12 volt fridge heating element, resistance value ?

[jockaneezer]

My Dometic RM7655L fridge freezer has stopped working on 12 volts. I'm getting 14 volts to the heater terminal block with the engine running and there is continuity through the element. I can borrow another meter which will let me check it's resistance but I don't know what it should be. There's no mention of the wattage or resistance in the manual.

 

[sallylillian]

My Dometic RM7655L fridge freezer has stopped working on 12 volts. I'm getting 14 volts to the heater terminal block with the engine running and there is continuity through the element. I can borrow another meter which will let me check it's resistance but I don't know what it should be. There's no mention of the wattage or resistance in the manual.

My element, admittedly on an 8555, is 170 watts (marked on the grey cable shield) and as it happens I have a spare element which came with the replacement cooling unit last week. If no one comes back to you I can put a meter on it tomorrow.

 

[bigtwin]

I'd expect it to be a couple of ohms.

Ian

 

[hilldweller]

is 170 watts .

So 170 / 12 = 14A,

R = 12/14 = 1 ohm (ish).

Need a good meter to measure that.

 

[bigtwin]

So 170 / 12 = 14A,.

Hence why 12v operation is generally restricted to when the engine is running!

Ian

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[hilldweller]

Hence why 12v operation is generally restricted to when the engine is running!
Ian

Yes indeed, with the average 100Ah using the average 50Ah of it that's 3 hours worth.

It shows how good gas is, the gas used is tiny but on electric it's a massive load.

 

[jockaneezer]

Ok then, borrowed my neighbour's auto ranging Fluke and it's showing 1.1 ohms on the 12 volt element, and the car type relay that switches the heater is operating correctly and letting through the 14 volts, so that's all fine.
On our 9 hour drive down to the tunnel in June, the fridge started to defrost after a couple of days running no problem on 240 volts on the drive and we had to resort to running it on gas whilst driving, it'd always worked fine up till then.

 

[sallylillian]

I had an issue with the fridge it got warm overnight, 15 deg, the element and shroud were hot. I switched to gas and the fridge dropped 3 degrees in an hour. Left on gas for 2 days then switched to AC again and fine.
Long story short several AC friends said that there could be a contaminate in the cooling unit causing it to not work efficiently and eventually it may fail. Using gas it heats better and the flame distributes evenly unlike the AC or DC tubes. So Dometic replaced my cooling unit under warranty. Not saying that is your problem but giving it a blast on gas for a couple of days then see if the it works on DC?

 

[jezport]

Check for a dodgy connection where the element plugs into the wiring from the fridge, or last van was intermittant and I could see it sparking on and off. I cut the plugs off and put a decent connector on it.

 

[HarryOverton]

I have got the same problem and I suspect a lot of other owners have but don't realise it. I only found out when I noticed some of the frozen food had gone soft after a full day's travel on 12volt. I put a thermometer in the fridge and the freezer and it confirmed both were not holding the temperatures I achieved by precooling on gas or 240 volt.
I have fitted a fan (to operate when the engine is running) to the top vent but before I had chance to test it I realised it would probably make the situation worse.
When I am on my motorbike in the high temperatures of Spain or Portugal I need a jacket and gloves due to the wind chill so I applied that principle to the fridge problem. I fitted the vent covers when on 12 volt and did a 5 hour trip this week and it maintained the temperature in the freezer but the fridge still went from 4 to 10 degrees.
Anybody got any ideas how to maintain the fridge temperature?

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[WillH]

I use gas when travelling in hot weather.

 

[DBK]

I have got the same problem and I suspect a lot of other owners have but don't realise it. I only found out when I noticed some of the frozen food had gone soft after a full day's travel on 12volt. I put a thermometer in the fridge and the freezer and it confirmed both were not holding the temperatures I achieved by precooling on gas or 240 volt.
I have fitted a fan (to operate when the engine is running) to the top vent but before I had chance to test it I realised it would probably make the situation worse.
When I am on my motorbike in the high temperatures of Spain or Portugal I need a jacket and gloves due to the wind chill so I applied that principle to the fridge problem. I fitted the vent covers when on 12 volt and did a 5 hour trip this week and it maintained the temperature in the freezer but the fridge still went from 4 to 10 degrees.
Anybody got any ideas how to maintain the fridge temperature?

