Dometic fridge 12v supply puzzle

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Jun 20, 2005
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I understand that Prof, mine is the same as it happens.Do you think it is relevant though to the OPs problem? I say that because as us old lags know if you do not isolate the caravan battery from the fridge when stored, the interior light stays on and of course the things you mentioned.
 
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Mar 14, 2005
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I understand that Prof, mine is the same as it happens.Do you think it is relevant though to the OPs problem? I say that because as us old lags know if you do not isolate the caravan battery from the fridge when stored, the interior light stays on and of course the things you mentioned.
Yes Dusty I do think its relevant. Ecleptic tells us the indicator light comes on, when connected to the car but it dosen't cool. As these two parts are supplied form different places the key factor is it's a new tow bar ( and I assume electrics) and its only since this change the fridge has stopped working whem coupled to the car. Its too much of a coincidence to ignore.

One of the first rules of electrical fault finding is to check the supply.

We have seen plenty of reports that standard fit tow bar electrics often don't include the fridge wiring, and all the symptoms point towards this as the most obvious thing to check out first.

As its all new then there is a contract with the fitters, and it may depend on exactly what was asked for, and whether it included fridge wiring or not.

I'm not sure why some people seem so reluctant to ask the people who have done the work and have the equipment to actually test it. Where as all we can do is guess based on the information we have been given.
 
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Hello all,

I have the following to report re the fridge operation/not.

Taking to heart comments concerning earthing and voltage i did the following.

Checked the voltage at the tug outlets with motor running - 14.4v
Turned on main beam head lights - 13.9v

Next the earthing issue.
With the 12 pin flat connector I have a choice of 2 earth outlets, one for heavy current and one for typical lighting current.
I had soldered together the two earth lines (main and lighting) and this did connect to the lesser amperage earth plug, with the fridge -ve line connected to the heavy amperage earth line.
Swapped them around, and screwed the holding screws in tight.

Bingo. Fridge works. I suspect the problem was lack of firmness in screwing down on the cable causing an indefinite earth, but am happier that I made the change of lines to the new setup.

Cooling fan is still not working but I will refer that one back to the installer.

"On the way to the colluseum" after finding the fridge working I thought I should test my luck and see if the electric heater would work (240v) and, scrub my knees - it has started working also. Beats me but am I happy, you betcha!

Many thanks for your great input and problem solving skills. We have a great community, here.
 
Jun 20, 2005
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Hello all,

I have the following to report re the fridge operation/not.

Taking to heart comments concerning earthing and voltage i did the following.

Checked the voltage at the tug outlets with motor running - 14.4v
Turned on main beam head lights - 13.9v

Next the earthing issue.
With the 12 pin flat connector I have a choice of 2 earth outlets, one for heavy current and one for typical lighting current.
I had soldered together the two earth lines (main and lighting) and this did connect to the lesser amperage earth plug, with the fridge -ve line connected to the heavy amperage earth line.
Swapped them around, and screwed the holding screws in tight.

Bingo. Fridge works. I suspect the problem was lack of firmness in screwing down on the cable causing an indefinite earth, but am happier that I made the change of lines to the new setup.

Cooling fan is still not working but I will refer that one back to the installer.

"On the way to the colluseum" after finding the fridge working I thought I should test my luck and see if the electric heater would work (240v) and, scrub my knees - it has started working also. Beats me but am I happy, you betcha!

Many thanks for your great input and problem solving skills. We have a great community, here.
Well done. Glad the guys on here helped.
 
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Jul 18, 2017
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Besides the fridge more importantly will the ATC be working correctly as that also relies on a 12v supply from the towing vehicle and if ou have a SMART alternator the 12v feed may not be thre all the time? Not being an auto electrician I do not know.
 
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Hi Buckman, there is no ATC.
Antisway is provided by Alko hitch friction lock at tow ball, plus the tug's anti-sway electronics.
The SMART alternator provides charging voltage until tug battery is at capacity, then adjusts to required voltage for standard operations. Does up the voltage and amps to provide additional power when signaled by the tug on-board computer.
 
Aug 11, 2018
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The BS7671:2008 known as 17th Edition wiring regulations decided for some reason to include the 12 volt wiring to a caravan, one of the things they stated was the supply to charge the battery and the supply to work the fridge need to be independent, however the caravan only has two supplies one permanent when ever the caravan is connected to car, and one only when the ignition is switched on, and both battery charging and fridge only want a supply when the ignition is switched on, so only way is to use a relay, so the non ignition supply is switched on when the ignition supply is made live.

This means you need both supplies, even when you don't need anything to work with the ignition off.

The ignition switch is not normally man enough to take the power to the caravan and what is important is when the start button is pressed, that the caravan battery can't supply power to the car, as the current from caravan to car is far too high, there is also the problem if left plugged in the caravan could discharge the car battery, but compared with the start current problem that is minimal.

