, there's a relay box above the air return where a 12V DC relay turns on the 120V AC power to the A/C unit. So you may have a problem there. Another possibility is that the thermostat wires go to a board on the A/C unit that is accessed from the roof, by removed the shroud.

If you get up on the roof, proceed carefully. Wasps often build nests under the shroud. Given that most people will jump back when encountering wasps, that can get serious when on the roof.

 

[GK2018]

Re: M33 AC blowing 15 amp fuse

The A/C units run off 120V AC (shore power or generator). But there's some 12V interaction. The thermostat uses either 5V or 12V DC to signal the A/C unit to start/stop. On many trailers, there's a relay box above the air return where a 12V DC relay turns on the 120V AC power to the A/C unit. So you may have a problem there. Another possibility is that the thermostat wires go to a board on the A/C unit that is accessed from the roof, by removed the shroud.

If you get up on the roof, proceed carefully. Wasps often build nests under the shroud. Given that most people will jump back when encountering wasps, that can get serious when on the roof.

That makes sense but would those two potential problems you mentioned cause the 15 amp fuse to blow when the converter is powered on? The T-stat won't power on because that fuse keeps blowing. I just dropped the underbelly to inspect the wires under there and all seems to be good there as well. I inspected them all although I'm not 100% sure which wire in the underbelly if any runs to the breaker/fuse box even though one does look to be the same gauge and color as the one in the box and it does run to the bus bar that is attached to the front of the frame and runs to the battery so that confirms its a DC wire on that bus bar as well as the one near the blowing fuse being DC just not sure if that is in fact the same wire on both ends or even if that is where the issue resides.

I'll still check out the two you mentioned and in the meantime do you know where the converter would be located in this camper? I presume it would be behind the breaker/fuse box? Thanks!!

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[GK2018]

Re: M33 AC blowing 15 amp fuse

The thermostat wires run to where my finger is pointing and I dont see a relay unless it is housed inside the grey box that they plug into.

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[GK2018]

Re: M33 AC blowing 15 amp fuse

I unplugged everything on the main AC excluding the romex and it still popped the fuse.

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[danemayer]

Re: M33 AC blowing 15 amp fuse

The thermostat wires run to where my finger is pointing and I dont see a relay unless it is housed inside the grey box that they plug into.

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The relay that sends 120V AC to the A/C is inside that box and is operated by 12V DC. Good chance the problem is inside.

You probably have to be on a chair or stepladder to get inside. Don't get shocked while on a ladder.

 

[GK2018]

Re: M33 AC blowing 15 amp fuse

The relay that sends 120V AC to the A/C is inside that box and is operated by 12V DC. Good chance the problem is inside.

You probably have to be on a chair or stepladder to get inside. Don't get shocked while on a ladder.

Gotcha, I'll disassemble that box in about 10 minutes after I take a break. I was just about to pull the wire from the breaker box and see where it goes and what shape its in but disassembling that box will be much easier so I'll start there. Thank you!

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[danemayer]

Re: M33 AC blowing 15 amp fuse

Gotcha, I'll disassemble that box in about 10 minutes after I take a break. I was just about to pull the wire from the breaker box and see where it goes and what shape its in but disassembling that box will be much easier so I'll start there. Thank you!

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Disassemble should equal taking the cover off.

 

[GK2018]

Re: M33 AC blowing 15 amp fuse

Disassemble should equal taking the cover off.

The relay i'm pointing at is the one your referencing?

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[danemayer]

Re: M33 AC blowing 15 amp fuse

The relay i'm pointing at is the one your referencing?

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Not sure I remember - it's probably been 8 years since I've had that box open.

 

[GK2018]

Re: M33 AC blowing 15 amp fuse

Not sure I remember - it's probably been 8 years since I've had that box open.

Instead of testing the relay since I have to find my multimeter I unplugged everything in that box and turned shore power back on and it still blew the fuse so I think that would rule that out as the problem area. I think it's down to tracing that wire from the breaker/fuse box and seeing where it goes and what connects to it and how all those wires look as well. What are your thoughts? I've currently blown about 25 fuses in my failed attempts at troubleshooting the source the issue. I only have 48 fuses left haha

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[danemayer]

Re: M33 AC blowing 15 amp fuse

I'm pretty sure the 12V DC goes from the fuse up to that box. So if the fuse blows with the wires all unplugged from the ceiling box, it would argue that the wire is shorted somewhere. It's not unheard of for a brad or screw to hit a wire. Next step might be to pull the fuse box away from the wall and look at the back side, and the wire in question.

 

[GK2018]

Re: M33 AC blowing 15 amp fuse

I'm pretty sure the 12V DC goes from the fuse up to that box. So if the fuse blows with the wires all unplugged from the ceiling box, it would argue that the wire is shorted somewhere. It's not unheard of for a brad or screw to hit a wire. Next step might be to pull the fuse box away from the wall and look at the back side, and the wire in question.

That's what I was leaning towards as well, I took the breaker box apart and can see the inverter and behind the breaker box and everything looks good so it must be somewhere along the lines of that wire. I unhooked the T-stat as well even though I really didnt think it was that. I even tried putting the wire in a different spot on the fuse panel in hope maybe it was a burnt post. I'm about to unhook all the short stops on that bus bar and see if I blow a fuse before I completely pull that entire wire out of the breaker box and wherever else it may lead. I've been on this non stop since 1200 today and these temps out here are 104 so I'm really hoping to get it fixed before bed tonight.

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[GK2018]

Re: M33 AC blowing 15 amp fuse

Problem solved! So the problem was obviously my fault. There was one screw I just had to put into the underbelly to make it "more secure" and there was one wire I didnt secure up with the rest and I figured aww itll be ok they were all hanging anyways and its only one wire. WELL with that logic I managed to screw ONE screw into that ONE wire costing me 8 hrs of painstaking labor in 104 degree heat to find and fix! I bet I couldnt do that twice and I sure dont want to find out either! Thank you so much Dane for the input because I learned about that relay and how that wire runs so I guess not all is lost, I gained knowledge haha!

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[danemayer]

Re: M33 AC blowing 15 amp fuse

Glad you solved it.

 

[GK2018]

Re: M33 AC blowing 15 amp fuse

Glad you solved it.

Thank you! So off topic a bit but were you a service guy for campers? You always seem to have so much knowledge that I'd love to somehow tap into.

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[danemayer]

Re: M33 AC blowing 15 amp fuse

Thank you! So off topic a bit but were you a service guy for campers? You always seem to have so much knowledge that I'd love to somehow tap into.

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Nope. Mostly just read the forum and take in what others have shared, combined with having to fix a few things myself.

 

[GK2018]

Re: M33 AC blowing 15 amp fuse

Nope. Mostly just read the forum and take in what others have shared, combined with having to fix a few things myself.

Wow, you must definitely absorb and retain the knowledge then! Well if you're ever in Oklahoma one of these days hit me up and we can grab a cup of coffee and dinner and maybe you can teach me a thing or two!

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[danemayer]

I should probably also mention that Heartland Senior Management, in support of this forum and owners, has given me face-to-face access to Sales Management including several General Managers, as well as Manufacturing Managers, Landmark and Bighorn Engineers, the Plant 2 senior line tech, VP of Quality and his staff, and the Director of Engineering. All of those people are unbelievably busy, yet gave me their undivided attention to share information that helps owners on this forum.

They've also given me free, unsupervised run of the Plant 2 (LM, BH, BC) and Plant 69 (Toy Hauler) manufacturing floors with product running through at what seems like 100 MPH so I could take pictures and better understand how they put these rigs together.