Welcome! I've fitted two fans to my fridge to help it work in hot weather. I'm not sure what your logic is for fitting vent covers and not fitting the fan. Mine are permanently wired so will work when needed - there is a temperature controller fitted and I would never fit the vent cover except in winter. The fridge generates heat from the condenser at the back of it and this heat is what you need to get rid of. Hence the fans and open vents.
The poor performance on 12 volt is simply because it runs at a low wattage is my understanding.

 

[dpsuk999]

I was led to believe that both 12v and 230v heating elements are both 170w but I maybe wrong

 

[DBK]

I was led to believe that both 12v and 230v heating elements are both 170w but I maybe wrong

You may well be right and probably are. A check a few moments ago of a few models online suggests they have the same wattage on both 12 and 240 volts. Personally, we've never noticed poor performance on 12 volts but many others do report it. Perhaps fridge models differ or I suspect they work less efficiently when on the move. It is generally accepted these fridges need to be level but driving, with acceleration, deceleration plus braking will expose the fridge to similar conditions as not being level. So this may be the answer to why they don't work well on 12 volts and the solution is to do what I do - drive gently.
Thanks for questioning my wrong assumption.

 

[sallylillian]

I was led to believe that both 12v and 230v heating elements are both 170w but I maybe wrong

My AC is 250watts my DC is 170watts, so a big difference.

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[HarryOverton]

Thanks for your reply DBK.
The logic of fitting the vent covers is to prevent the wind chill causing the temperature at the rear of the unit to drop below 8 degrees C, as I understand the fridge will not run effectively at low outside temperatures.
The fridge runs fine on gas or 240 volts in hot weather without a fan.

 

[jockaneezer]

Still haven't got to the bottom of this, but I found an analogue 40 amp panel ammeter in one of my boxes of bits and decided to put it in circuit with the heating element to see how much current it was drawing. Now I don't know if the ammeter is suitable for AC or DC and I've since read that I may have needed to fit a shunt for DC, but haven't got a clue what that means, so I just wired it inline anyway ! The result was that when I switched the fridge onto 12 volts with the engine running, the meter registered 15 amps for about a second, then dropped back to zero, then back up to 15 amps then back to zero and stayed there. Now I was expecting a load of around 15 amps but I'm confused as to the pulsing nature of the reading, my first thought was I've probably fried the meter, which was a risk I was prepared to take, or that it shows a fault with the heater element breaking down under load, does anyone think that's a possibility ?

 

[Richard n Ann]

Yes certainly possible. Does the element get hot?

 

[jockaneezer]

Yes certainly possible. Does the element get hot?

I haven't been able to get up to the element via the vents, removing the fridge isn't something I'm in a hurry to do at the moment. I'll see if I can poke my K type thermocouple up there tomorrow and get a reading.

 

[Chablais]

Hi Jockaneezer
I have a similar problem with the same fridge model, no cooling on 12v. It looks as if the fridge will need to be removed to access the 12v heater element, and like you not in a hurry to do that yet, as in the middle of a trip. It currently works fine on gas. New 12v elements appear to be about £60.00!!
How have you faired?
Andy

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[bigtwin]

Are you sure that you need to remove the fridge to access the heating elements?

I've replaced mine through the lower vent.