So early in the caravan usage we got the blocking diode, this seems good, but the diode reduces the voltage by 0.6 volt, with the old CAV 440 regulator this was OK as you just moved the spade connector from Med to Hi and lifted the alternator output to suit, but that may have been OK with wagons, but cars did not have an adjustment on the alternator regulator. So the blocking diode lost favour, the next was the relay, by powering the relay from the warning light output from the alternator, it insured contacts open until after engine started.

However may French cars used single phase alternators and the system would not work with them. So came the voltage dependent relay, it only switched on at 13.8 volt and off at 12.8 volt so it would ensure caravan battery not connected.

Then came the schottky diode, unlike the old blocking diode these don't have a 0.6 volt drop across them, so worked far better, however not cheap, Sterling power version £40.80 and although not 0.6 volt there is a small amount of volt drop, however with the use of a battery to battery inverter they are not required.

So with a battery to battery inverter the power can't flow caravan to car, it can flatten the car battery if left running, but it does two things, one is it charges the caravan battery faster and two it stops the back power caravan to car, and the best location is in the caravan.

So now we have a problem, there is no standard way to wire, if you have a simple relay connected to ignition supply that will stop the caravan being supplied when ignition is off, so it will stop the car battery discharging, but not all cars turn off the ignition when cranking, and if it does not turn off the ignition then it could burn out wiring between car and caravan, so the only way with many cars is to fit the schottky diode as that is only sure way to stop supplying car from caravan.

I know the subject is fridge, but it is not the fridge which is the problem, it is charging the caravan battery which is real problem, personally I have disabled the charging of caravan battery from the car, with a tow time of maximum 4 hours, between sites, going home does not matter, and going from home does not matter as former battery can be charged once home, and latter battery fully charged anyway. So 4 hours between sites, and with a split charging relay it may start at 30 amp, but within a mile down to less than 5 amp, so that 4 hours is likely just 10 Ah into the battery, hardly worth bothering about, so it is either a battery to battery charging inverter which means you may get 30 Ah in 4 hours, or solar panels, and if you are living off grid the solar panel seems the way to go.

But this means some discussion with the tow bar installer as to what he is going to do, simply I want caravan electrics fitted is not enough, you need to say what you want, the two bar fitter can guess, and he has three options 1) schottky diode. 2) voltage dependent relay. 3) simple fuse and ignition relays. With the latter should the engine be cranked with the caravan battery connected it will simply rupture the fuse.

There is nothing oddly in BS7671:2008 (yes I know now 2018 but I don't have a copy of that) to say about the problem of reverse power caravan to car, and some cars pressing the starter does switch off ignition power, but to be fair all the tow bar fitter can do is fit the manufacturers wiring loom and follow their instructions, and many German cars the loom does not support the two supplies to the caravan.

The most important thing is where there is an electric anti snake device that the battery charging does not cause the supply to this from being interrupted.

But you can't blame the electrician who wired the car, unless the caravan was there with the car when he wired it, He has not got a crystal ball, he does not know how your caravan is wired.

Personally I see no future in split charging, my wife's car has the charging controlled by the engine management, and I have during the lock down needed to use a smart charger to recharge the battery as car not used enough, it can't even keep its own battery charged never mind one on the caravan.
 
Oct 10, 2018
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The BS7671:2008 known as 17th Edition wiring regulations decided for some reason to include the 12 volt wiring to a caravan, one of the things they stated was the supply to charge the battery and the supply to work the fridge need to be independent, however the caravan only has two supplies one permanent when ever the caravan is connected to car, and one only when the ignition is switched on, and both battery charging and fridge only want a supply when the ignition is switched on, so only way is to use a relay, so the non ignition supply is switched on when the ignition supply is made live.

This means you need both supplies, even when you don't need anything to work with the ignition off.

The ignition switch is not normally man enough to take the power to the caravan and what is important is when the start button is pressed, that the caravan battery can't supply power to the car, as the current from caravan to car is far too high, there is also the problem if left plugged in the caravan could discharge the car battery, but compared with the start current problem that is minimal.

So early in the caravan usage we got the blocking diode, this seems good, but the diode reduces the voltage by 0.6 volt, with the old CAV 440 regulator this was OK as you just moved the spade connector from Med to Hi and lifted the alternator output to suit, but that may have been OK with wagons, but cars did not have an adjustment on the alternator regulator. So the blocking diode lost favour, the next was the relay, by powering the relay from the warning light output from the alternator, it insured contacts open until after engine started.

However may French cars used single phase alternators and the system would not work with them. So came the voltage dependent relay, it only switched on at 13.8 volt and off at 12.8 volt so it would ensure caravan battery not connected.

