# Where would Dometic fridge get its 12 volt supply



## andytw (Sep 30, 2009)

Hi,

Pilote Galaxy 27 1998.

I am looking to investigate poor cooling whilst on the move and the Dometic fridge is powered from a 12v supply. The heating element is OK but the voltage, measured at the top of the fridge is low, at around 10v. I am thinking that the wiring needs uprating but was put off by the thought that it is a long run from the fridge to the vehicle battery but just realised that it may be wired to the leisure battery which is quite a bit closer.

Just wondered if there is a best practice for where power would be taken from for a fridge. i.e. would it be 'normal' to use the leisure battery or the vehicle battery ? It seems to me that it doesn't matter, as once the engine is running and the relay cuts in then both batteries are being charged anyway.

Thanks,

andytw


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## boringfrog (Sep 22, 2006)

*Fridge*

Fridge will get it,s power from the leisure battery but only when the engine is running as you said. Are you pre cooling the fridge on hookup before driving?


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## philoaks (Sep 2, 2008)

andytw said:


> Hi,
> 
> Pilote Galaxy 27 1998.
> 
> ...


Your fridge will almost certainly be wired from the vehicle battery, but via a relay that ensures the supply is only available to the fridge when the engine is running. This ensures that you don't flatten your battery as the fridge will be drawing around 8 -10 amps when on. As you say the length of the cable run could well be causing your volt drop but you should also check the connections along the way ( fuse. relay. etc. ). Assuming that your engine battery will have around 14v across it with the engine running then your losing a fair bit of that if you are only getting 10v at the fridge.


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## GerryD (Sep 20, 2007)

The fridge is designed to run on 12V only whilst the engine is running, but as for lowering the temperature; you can forget it.
The sole purpose of the 12V feed to the fridge is to at best maintain temperature and at worst not allow it to raise too high.
If you want a fridge to cool under a 12V feed then you will need to look at compressor fridges. They are much smaller and much more expensive.
Gerry


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## andytw (Sep 30, 2009)

Hi,

Yes pre-cooled in that we are happy with it when on a site but on moving site find it is not as cold on arrival with previously well frozen food starting to de-frost.

Yes, there is a big difference between 10v and 14v.
As the power follows the square of the voltage, in the first instance we have 100 divided by the resistance of the element and in the second it is 196 divided by the resistance...virtually twice the power !!

Will do some checking of connections etc, biggest problem is that to access the top of the fridge, I have to remove the sink.

andytw


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## andytw (Sep 30, 2009)

Hi,

I was a bit lazy last night in asking here, as I have now dug out the diagram for the control panel and charger unit.
I was surprised to see that the fridge wiring goes through this unit which also includes the relay which is energised when the engine is running.
The fridge power comes form the vehicle battery and there are plenty of connectors in and out for me to see where the voltage is getting dropped off.

Thanks,

andytw


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## rayc (Jun 3, 2008)

andytw said:


> Hi,
> 
> I was a bit lazy last night in asking here, as I have now dug out the diagram for the control panel and charger unit.
> I was surprised to see that the fridge wiring goes through this unit which also includes the relay which is energised when the engine is running.
> ...


My guess is that the voltage is being dropped throughout the run from the vehicle battery to the fridge via the control panel due to the size of the wires.
To get a it any better you may need to revert to the 'old' system of having a seperate voltage activated relay mounted in the engine compartment and 4mm wiring directly to the fridge. There is no reason for it not to work, all my caravan fridges did and it is a long run from the car battery to the caravan fridge.


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## spykal (May 9, 2005)

Hi andytw

Having read through the thread I would firstly go with your own suggestion to trace back from the fridge through the wiring route and look for a bad connection somewhere. On my sons old Autosleeper we solved exactly the same problem when we found a corroded and melted connector on the top of the fridge ( the main connection point on that model fridge). Once we replaced that the fridge worked as it should on 12V.


Mike


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## Tobysmumndad (Nov 12, 2007)

Just a thought: have you checked the condition of the Maxi fuses in the engine compartment?


