Dometic 9-Series Fridge Burning (12V)

My van has a Dometic RML9335 fridge in it. About 4 weeks ago, the van started smelling when the fridge was running on 12V – an electrical  burning smell. I have since not been able to use the fridge on 12V which is a pain because the freezer keeps defrosting while you are on the move!

Between Xmas and New Year, I had a couple of days to investigate and sort this out. I had already ruled out the distribution board as the source of the smell, so it was definitely the fridge, I had assumed that the element had just burnt out. Took ages to get to it; the outside vent does not permit reasonable access, so the fridge had to come out inside the van. I disconnected the gas and electrics, and eventually managed to drag the bloody thing out of its recess.

The problem was immediately evident, the cables for the high current 12V feed for the element had melted through as they entered the control box. I am an electronics engineer  – my professional opinion is that this failure is down to poor engineering on the part of the fridge manufacturer, Dometic. These connectors are designed for 15A maximum, the element can draw 18A; the PCB tracking was woefully inadequate for the current involved, and its never, ever a good idea to run a system at, or slightly beyond its rated current for any lengthy period of time. The result is that the connection gets hot, the constant heating up and cooling down likely leading to an even worse connection, self-perpetuating towards eventual failure. The plastic cover contained the damage, possibly preventing a fire.

The failure mode was essentially that the heat melted the connector and burnt the insulation off the cable. The solder retaining the spade receptor in the PCB then melted (the PCB track is burnt through), and gravity then ensured that the upper (-ve) connector touched the lower (+ve) connector, thus blowing the 20A supply fuse. This shows the importance of properly fused feeds – this may have prevented an actual fire from occurring.

I only use my van for touring, some 22K miles in 2.5 years. At say an average of 45MPH, this equates to 20 days operating solely on 12V. Given the nature of the fault,  I would expect many other folks to encounter the same issue with their fridge after a similar running time. I would have expected Dometic to have performed an  accelerated life test for both gas, 240V and 12V operation – random fridge on/off times, eg 1 min to 5 hours, in a moist-dry-moist environment like would be experienced in the real world.

The correct way of connecting a high current 12V supply to a PCBA is via sizable soldered-in studs, with the connection for both the element and the high current 12V feed made via ring terms and split washers. The design current would be for 30A allowing 18A to be carried with ease, and zero heat. Heat here is simply wasted energy – in a Motorhome there is zero excuse for wasted energy.

While I had the fridge out, I took the opportunity to service the gas side of things. I removed the burner and uses a fish tank pipe-cleaner to brush the flue and chimney. The jet (orafice) and the burner were then soaked in Isopropyl Alcohol for a while before being re-assembled (I used a cheap jewellery cleaner to agitate). I also fixed up the poorly attached fridge-fan, and tidied all the cables.

I was heading away to Mull for new year, so after gathering photos, and writing to Dometic, Campereve and my dealer re this issue and the potential danger it poses, it was time to make the fridge  safe, and to make it work on 12V with AES again. This was easy, its just a 12V element after-all. Earlier Dometic fridges had no thermostatic control on the 12V element, so, if this one did have – I reckoned it could do without it! I obtained a studded (!), voltage sensitive relay, and fitted this adjacent to the control box under the bench seat, linked to the fridge 12V high current supply cable. I fed this with a new feed directly from the van battery, fused at 20A. Behind the fridge it was simply a case of making a good connection to the element which was done with a crimp joiner, and taking a fused (1A), low current feed to the lower (+ve) 12V input on the burnt out panel such that the AES would work.

All that then remained was to manhandle the fridge back into its vestibule, reconnect the gas, cables etc and test.

Update: Everything worked fine while away for new year – should Dometic wish to charge me for a new control box, then I can see this becoming a more permanent fix – its certainly safer than their factory solution.

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