Ian

 

[Chablais]

Hi Ian

No not entirely sure, it looked tight, as the wires feeding the element went up out of sight, and possible reach. Was it the same model fridge?
I have one vent on the outer skin of the van and one roof vent.
Looked at leisure spares and they are £60.00 for the element, so was hoping it was a fuse or relay, but fitted new fuses earlier today and still same problem.
Need to check my S+ connection is secure and making good contact. Not sure how to check this is actuating the switch to 12v from the leisure bat.
Andy

 

[bigtwin]

Hi Ian

No not entirely sure, it looked tight, as the wires feeding the element went up out of sight, and possible reach. Was it the same model fridge?
I have one vent on the outer skin of the van and one roof vent.
Looked at leisure spares and they are £60.00 for the element

Yes, I think that it's the same model.

My lower vent is a double height vent so that affords a bit more access than a single height vent.

However, before you start trying to remove the 12V element have you checked to see if there is warm air exiting from the flue/top vent? If there is heat coming out of the top end that would suggest that the element is providing heat.

The 175W elements seem to be about £50 on e-bay. A bit less for the lower wattage elements.

Ian

 

[jockaneezer]

Looks like I've finally got to the bottom of the problem and I'm not best pleased. When we first got the van 6 years ago, we had a couple of electrical issues, mostly around charging. The van went back under warranty and they replaced the Reich Ebox as some of the circuit board tracks had apparently burned out. I still had the old box so took it apart and the fridge circuit was burned out too. I went straight to the Ebox and pulled the heavy 12 volt heating element wires off and one of the terminal spades is like a kid's loose tooth, so it looks as if the same thing has happened again
I would have got to it sooner but we've not had any charging issues since the replacement and I fitted solar soon after the repair which meant the van is hardly ever on hook up.
It looks like the unit is only good for 6 years and I was told the replacement was around £750 ! I'll have to start a separate thread about Ebox repairs and issues now.

 

[two]

The main problem with a 12V supply is voltage drop. You don’t need much drop to lose effectiveness and there are many things en route that conspire to do so (low volts, thin wires, poor connections, …). Most fridges don’t even bother to control the 12V side by the thermostat because it has been found unnecessary (it cannot overcool).

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[tonyidle]

The main problem with a 12V supply is voltage drop. You don’t need much drop to lose effectiveness and there are many things en route that conspire to do so (low volts, thin wires, poor connections, …). Most fridges don’t even bother to control the 12V side by the thermostat because it has been found unnecessary (it cannot overcool).

If you get the wiring right the lack of a thermostat on the 12v element feed certainly can cause problems. I've had milk freeze on a long trip with a caravan after running decent size wire.

 

[jockaneezer]

@tonyidle, I've just been out to measure the cable, it looks to be 2.75mm diameter which is almost 6 mm2. The cable run is about 4 metres and I get 14 volts at the fridge so the cable seems to be specced correctly, what is an issue is the track on the circuit board maybe isn't up to handling the current as it's only 2 mm wide, or the Lucar connector and spade terminal are creating heat and frying things that way, although the connector wasn't an unreasonable fit on the terminal. All the other larger cables on the box are ring and screw terminals. Here's a couple of pics of the damage on the original board. I'll try and find an email address for Reich and see what they have to say.

 

[tonyidle]

@tonyidle, I've just been out to measure the cable, it looks to be 2.75mm diameter which is almost 6 mm2. The cable run is about 4 metres and I get 14 volts at the fridge so the cable seems to be specced correctly, what is an issue is the track on the circuit board maybe isn't up to handling the current as it's only 2 mm wide, or the Lucar connector and spade terminal are creating heat and frying things that way, although the connector wasn't an unreasonable fit on the terminal. All the other larger cables on the box are ring and screw terminals. Here's a couple of pics of the damage on the original board. I'll try and find an email address for Reich and see what they have to say.View attachment 183355View attachment 183356

I agree with you. If Reich don't come up with a satisfactory answer it is quite possible to repair the circuit board then fit a relay alongside to prevent it happening again. I had a similar problem with my Hymer. The circuit board was up to the job but the output is via a 2mm pin in a multi-connector. No doubt it was adequate when brand new & spotless but given very minor oxidation & it overheated and dropped 3v. I connected a larger 3-pin connector directly to the board to cure it forever.