Dometic fan/ventilator relay

First: Tracing out issue with fridge, discovered a disconnected blue wire from an unidentified 'black box' at the rear of refer. Before I confuse with my issue and how I discovered, a picture of the ventilator 'mysterious black box' (auxiliary fan) control. It’s an option from Dometic, nothing in my documentation showed this mysterious lil black box. When tracing out other issues I 'found' a lil blue wire disconnected. Searched every where but could not identify what the black box was that blue wire led to. Though it had nothing to do with issue I was having, the black box had a red wire connected to12v heater output, ground screw on burner-so the blue wire went somewhere?. Need to resolve since I’m here. Fortunately Chris of Bryant RV sent this diagram identifying...


The purpose of the box is to supply power for aux. fan that is triggered by refer board but doesn’t 'use' power thru board. Sort of like a relay, power is delivered from main 12v terminal (independent of refer), goes to thermal button, then to fan, ground from fan connects to the blue wire of ventilator box/relay. When refer is on power is sent via red wire to black box and connects the blue ground wire from fan to black ground wire. Simple, lil black box controlled by refer simply connects the ground wire.

Just FYI on the little black box-its purpose and how it’s wired. Lost day & half because no documentation for this unrelated issue- my disconnected blue wire connects to ground wire from fan, simply got pulled loose. But as described below while identifying my real problem I also discovered this black box had been -miswired sometime in the past. It also explains why since 2001 my factory fan hasn't/ or couldn't come on, power was miswired, getting power from 12v heater. Just FYI on the lil black box.

So on to my actual issue...

Dometic RM3663 3 way refrigerator-not working on 12v.

Just returned from outing and on the way up refer stopped working on 12v which is what we use on the road. Usual auxiliary. amp draw of 10-12 not showing on amp gauge. I assumed a relay/bad connection, failed constant duty solenoid or something else that would wait until we got home. It worked fine on gas while camping.

Once home after tearing everything apart- starting with auxiliary 12v charge from truck. traced all the way to fridge. Meter showed truck battery voltage at every connection-thru every switch/fuse/relay. I simply found no fault with delivery of 12v power, everything is functioning as it’s supposed to-?? Last test.. as I assumed delivery problem, pulled the wires on the refer 12v heater, 0volts-its dead. Not working, -gosh I wonder why no 12v power usage shown. DA 2 hours later...

So pulling the 12v heater to replace I note a red wire soldered to the 12v heaters plug on board that’s preventing me from removing-traced it up to a small mysterious black box. The red wire enters it and is potted. Out of this box also is a black ground wire fastened to gas nozzle bracket, and a blue wire that disappears in loom. I cut the red wire to remove heater wire and it pulls out other end of blue wire? It’s not connected. I've searched everywhere and can find no mention of this lil box-the part number on the box does not show, nor any Dometic diagram shows it. I'm simply at a loss as to its function or if it even was ever hooked up (other than ground and power to it?), end of the blue wire looks like a clean cut. Pics below, new heater to get ordered shortly. Meantime I need to figure out this mysterious black box and dangling blue wire.


I searched every where trying to find. Posted query on RV.net- I can only hope at this point that someone has something similar and can see where (if) the blue wire is connected to, or might have an idea what the lil box might be.

Just another view of mysterious box. Note it sets beside the ignitor box, also the unknown blue wire in foreground.

pic of red wire from mysterious black box that is soldered to heater plug. Unless I can find purpose I'm not reconnecting?

Well as mentioned at beginning, Chris of Bryant RV uploaded the diagram of the box. Solved the mystery and explained purpose and wiring of box. Blue wire was supposed to be connected to the ground from fan but had been simply pulled loose. But looking at the diagram it also showed that the red wire that had been soldered to my 12v heaters lead wire at board was wrong?

I've been behind my refer for several reasons, backtracking old photos this "mis-wired" red wire shows up being attached to 12v heater plug so pre stuff I've done? regardless- While it works (if running refer on 12v) its wrong- mostly because only time fan could work is when using 12v on road, kind pointless. So moved the red trigger wire to the internal light terminal. Though we don’t rely on factory fan, this allows fan, if temps reach high enough to activate thermal button, to be energized in any mode as designed (gas, AC, DC), yet still be deactivated if recall safety switches tripped.

Only time anyone else has worked on camper is when Dometic recall performed. I can’t imagine them moving trigger wire to the 12v-heater output. I verified that fan goes off if recall power interrupts tripped. So now that lil black box is identified/rewired as intended, I'm removing fuse from the power wire to fan. I had added aux. muffin fans to back of refer a while back. I wanted to correct the wiring but I don’t want factory fan coming on when off grid.

back to original 12v heater issue...

The 12v heater come in various wattages. Located a replacement heater for our 235 watt, but I also found a slightly higher one at 250 watts. It would generate a bit more heat so might cool a bit better on 12v. It’s still low enough to be easily supplied by our charge system.

However I stumbled on a write up stating how difficult it was testing resistance on the 12v heater with most multi-meters? and that zero resistance was NOT an indication of bad heater. Hmm I found it pretty easy but I retested. Showed zero -retested the AC heater--yep .44 ohms...ohhh good grief- I'm on wrong calibration. AC should read 44 ohms not point44. So turned the dial, got exactly 44 ohms. Retested the DC element got 0.7 ohms. That’s close enough. So heater is OK. Odd if heater is ok and everything is delivering power, including output at board-why isn’t it working?

In shop hooked heater to 12v power supply, HOKEY SMOKE that’s hot...

Took back to camper, plugged into board-supported so it wouldn’t touch anything, turn the fridge to 12v-instant hot. She’s working? At this point I can only assume I had a bad connection. In tracing/testing power I disturbed enough to get working. Where/which one no idea. Testing OHMS incorrectly of heater and getting online to order heater I stumbled on the testing procedure and realized I had erred. Going back thru everything I only accidentally discovered the real problem-bad connection. Had I ordered new heater it would have worked-but only because I had inadvertently disturbed bad connection. Or worse bad connection reappeared and new heater didn’t work.

To ensure problem, at least fixed more than just being wiggled, I start back at the truck, taking apart every connection-cleaning and reassemble every connection in path between truck batteries and refer output.

Obviously the problem was one of the connections. Solar showed batteries at 13.7v. Start truck-let it idle a minuet. With truck running, turn on fridge, Amp meter (on aux. charge line to camper) climbs to about 14amps to camper, turn off fridge, drops to 2 amps, turn fridge back on it jumps to 14. Let fridge run for couple of minuets with aux. charge turned off. Turn charge back on climbs to 16 amps then drops to bout 14. So fixed. Last couple of years the amp gage only showed 10-12 amps being pulled when refer on.

Hopefully-as with everything now working & I can’t 'fix' anymore, problem won’t reappear on the road. If it does Ill check resistance at each connection and try to figure out which connection is culprit. Now that its working, obvious to me what I should have been looking for, a bad connection, because I do recall seeing the amp fluctuate up 6-8 amps then fall back down. Hindsight.

Well other than to remind myself 'to assume is to error' mostly this page might possibly pop up showing the picture of the 'ventilator control box' for someone else who might be searching as too what that mysterious lil box is. I also had assumed that the fan power/ground was direct & not going thru the lil box. If the blue wire had been connected I guess that would have been obvious.

Happy fridge-happy amp meter.

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