Dometic AC and aqua hot heat problem

[Adam Hicklin]

Hoping for some insight.  Coach sat for about 6 months.  Went to fire up the AC and couldn’t change the zone (zone indicator was not visible) on the Dometic 4 button thermostat.  Other indicators were visible and could be cycled through but AC or heat would not come on.  4 button tstat is problematic so sent it out to be repaired.  Repair tech said when he hooked it up, it seemed to work fine, although he couldn’t figure out why it had 3 zones with a 2 AC system.  Explained the Aqua Hot heating but he didn’t quite understand. He changed the buttons anyway.  Got it back and now it will run zone 1 both heat and AC but the zone indicator still is not visible and it won’t cycle through the other zones.  I’ve checked all fuses although there must be another board for Zone 3 (heat only zone) somewhere.  I can’t figure out how and where that is connected to tstat. Front AC has 2 communication wires hooked in the ports, rear has one, and of course there is the wire that hooks up the tstat.  Aqua Hot is working on the electric side, but not diesel. Diesel side is completely dead.  No humming, buzzing or burner fan  activation. Completely dead.  When the coach was parked, all was working perfectly.  Can’t help but think it’s all related somehow.  Thanks in advance. 

[Steve Huber]

Adam,
If you haven't, suggest you remove the t'stats and clean the phone wire connections with contact cleaner as the connections can build up resistance if unused for a long period of time. I'd also do the same to the control switch at the bottom of the stat. From what you said, I assume the tech replaced the remaining switches so they should be OK. The AQH issue also sounds like a possible bad connection since the electric side is working but the diesel side is dead. The normal issue with the diesel side not running after a long storage period is fuel gumming up in the fuel filter. However, the fan will normally start running until the control board  senses a no fire situation. I would start by checking the AQH control board to see if there is a fault condition indicator lit or if you are getting a start signal from the diesel switch in the coach interior. I don't know what model AQH you have but access to the service manuals for discontinued and current models is located in the HVAC section of Coach Assist on the BAC website. Access it by clicking the www.beaveramb.org at the top of the page. Then log in on the club website, select Technical, then Coach Assist.
Steve

[Joel Ashley]

Our two 5 button Duo-therms have zone buttons, but each thermostat is in the zone it controls, front and bedroom, so the zone button is functionally of no use that I’ve found.  I can’t use the front tstat to run the bedroom AC or exchanger, and vice versa. 

A 3rd zone represents the basement heater, which has its own automatic thermostat that kicks in if the hydronic heat is on and bay temps drop to around 40F.  I’d surmise that a 3rd zone shown on a living area stat could also be there for long coaches that may optionally have a third roof AC. Regardless, I’ve never been sure why the “zone” button is on coaches with multiple tstats, except that it’s simply a standard tstat used regardless of the number of appliances controlled.

But as to Steve’s query, did you check for fault lamps on the AHot control circuit board in the bay, prior to sending the tstat for service?  Usually a low battery, low coolant, or other malady is cause for the system to not even start the firing process, and a fault lamp will light on the board accordingly.  I fret that your tstat remake may have been unnecessary, esp. if the tech was not aware the device can be used in three AC/three exchanger area rigs; and his “fix” may now befoul normal tstat functions if and when a correct appliance diagnosis is engaged.

Joel

[Adam Hicklin]

AQH model 421/12.  I’ve cleaned every connection on comm cables.  Fuses checked on both AC’s and all AQH fuses.  This model AQH has no fault indicator.  I’ve verified all 4 AQH tstats are good (diesel control tstat). Fluid is topped off. 

Joel, on this vintage coach there are 2 AC’s and 3 heating zones inside the coach, controlled by ONE tstat in living room area.  Basement zone is controlled automatically. I can’t figure out how the thermostat communicates with the AQH and heating system. I can’t find any type of control board like that on the AC units with any kind of comm cable.  But they have to communicate somehow.  I have the service manual for the AQH but that gives no indication on how they all communicate.

Appreciate all input.  Unfortunately, there aren’t many people left that know much of this vintage coach.  I’ve talked to a few local techs, but apparently I’m speaking another language. 

