Furnace cuts out on shore pwr/Run all day on batts

[TxLoser]

First off this is not a Jayco, but trailers are trailers and theirs some smart folks here.

Recently purchased a new FW. I have another thread on that. Having an odd issue. My Furnace(Suburban) will run all day on batteries alone. However, hooked up to shore power, it'll quit after about 20 minutes prior to reaching selected temp and not come back on without me cycling the heater at the thermostat. This is both on a home 20amp circuit, and campground 50amp. So just my chord and 50amp surge protector dog bone are common between the two scenarios. Furnace has been bench tested, taken apart and put back together. No issues according to the dealer, and I agree since it runs like it should on batteries.

The chord is new, my 50amp surge protector is 5 years old. Not the spendy EMS type. Just surge. Could this be something with my surge protector or should I look within the trailer electrical somewhere.

This isn't a huge deal as summer is approaching, but it's a head scratcher that's driving me nuts. Just trying to troubleshoot a bit on my own before surrendering it to a dealer for 3 weeks so they can tell me it works fine.

[JFlightRisk]

It sounds like your surge protector is hard wired in? If you could disconnect it and bypass the power directly from the cord to the FW, it will tell you if it's the culprit or not.

[TxLoser]

It’s just the cheap external dog bone style. I went back to the trailer and took that out of the mix. Same results. Electrical stuff baffles me and I’m at least semi intelligent.

School me a little here. On shore power, my understanding has always been the converter is power the fuse/cb panel. However if I had a converter issue the lights would flicker or other 12v stuff might shut down. Second question, would there be different grounding points depending on if the heater is running on shore power or batts?

I first thought maybe converter was getting hot since it’s parked right by a heating duct and the fan runs frequently. Thus dropping voltage or something. Stupid sideways photo. lol.

[RogerR]

Have you checked the output of the converter? It may be low voltage output and the furnace times out after a while. PS, My ground bar screws were never tightened at the factory. Random light flickers went away after doing that.

[craigav]

I haven't heard of reports where the furnace cuts out on shore power but not battery power, so that is a bit uncommon. However, I have some thoughts to share with how I would proceed to correct this issue.

First point is that the 12vdc wiring for every RV I have seen is that that the 12v system is directly connected to the battery and the 12v components in the RV will never lose power when connecting or disconnecting from the shore power. I caveat this with saying I am aware of the converter, the battery, and how it is all wired, so my point here is not to overly simplify the circuit but to say at a high level, the 12vdc power should never get interrupted if you have a working battery installed, even if the converter turns on and off repeatedly. So, with that I would discount the converter being the issue (I wouldn't completely discount the converter as I would first monitor the voltage to see if it is fluctuating on shore power, or outputting a very high voltage (14.6+), and if so then my thoughts would change).

Second point, what would change then if not the loss of power? What changes is the voltage on the 12v circuit, if you were to measure the voltage you would see that it is higher when the converter is powered on and lower when it is powered off (but should be somewhat steady).

Third, DC motors unlike AC motors (most all anyway), an AC motor decreases the current flow (amps) as the voltage increases (in other words requires more amps at a lower voltage). This causes more heat and can damage AC motors, and why you should never allow you A/C (air conditioner) to run on low voltage (<~106v). However, a DC motor increases in current (amps) when the voltage goes up (vs when the voltage goes down) this is why a common DC motor runs slower with lower voltages than it does with higher voltages.

To put the points together, I suspect the furnace motor is at the threshold of failure and when on battery power only, the voltage to the motor is a little lower and the resulting current is also a little lower. Inside the motor are brushes that are possibly on the edge (worn or dirty) of their ability to transfer the higher current (which occurs with higher voltage, such as when the converter is powered on via shore power). I wouldn't be surprised to find that at the higher voltage the motor momentarily looses connection via the brushes for maybe just a moment of time, but enough time for the control board to shut down the furnace.

I spoke about the motor itself and that is what I would likely replace, especially if I was to not do any testing. I want to add though that it is certainly possible that the issue could be with the control board or the relay to the motor (and some control boards are sensitive to rapid changes in voltage). I am not an RV Furnace repair person by any means, but if possible and if I had this issue, I would hope I could recreate (repeat) the problem in testing and if so I would attach my voltage meter to the motor's input wiring and see if the voltage dropped at the time of the problem, if so I would focus more on the control board (also, I thought I had read some RV furnaces have diagnostic lights on the control board that can report a problem, not sure, maybe only some of the newer ones).

In any case, I would attempt to recreate the problem while monitoring the voltage, if the voltage doesn't drop when the furnace shuts down then I would have more confidence that the issue is internal to the furnace motor. If I couldn't easily perform such a test or if the results were not clear, I would go ahead and replace the blower motor anyway as while I know what you describe could be caused by the control board or relay (likely on the board) I would suspect that the issue would be more likely to be with the motor itself but I can't say for certain it is the motor as again it could be with the control board, so I am saying only what I would replace first absent of testing that indicates the issue is not with the motor as I believe for what you are reporting would be more likely to be with the motor first, then the control board second. ~CA

[TxLoser]

Thanks for taking the time to write that up. Great info there. We're headed out tomorrow for a four day trip. I should have some down time at some point and I'll see if I can figure anything out with my meters. The heater is new along with the trailer so it shouldn't be dirty. However, the cage is bent at the top. Doesn't impede any function, but that doesn't mean it wasn't dropped at the factory or damaged during shipping. Could very well be the control board or the blower was damaged just slightly.

I think if I pull the external cover I may have access to the wires coming into the unit. The control board and blower would require me pulling the unit out, but that's pretty straight forward thanks to the external and internal access.

I suspect your correct that it is not the converter. Everything else electric in the trailer works as it should. Only other electric hiccup I've had was front landing legs that wouldn't extend the other day. Suspect the 80amp self resetting breaker there. I'd been installing snap pads and manually working the system. Works just fine now along with everything but the dang furnace.

I'm half considering paying a mobile tech to look at it on my dime, and then settling with GD after the fact. Even if I'm only reimbursed a portion at least it may be fixed.

Anyway, thanks again.

[TxLoser]

Finally got to the bottom of this. I decided to take the dealer out of the equation and take it to an independent RV repair shop. My thought was that if I'm paying someone, they will have some motivation to make it work.

Turned out to be both a bad board, and bad blower motor. The batteries vs shore power was just dumb luck. They found to board was bad and replaced that. The issue persisted, and then they found that the motor was pulling 12amps and climbing till it amped out and shut down the whole system. 10 is the max per the manufacturer. It'll cost me some money, but again worth having someone with a vested interest in things looking at it.

Unfortunately, since I live in Texas and we have some odd franchise law, GD can't reimburse for anything outside of a GD dealership. If I lived in another state or was in another state, they'd at least pay for parts. Still worth it.