Then came the schottky diode, unlike the old blocking diode these don't have a 0.6 volt drop across them, so worked far better, however not cheap, Sterling power version £40.80 and although not 0.6 volt there is a small amount of volt drop, however with the use of a battery to battery inverter they are not required.

So with a battery to battery inverter the power can't flow caravan to car, it can flatten the car battery if left running, but it does two things, one is it charges the caravan battery faster and two it stops the back power caravan to car, and the best location is in the caravan.

So now we have a problem, there is no standard way to wire, if you have a simple relay connected to ignition supply that will stop the caravan being supplied when ignition is off, so it will stop the car battery discharging, but not all cars turn off the ignition when cranking, and if it does not turn off the ignition then it could burn out wiring between car and caravan, so the only way with many cars is to fit the schottky diode as that is only sure way to stop supplying car from caravan.

I know the subject is fridge, but it is not the fridge which is the problem, it is charging the caravan battery which is real problem, personally I have disabled the charging of caravan battery from the car, with a tow time of maximum 4 hours, between sites, going home does not matter, and going from home does not matter as former battery can be charged once home, and latter battery fully charged anyway. So 4 hours between sites, and with a split charging relay it may start at 30 amp, but within a mile down to less than 5 amp, so that 4 hours is likely just 10 Ah into the battery, hardly worth bothering about, so it is either a battery to battery charging inverter which means you may get 30 Ah in 4 hours, or solar panels, and if you are living off grid the solar panel seems the way to go.

But this means some discussion with the tow bar installer as to what he is going to do, simply I want caravan electrics fitted is not enough, you need to say what you want, the two bar fitter can guess, and he has three options 1) schottky diode. 2) voltage dependent relay. 3) simple fuse and ignition relays. With the latter should the engine be cranked with the caravan battery connected it will simply rupture the fuse.

There is nothing oddly in BS7671:2008 (yes I know now 2018 but I don't have a copy of that) to say about the problem of reverse power caravan to car, and some cars pressing the starter does switch off ignition power, but to be fair all the tow bar fitter can do is fit the manufacturers wiring loom and follow their instructions, and many German cars the loom does not support the two supplies to the caravan.

The most important thing is where there is an electric anti snake device that the battery charging does not cause the supply to this from being interrupted.

But you can't blame the electrician who wired the car, unless the caravan was there with the car when he wired it, He has not got a crystal ball, he does not know how your caravan is wired.

Personally I see no future in split charging, my wife's car has the charging controlled by the engine management, and I have during the lock down needed to use a smart charger to recharge the battery as car not used enough, it can't even keep its own battery charged never mind one on the caravan.
Thank you, Ericmark. Very insightful with historical background.

I shall certainly have a chat with the wiring installer.

Appreciate your input

Regards
 
Oct 10, 2018
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We solved the non-functioning fan.

Took the van to the installer who found a lead had disconnected from the switch - guess I wasn't going to find that one anytime soon....

Reconnected with more slack in the power lead, and we are finished. Hooray!

You may recall my comment about electric heater started working when I got the fridge working - huh, must have been a dream....
 
Oct 3, 2013
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Dometic RN4271 fridge runs in 240v, and gas.
The fridge 12v supply from the car is operated via ignition relay, and the +ve line to the pin socket shows normal voltage. Of note is the relay inside the car engine bay and the regular relay in the distribution area in the van on the 12v+ve feed. I don't think this is an issue.
The return line (-ve) from the fridge shows .07 volts and I have connected that to the the earth side of the car battery.
Running the car, I get normal indicator light showing when I turn on the 12v switch, but no cooling.
Yes, have checked the 12v fuse at the onboard distribution.

Van has not been used for some 5 months, and the tug is a different one from previous excursions. Had to change the plug setup to a 12 pin flat to match the towcar.

Perhaps the 12v heating element has failed?

Suggestions welcome.
Hello Ecliptic,
The 12 volt supply is only supposed to keep fridge cool (ie "topped up" ) after it has been cooled down from a gas or mains electric supply source.It won't cool a fridge down from scratch.
 
Nov 6, 2005
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Hello Ecliptic,
The 12 volt supply is only supposed to keep fridge cool (ie "topped up" ) after it has been cooled down from a gas or mains electric supply source.It won't cool a fridge down from scratch.

A fridge 12v element is around the same power as the 230v element so very capable of cooling down from scratch - but both need around 24 hours which is much longer than the typical tow.
 

Damian

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Mar 14, 2005
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Hello Ecliptic,
The 12 volt supply is only supposed to keep fridge cool (ie "topped up" ) after it has been cooled down from a gas or mains electric supply source.It won't cool a fridge down from scratch.

