Thermostat troubles

Minniecraig · 12-18-2005, 02:21 AM

I seem to have lost power to my thermostat. It's a digital Coleman Mach for heat and A/C only. I have checked the fuse on the thermostate and it has continuity and I read 10.2 volts across the fuse holder. Any one have any other thoughts? Funny thing is I just got it back from the dealer, had to get a new cab-over window (it leaked), and now this.

Minniecraig · 12-18-2005, 02:21 AM

I seem to have lost power to my thermostat. It's a digital Coleman Mach for heat and A/C only. I have checked the fuse on the thermostate and it has continuity and I read 10.2 volts across the fuse holder. Any one have any other thoughts? Funny thing is I just got it back from the dealer, had to get a new cab-over window (it leaked), and now this.

Tenn.Vol. · 12-18-2005, 07:48 AM

Lack of a GOOD ground???? TENN.VOL.

MrTransistor · 12-18-2005, 09:14 AM

Hi Minniecraig,
The fuse may show continuity but if you read 10.2 volts across the fuse holder, the fuse is bad (high resistance). Fuses when good should drop from several mille volts to a half a volt or so for very low current fuses, i.e. 50 mille Amp. I saw one High Voltage fuse that was rated at 5 mille Amp. that dropped about 10 volts, but at 1000 Volts that was insignificant.

Minniecraig · 12-18-2005, 09:59 AM

Let me preface this by saying I am not an electrician... I went back again today to recheck and found that when I read voltage with the fuse out it was 10.2 across the fuse holder but with the fuse in I got nothing? Is that correct? What is weird is that everything was working fine, ran the furnace for a week in Tenn. over Thanksgiving got home dropped off to dealer and know nothing. Oh Well.

tomsm · 12-19-2005, 01:59 AM

Craig,

You are correct. If you read voltage across the holder with the fuse out and no voltage with the fuse in, the fuse is good. Assuming your batteries are charged, you seem to be losing a couple of volts somewhere. Check for loose connections as suggested by Tenn. Vol. - good luck...

Minniecraig · 12-19-2005, 02:27 AM

Thanks... will keep on it.

Dsouthw524 · 12-19-2005, 10:03 AM

You did not mention if there was voltage at the thermostat. I suspect that you may have a grounding problem in the circuit. I believe that is why you are getting the voltage drop.

Minniecraig · 12-19-2005, 01:34 PM

I was getting the 10+ volts across the fuse holder on the thermostat itself.

Minniecraig · 12-22-2005, 05:42 AM

Here's an up date.

I pulled the thermostat off the wall, when I check voltage at the wire harness on the supply side I have 12 volts from the supply from the furnace and ground(according to Winn. Wiring Diagram), when I plug the thermostat harness into the supply harness AND the fuse is in the thermostat, the voltage on the red and blue wire at the thermostat (connects 12 volt supply and ground on the harness)the voltage drops to about 1.25. If I remove the fuse from the thermostat fuse holder I will get 12 volts again.

Any other thoughts?

tomsm · 12-22-2005, 07:34 AM

Craig,
It still sounds like a loose (high resistance) connection. It can read 12v with no load applied, but the slightest load (like the thermostat) can cause the voltage to drop to almost nothing. Very classic symptom of a bad connection. There is weak continuity across the bad connection, but it can't support current flow.

The problem with the meter is that it doesn't put a load on the circuit. You probably will need a 12v trouble light. If you don't have one, just solder a couple of wires to a 12v bulb. Touch the wires to the battery to see the normal brightness. Then try it across the fuse holder with the fuse out. If it's dim there, keep working your way back toward the battery.

Then again, it could just be elves stealing power for the sleigh

John_Canfield · 12-22-2005, 08:17 AM

Tom is 100% correct. (not so sure about the elves part...) I wonder if the thermostat cable is one piece back to the a/c unit. If it is, then I think (failing Tom's test) attention needs to diverted to the other end of it. Do you have basement air like the bigger coaches? What exactly is at the other end of the thermostat wiring? A control board maybe?

Minniecraig · 12-22-2005, 08:49 AM

I will get my test light out and try that. What are your thoughts on a bad ground?

I will set elve traps too, just incase!

3huskies · 12-22-2005, 09:02 AM

Here's a link for troubleshooting Coleman thermostats.
http://www.rvcomfort.com/pdf_documents/1976376.pdf
Or give their tech suport a call
(316) 832-4357

Minniecraig · 12-22-2005, 11:17 AM

Thanks 3huskies, looks like some bathroom reading tonight

.

3huskies · 12-22-2005, 11:33 AM

Hey, That's where I do my best work.

Minniecraig · 01-10-2006, 03:05 AM

Here is a follow up on what the problem was. It turns out that Winnebago had a group of wires running outside the coach to the furnace. The wires were in the black corrigated covering. This was on top of a support for my airbag fill valves. It seem that one of the furnace wires vibrated its way out of the slit in the plastic covering and rubbed itself on the support.

Of course I was just out of warrenty and it took the tech 2 hours to trace all the wires and splice the bad one together. THANKFULLY Winnebago agreed to extend the warrenty on this incident since I have had so many troubles in the past.

Just thought some would like to know... check where wires come in contact with frame members, thats the lesson I learned.