Dometic Duo-Therm A/C Troubleshooting help please!

Hi!  I joined the forums because I hoped you folks could help me out here before I plunk $600 down for a new A/C unit...

Here is what happened - Last night, we had lightning strike a tree about 50 feet from our travel trailer (2009 V-Cross Front Kitchen, 2 Slide outs, 32ft).  It blew up not only the tree, but also the transformer box that was about the same distance away from the tree as our TT.  They came later last night and replaced the transformer (after making our power flicker on and off a couple times - possible short??) and the A/C and Furnace would not function at all or make any noise while in auto-mode no matter whether the temperature slider was moved up and down.  When I turned the fan switch (Duotherm Analag thermostat) to "ON" and flipped between hi and low fan speed, the fan works fine.

What I've done so far: I've found the repair manual online for duotherm and checked the compressor, and the capacitor (I think I did it right, but I don't really know much about using multimeters... I did at least figure out how to check continuity), and the overload thing.  They all seemed to be normal, minus a black widow spider hiding in my capacitor area....  I pulled the thermostat off the wall and disconnected all the wires from it... Touching the green wire to the blue and orange makes the fan go hi and low (with both touching the green together), and touching the green and white makes the furnace/heater start up just fine.  However, touching the green and the yellow (A/C) doesn't do anything at all.  I haven't been able to get the red (7.5) wire do anything at all - what is that for?

So, I'm rather confused - I had read all over that if you can bypass the thermostat then the A/C should run... but it isn't... And given that the thermostat wouldn't operator the furnace but using the wires makes it work leads me to believe that the thermostat is bad.  I'm planning to run to walmart and get a new thermostat to try here shortly, but if you guys have any opinions or things I can check...  I have the control box taken apart, but I don't see anything wrong with it... and I don't know how to check it either.  I can upload pictures if that would help.  Can't find a model number on the A/C - the paper is all faded.

Thanks!

Edited to say, I've been researching all day and can't find much that seems to help...

I assume you have read the various write up on how you can install certain Hunter or Honeywell branded thermostats that have their own internal batteries in place of the Dometic Analog thermostat.  As to the red 7.5V wire, it provides power for the logic chip in the Dometic "Analog" thermostat, as it is really digital inside, not an old style mechanical thermostat, it is therefore not used on a Hunter or Honeywell thermostat with an internal battery.

I have read through those, yes.

So, update - I installed the Honeywell non-programmable thermostat from walmart.  Took a bit of playing with the wiring, but I now I the heater working as it should, the fan on switch working as it should (on high only, since its only one setting) and when I turn cool on, it turns the fan on... But still no cold air.  I found another guide for checking the control box, and everything I can figure out shows the box is still good, even if the lightning somehow fried the thermostat (how likely is that?)

Tomorrow, I'm going back up and rechecking the capacitor - I didn't disconnect it, and it reads like I should have. 

Hopefully then it is something repairable in the control circuit, or a bad capacitor on the compressor or sensor and not the compressor itself.  If it does turn out to repairable you might want to add a high / low fan switch to the thermostat.  Here is a photo of how I did it on a Honeywell Thermostat when I changed out my Dometic Analog thermostat last winter using a cheap toggle switch (seen on left of thermostat) with pre-wired pigtails off ebay (I think the price was something like 5 switches for $4 shipped), it was a simple matter of finding a place where the switch would fit inside the case drilling a mounting hole, and nibbling out a couple of tiny bits of plastic to route the wires.

I've found the best way to troubleshoot a problem is to break it down into easily testable blocks.

First, is the compressor motor getting voltage when the thermostat calls for cooling?  If it is, the problem is in the compressor.  If it isn't, the problem is in the control circuits,

Look in your repair manual for the compressor power pin on the plug going from the control box to the a/c unit.

Set your voltmeter to AC Volts and check for 120 volts between the compressor lead and common (blue and white wires in the manual I'm looking at).

If there's voltage there when the thermostat calls for cooling, the problem is in the compressor.  If there isn't voltage there, the problem is in the control circuits.

Lou Schneider said:

I've found the best way to troubleshoot a problem is to break it down into easily testable blocks.

First, is the compressor motor getting voltage when the thermostat calls for cooling?  If it is, the problem is in the compressor.  If it isn't, the problem is in the control circuits,

Look in your repair manual for the compressor power pin on the plug going from the control box to the a/c unit.

Set your voltmeter to AC Volts and check for 120 volts between the compressor lead and common (blue and white wires in the manual I'm looking at).

If there's voltage there when the thermostat calls for cooling, the problem is in the compressor.  If there isn't voltage there, the problem is in the control circuits.

Click to expand...

There is proper voltage coming out of all the pins from the control board leading to the A/C unit.  So I guess that means it has to be in the A/C unit, eh? 

Not so fast, have you tested the run / start capacitor for the compressor yet,  you probably have 2 run / start capacitors, one for the fan one for the compressor, and is it getting power, as it may also be the relay. 

Isaac-1 said:

Not so fast, have you tested the run / start capacitor for the compressor yet,  you probably have 2 run / start capacitors, one for the fan one for the compressor, and is it getting power, as it may also be the relay.

Click to expand...

So, I've pulled the only capacitor I can find out and tried to test it - but I'm still not sure how to determine results...

It's a large round capacitor, with 3 terminals marked C, HERM, FAN.  If I'm understanding correctly, this is a combination fan and compressor run capacitor, right?  I'll see if I can upload a picture of it.  I was trying to test the terminals with an old style multimeter with the needle - it says connecting the terminals together should make the needle go to 0 and then back to infinity?  The needle doesn't move no matter which terminals are paired together with the meter. 

However, I thought I also needed to have a compressor start capacitor?  I can't find another capacitor anywhere.

Thanks for all the help so far

here are pics

Short out the C and Herm terminals to discharge any stray voltage.  Then test it with the ohmmeter set to the 1000 ohms range.  This is the compressor capacitor.

If the meter doesn't deflect to zero and then fade back towards infinite resistance, the capacitor is bad.  Do the same test between the C and Fan terminals - since the fan is working this part of the capacitor should be working.

Dumb question, but the ohmmeter does go to zero (full scale) when you touch the test probes together, right?

You should also see continuity between the two terminals on the motor where the power enters, and infinite resistance from either terminal to the metal case.  Scrape off a little paint so you're making good contact with the case.  If you see continuity to the case you have a ground short in the motor and it's toast.

Ok, so I found out a friend of mine on the other side of the US is quite knowledgeable with electricity lol.  He walked me through testing the capacitor, and everything.  We've narrowed it down to the main power relay between 110 and the 110 wire that goes to the compressor.  So, time for a new board - I'm not playing with solder lol.

We figured a way to hotwire the power to the fan and the compressor, so it'll run for awhile off the breaker until I can get the new board ordered.

I am glad to hear that it is repairable

Gee. Not playing with solder..... I have two soldering irons (or is it 4) a GUn and a couple of gas powered devices.. I play with lots of solder.  Not hard to do. but then I've been doing it for fifty years.