[Steve Huber]

Adam,
The communication between the Tstat and the AQH is the same for both elect and diesel (Fig 18 in the service manual) so if the AQH responds to the tsat setting on electric, it is getting the signal it needs for diesel also. The owners manual for the 431/12 (no 421/12 listed) lists possible causes for the diesel no start symptom. It breaks it into 2 cases depending if the light in the diesel switch comes on or not when the switch is activated. Low battery voltage (10V) can cause the problem in both cases. It the light comes on there are 2 fuses in the AQH that could also be bad, although you indicated you checked them. I'd also check the switch itself and the connectors listed in the owners manual troubleshooting guide. But before that suggest you verify you are getting 12V at both battery sets and to the AQH, if you haven't done so already.
Steve

[Eric Maclean Co-Admin]

Adam
Page 59 in the 1996 Patriot wing diagrams should get you what your looking for.

Eric

[Adam Hicklin]

Thanks Steve and Eric.

My mistake.  It’s the 431/12 AQH.(stupid fat fingers)

Steve, I’ll investigate the switch today. The indicator light by the switch has never come on, even when everything was working fine, so it’s never been a help.  I’ll dive in to the switch today.  But all fuses are good and voltage is good.

Eric, thanks for that.  I have the diagram for the 431/12. I have one thermostat in the coach.  With one comm cable to the back ( much like the old phone, 4 wire cable, which comes from the AC.)  The tstat wires from the AQH don’t run directly to the tstat in the coach.  They have to interface somewhere else.  Maybe on the AC board?  Maybe on some buried circuit board somewhere else? But as Steve said, I can get heat to the zone 1, but zone 1 is where the tstat is stuck.  Can’t change off zone 1.

I’ll keep searching.  Keep the suggestions coming!

[Steve Huber]

Adam,
Bear with me on this; You said the AQH is working on electric. I assume (maybe incorrectly) that by working you mean that the AQH responds to the t'stat settings. If so, the wire (signal) from the t'stat is getting to the AQH and since you have a single t'sat and the AQH uses a single t'stat input for both electric and diesel, it's not clear to me why you are looking for the t'stat wire. The connection between the t'stat and the AQH is working.
The light in the Diesel switch is powered by a signal from the AQH so it may be a bad switch or a problem in the AQH itself. The AQH has separate internal control t'stats that sense the boiler fluid temp for diesel and electric functions. If the diesel t'stat is bad it could result in the symptoms you are reporting. See pg 14 of the service manual.
The zone selection problem sure sounds like either a faulty t’stat or that the dip swiches in the t’stat aren’t set correctly.  I’ll email you the manual for the 4 button t’stat in case you don’t have it. (I’ll also add it to Coach Assist).
Steve

[Adam Hicklin]

Thanks Steve.

Yes, the AQH is working on electric, in that the electric element is heating the fluid.  As far as the tstat goes, it will kick on the heater blower fan of zone 1 although I haven’t left it on long enough to see if it is actually circulating fluid for heat.  I’ll do that tomorrow.

I found the elusive 3rd zone control box.  This is a box containing circuit board and comm connections exactly like the AC units.  Mine is in a compartment just behind the AQH control switch above the kitchen table. Panel also contains generator start switch, water pump and water fill switch and kitchen fan.  It’s accessed through a panel in the neighboring cabinet.  My thinking was that perhaps that control board had a fuse blown and it was affecting the other zones.  This whole problem started because the t stat was only seeing zone 1.  Unfortunately there’s no obvious problem with this circuit board. Dip switches are all in the correct position.  Incidentally, the comm cables in to this 3rd control box are marked front and rear. 

I checked the switch for the AQH today.  It is good.  All the terminals on the AQH are getting power.  Voltage is good.  Tomorrow I’m going to try to bypass the diesel control thermostat on the AQH.  It checks out good, but I’ll try anyway. 

Gent who checked out the tstat suggested testing the comm cables.  Not sure how I’ll do that but, that’s on the agenda for tomorrow also.

