Dometic HVAC expert help needed.

[jlyon69]

My '99 has the CCC and has been upgraded to the 5 Button CCC and control boards. Front AC Zone 1 handles furnace. Rear AC Zone 2 is just that....nothing else.

Last night I fired up the furnace (very cold outside) to do some volt readings on the rear AC. I was just looking for a good possible 12v source to power a 12v 2amp LED TV. The only 12v I found was at the 3amp fuse. At some point when I tested that circuit again, there was only 2.6 volts. At that point I went forward to look at the CCC and it was dead. I then went to the front AC and it only had 2.6 volts also. Both 3amps fuses were fine. I figured with 2.6v, I hadn't somehow blew that circuits main 12v fuse. I briefly checked the fuse panel (cold and dark by then) and no LEDS were lit (indicating a blown fuse) I will check this again when it warms up again, as well as test everything I can with the Dometic manual/trouble shooting guide in hand.

It doesn't seem likely I fried something just using a voltmeter. The only thing I can think of, is at a point before this cropped up, I tried to test voltage on the data cable. There wasn't much light and those connections are small. I'm thinking I might have shorted 2 of the lines which took something out at Zone 1. Of course that data cable was plugged in during most of my testing in Zone 2.

What ticks me off is, I knew the 12v system is powered via the data cables, but that never dawned on me before I started testing

I'm hoping someone with more experience can give me some useful info before I go out again and freeze my butt off for hours making unnecessary tests.

[RV Wizard]

There are 12 vdc circuits feeding both units control boards. Usually white wires are going to be battery grounds and accompanying colored wires will be the 12vdc positive conductors. It sounds like you may have lost a good ground connection in the ceiling assembly(s) or possibly at the main ground bar near or in the 12 vdc distribution panel. Find these conductors and check voltages and connections. Also check the T'stat for a fuse. I cannot remember if the 5 button CCC has a fuse in it or not. Let us know what you find.

[jlyon69]

I went out for a bit to start 2 elec heaters and did a few checks. Zone 2's box is grounded via the 120 line. If the board is grounded different, it's via the data cable. I used the box for ground on initial 12v test, no problems. I need to re-confirm this, but if I trip the 120v breaker for Zone 2, there is no volts on the DC circuit???? I do have an EMS, I just don't know if it uses 12v control or 120v control over the 4 120v circuits it manages. It doesn't however show any circuits turned off.

Zone 1 - same 2.6 volts. I have a 12v circuit there I cut off from an old solar panel. A test there shows 13+ volts using the box as ground. I've found 1 wire so far (out of 5-6 ) that enters this control box that I have tested. It has 13+ volts also using the box for ground. That's as far as I've gotten so far....going back out in a bit.

Zone 1 has an inbound 12v circuit that feeds 12v to the thermostat and Zone 2 via the data cable. I had a mouse once chew a data cable (between Zone 1 and the CCC) which also knocked the whole system out, just like this. My thoughts at this point are, whatever happened, it's affected something in Zone 1. When Zone 2 is disconnected, the CCC and Zone 1 still isn't working. I still need to do more checking on those inbound 12v wires. However, if there is one 12v circuit going into the box @ 13+v, then something is happening between there and the 3amp fuse. Which leads be to believe I may have zapped the control board? Oh, no fuse on the CCC.

[PandK]

Sounds like RV Wizard has the right track. I'll attach the installation directions just in case you don't have them. May help with the 12 volt connection points.
If I understood your post correctly you lost the 12 volt supply when you turned off the 120VAC supply. If that is correct you may want to check to see the hot and neutral wires are not reversed somewhere.

[jlyon69]

Quote:

Originally Posted by PandK Sounds like RV Wizard has the right track. I'll attach the installation directions just in case you don't have them. May help with the 12 volt connection points. If I understood your post correctly you lost the 12 volt supply when you turned off the 120VAC supply. If that is correct you may want to check to see the hot and neutral wires are not reversed somewhere.

Thanks for the link, I did already have a manual and trouble shooting guide.

I think Wiz is correct regarding a ground. I just got back in and here's where I'm at. Zone 1 (AC, Fan, Furnace) is now working normally as well as the CCC. I pulled a lot of connections for testing (Zone 1) including removing the control box as I couldn't test for DC at the data cables themselves. While testing for DC on the 2 data cable output jacks, I blew the 3amp fuse. Used the one from the rear and buttoned it up. Everything in Zone 1 checked out and was working.

The CCC did recognize Zone 2, even without the fuse. I put a 10amp fuse there just to test. At this point there was about 8-9v there. I switched the Z2 cable and the CCC cable at the Z1 box cable outlets. I thought I may have damaged one of them.... No difference.

Back at Z2, I pulled the fuse and it was jumping from 6v all the way to 13+ v back and forth continuously. With the AC and furnace now working again (relieved -heading to FL in a few weeks), I quit for the night and will tackle the suspected ground tomorrow. I do suspect it COULD be the data cable between Z1 and Z2. The documentation I have isn't clear on what is used for ground. Since the CCC has no local ground, it must go back to Z1 to use it's common or ground. So maybe, so does Z2???

I'll go to Radio Shed tomorrow to get the stuff to build a few new cables for testing and spares. I'm not an electrician nor haven't even played one in a movie, come to think of it, haven't been at a Holiday Inn Express in quite awhile either... So I'll take all the help and insight I can get!

[PandK]

Sounds like a generally good day. Page 10 of my attachment shows the 4 wire cable crimp connections. Figure you already know this but if you get the pins crimped on wrong it will be very aggravating.

[jlyon69]

Quote:

Originally Posted by PandK Sounds like a generally good day. Page 10 of my attachment shows the 4 wire cable crimp connections. Figure you already know this but if you get the pins crimped on wrong it will be very aggravating.

Thanks P. I'm going after a grounding problem at the box first. Failing that, i'll go to RS and get some testing cables to test that theory My main fear was needing to take to a repair place and being delayed going to FL. At least now we can go as is, if necessary....

[jlyon69]

Update: The box ground tested fine on the 120v connection...no variation in voltage....constant 119v. I was still getting about 9v on the 3amp fuse. Of course by now I was fairly confident the common for the 12v system uses the data cable.

I'll back up a bit here. I have always thought that the two RJ11 outlets on the control boxes were set up so 1 had to be for the CCC and the other to connect the 2 Zones. When working on Z1, I discovered it didn't make any difference.

I found an old 30' telephone extension cable and I had a few spare RJ11 plugs. I cut one of the old plugs off and install a new one. As info, these data cables however have to be made so one plug has to be reversed from the other. So I plugged in the new data cable and basically the same low voltage problem.

All time previous to this, I was only using the 1 data outlet. I then plugged the cable into the other outlet, boom, I had 12.6v, I removed the test cable and used the original cable....everything as it should be!

So it would seem 1 bad outlet in Z2. I have no idea what took out Z1. Had I been more methodical in my testing I may have known.

Panic does not help when faced with a problem I guess