Wrong in every respect.
The 12v element works as hard as the mains element as they are both of the same power, so it does not need to be cooled down prior to use, and it will cool a fridge from scratch if you are prepared to drive long enough for it to do its work,,,as Roger said, but time wise, around 6 hours to produce Ice.

As most people do not tow for a solid 6 hours, then it appears that the element is not working as it should.
 
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Hello Ecliptic,
The 12 volt supply is only supposed to keep fridge cool (ie "topped up" ) after it has been cooled down from a gas or mains electric supply source.It won't cool a fridge down from scratch.
Hello Bertieboy1,
Yes the manual says to chill the fridge down using gas or 240v, and the 12v sys will keep it cool.
My experience is that cooling the fridge on 12v only does take time, but it will happen, eventually...
 
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Thanks for your input Damian and RogerL.

Interesting that the 12v element is of a similar (same) power.

I'll still ensure the fridge is properly chilled before heading off into the outback :cool:
 
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For anyone who may not know, these fridges use a process called absorption to chill, and counter intuitively it actually needs heat to drive the system. Because the heat is applied to the outside of the refrigerant tubing its called "externally heated" and it doesn't care how that heat is made. So which ever heat source is used, provided the same amount of heat (Watts) is applied at the correct point in the cycle, the fridge will produce the same cooling effect.

There are some differences becasue the heat sources cannot physically be in exactly the same place but these are relatively small in comparison to the total heat input from each source.

The performance of the fridge is affected by a number of external factors, some of which will be more related to where the power is coming from. It will work best when it's stationary, correctly levelled and settled. This will be the case for use with mains power or gas.

The 12V supplied from a tow vehicle is normally only available when the outfit is coupled to the tow car when the engine is running. This is also usually when the caravan is in motion. The 12V may be reduced or interrupted if the electrical demands of the tow vehicle require power. The physical motion of fridge in the trailer may impede the correct flow of the refrigerant through the fridge. And when a fridge is towed the external air flow used to cool the condenser coil on the back of the fridge will be disturbed and may be less than ideal.

The consequence is a fridge when being towed is likely to be less effective at cooling down, the fact this only happens when its on 12V supply is not cause of the reduced performance, but just a practical coincidence.

Some of these fridges are used situations where the only power source is 12V, for example solar powered where no mains power is available (e.g. Storage of of some medical supplies in sunny climates) and they can be effective and durable.
 
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I wonder if the fridge would have a better chance of cooling if the winter covers were fitted whilst travelling to prevent wind chill behind the fridge. I’ve never had winter covers ( or needed additional fans) on any of my vans so don’t know if they are of a secure enough fit to stay on. Just a thought
 
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Damian

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I wonder if the fridge would have a better chance of cooling if the winter covers were fitted whilst travelling to prevent wind chill behind the fridge. I’ve never had winter covers ( or needed additional fans) on any of my vans so don’t know if they are of a secure enough fit to stay on. Just a thought

Not really.
Winter covers are just that, for use when the ambient temperature drops to about 8 degrees or less.

The only advantage of using them at other times is to stop, or reduce, the ingress of water or foreign matter into the void the fridge sits in, and when washing the van.

Properly fitted they are secure enough to stay on.

Fitting them in higher temperatures will reduce the effectiveness of the fridge, which is why people who have awnings which enclose the fridge often find that it stops working as it gets too hot, that is when extra fans sometimes are useful.
The fridge must have good air flow to work effectively, and fridges supplied in the UK are made to work in temperatures up to 30 degrees, above that, which is common in an awning, reduces the effectiveness.
 
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I’ve used winter covers in low temperatures and never had any problem with integrity whilst towing. They are positively retained.
 
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I see, thank you. Just wondered whether if, when towing, the turbulent air might reduce the heating effect of the element ergo diminishing the cooling effect of the fridge.
 
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I see, thank you. Just wondered whether if, when towing, the turbulent air might reduce the heating effect of the element ergo diminishing the cooling effect of the fridge.
It certainly might, but there is an almost equal possibility the increased windage through the condensing coil might actually improve that part of the cycle, but also when being towed the movement of the caravan can shake up the refrigerant in the coil that will reduce its efficacy.
Its very much a moving target, but its certainly better to try and cool with 12V dc when towing rather than leaving fridge turned off.

Incidentally it is not recommended to use gas when towing, and of course gas must be turned of when at filling stations and some ferries and train operators may require no gas to be used.
 

Damian

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some ferries and train operators may require no gas to be used.

It is illegal to have anything gas powered ON when on board a ship or train.
Cross channel operators do check each vehicle to ensure this is complied with, and if a unit is found to ignore this mandatory requirement the owner can be charged with putting the ship and passengers in danger.

So it is not a question of "may", it is MUST
 
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