I just think it’s odd that I’m getting nothing from the diesel burner, along with the tsat failure.  I’m going to try to check in to the wabasto control box, although that may be above my skill set.  I’ve called all around the Houston area and there isn’t anybody I’ve talked to who is familiar with this vintage of system. And I don’t feel like paying someone $125/hour to learn about it.

These aging coaches may be timing out.  (Sorry.  Feeling very frustrated)

[Steve Huber]

Adam,
You might want to try toggling the t'stat dip switch settings back & forth a few times to make sure there isn't a dirty contact on one of them.
If you haven't you may want to follow the steps in Section 8 on page 50 of the AQH service manual to see if the burner motor / controller is functional.
You may also want to call the AQH factory in CO if all else fails. It's been many years since I spoke with them but they were helpful at that time.
Steve

[Adam Hicklin]

Thanks Steve. Today, I bypassed the diesel control thermostat and the AQH fired up, came to temp.  I let it run for a short while and turned it off.  I re-connected the thermostat and fired it up again.  It came up to temp, and turned off.  For whatever reason, I think the bypass was coincidental.  But it seems to be working properly now. I turned it off to let it cool.  Hopefully it works tomorrow.

AC and Heat tstat is still on the blink. My hope was they were all tied together somehow and if I fixed one, it would fix the other.  No such luck.  I’ve tried everything I can do.  May have to call in a professional.

Question:  is there a way to get the rear AC on by bypassing the tstat all together?  I’m thinking literally a rigged up, temporary on/off switch dangling from the ceiling I can turn on. Not sure how I would go about that, but maybe someone here knows.  Anyone?

A little explanation for my urgency…my wife and I are supposed to start living in the coach this coming Friday as our house is scheduled to start a remodel.  Can’t live in the coach, in TX summer, with no AC. 

Thanks all.

[Fred Brooks]

   Adam,
When the 4 button t-stat gets unplugged as yours has, it becomes virgin again. It has lost its connectivity to everything that was attached to it. The default setting is zone 1 only. Perform the "reset" procedure on page 7 in the owners manual that Steve just put in Coach assist. When you do that the t-stat will recognize the rear air conditioner "zone 2" and the control box dedicated to the aqua-hot "zone 3". Incidentally the pc boards on the air conditioners have "dip switches" that tell what functions the t-stat will recognize. The word "furnace" dip switch tells the t-stat to regulate the heat temperature in that zone by the aqua-hot. Hope this helps, Fred

[Adam Hicklin]

Thanks Fred.  Yes, I’ve tried resetting the tstat several times now. Followed the process and it did default to the default temps, so I know it was reset. It’s just not seeing zones 2 and 3 for some reason.  And that was happening before I ever unplugged it.  I’m at a loss.  Had one tech suggest that maybe one of the cables are bad.  If they are, there’s no good way to run a new cable through the ceiling.  I’ll try to test those some how.  Good news, AQH seems to be working normally, so at least we’ll have plenty of hot water.  And heating on zone 1 as we head in to winter. 

New, or possibly related problem…I have no power to the switches on the front blinds in living/kitchen area (3), all breakers are set and functioning.  Do the blinds run through a relay maybe?

[Fred Brooks]

  Adam,
The front A/C is the main source of information. It is where the rear A/C comm line connects to one of the beige plugs. The other beige female plug is the t-stat. I do not remember if the comm lines plugs are mirror images of each other or wired identical. Check it out. You can buy the plugs and crimping tool at home depot. Lets try and get zone 2 working first and don't worry about routing for now. Incidentally, if you find 2 comm lines connected to the bedroom A/C that is where the aqua hot comm line from that 3rd box is coming from. Determine which comm line from the bedroom a/c is the one plugged into the front a/c and replace the plugs exactly as configured: red,green, blue, white. Next turn of the house batteries for 2 minutes then back on. Reset the t-stat and see if zone 2 appears. Call me at 520-309-1260 for further info. Fred

[Adam Hicklin]

Thanks Fred. I’ll give you a call tomorrow.  Appreciate it.