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## Touchwood_RV (May 1, 2005)

Hi disregarding the pre-cool issues etc, the key issue here is the voltage drop, with the engine running you are loosing around 30% of your voltage! I.e. 4.4 volts, that is not to be ignored, that is a substantial loss, and like Mike I recommend a through check of the wiring, starting from the fridge back to the source, checking at all convenient points along the way, connectors, fuses etc. Something, my guess, a dirty connector such as a fuse is the cause and just needs cleaning.


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## Pat-H (Oct 30, 2009)

The other worry is that the volt drop could also risk overheating.
If it's just one poor connection it could be disipating that power as heat and that's a fair bit of power at 2-4V at 8 AMP or so.


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## safariboy (May 1, 2005)

Don't forget the earth return line. I think that a too small wire is unlikely if the fridge was installed as new. It would have worked once. Connectors and fuses deteriorate but the wire is unlikely to get thinner!

The best way to find the problem is to firstly check the voltage at the relay output. Should be 14V when the engine is running.
Then connect a long wire (any thickness) to that joint and take the meter and a short length of wire with a probe set to 20V and work up the wire checking available connections. As soon as the voltage shows more than say 0.5V you have at least one problem. Try also frame to ground on the fridge same applies.

Hope this helps.

Safariboy.


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## andytw (Sep 30, 2009)

Hi,

Found a burnt connector on the back of the 16A breaker in the control unit. Now have 12.3V at the fridge. Am losing half a volt on each of the +ve and -ve cables from the control unit to the fridge.
Will think about bigger cables on this last run but inclined to leave things as they are.

Incidentally I am seeing 12v power to the fridge when the ignition key is turned to on, but prior to starting the engine.
Is this correct or should it only be there once the engine is running ?

andytw


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## Jezport (Jun 19, 2008)

andytw said:


> Hi,
> 
> Found a burnt connector on the back of the 16A breaker in the control unit. Now have 12.3V at the fridge. Am losing half a volt on each of the +ve and -ve cables from the control unit to the fridge.
> Will think about bigger cables on this last run but inclined to leave things as they are.
> ...


AFAIK the 12v should only be switched when the alternator is running.


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## philoaks (Sep 2, 2008)

andytw said:


> Hi,
> 
> Found a burnt connector on the back of the 16A breaker in the control unit. Now have 12.3V at the fridge. Am losing half a volt on each of the +ve and -ve cables from the control unit to the fridge.
> Will think about bigger cables on this last run but inclined to leave things as they are.
> ...


Normally the operating coil of the fridge relay would be connected to the sense wire of the alternator. This would mean that it wouldn't operate until there is an output from the alternator (i.e.the engine is running). It sounds as though yours is just wired to a ignition switch controlled live. This shouldn't be a major problem as long as you don't leave the ignition switched on for long periods without the engine running.


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## safariboy (May 1, 2005)

andytw said:


> Hi,
> 
> Found a burnt connector on the back of the 16A breaker in the control unit. Now have 12.3V at the fridge. Am losing half a volt on each of the +ve and -ve cables from the control unit to the fridge.
> Will think about bigger cables on this last run but inclined to leave things as they are.
> ...


This would indicate a total of about 0.1 Ohm. Taking into account that the fridge 12V should only operate when the engine is running this look reasonable. I suspect that you have restored it to as new and will need to put in much larger wires to produce an improvement.

I would be concerned that the relay closes when the ignition is on because the relay that connects the two batteries together is likely to operate off the same line. The result is that when the starter motor runs it can take current from both and the leisure battery is not designed for a heavy discharge. There should be a fuse that might blow and the contacts of the relay will get hammered. So I would try to get it changed if possible.

Safariboy.


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## andytw (Sep 30, 2009)

Hi,

Thanks for the help and suggestions.
Am not worrying any more about the voltage at the fridge as this appears to be OK.
However, re the comments on the batteries getting joined at ignition on time, this seems likely to be happening.

With ignition off I can see voltage of each battery at the control panel and they are different, e.g. 13 and 13.5v. If I now turn on the ignition then both voltages at the control panel appear to be the same.

I realise this is hardly foolproof but, in addition, I see that with the ignition switch on and the habitation lights on, if I turn the fridge on and off with the 12v switch on the fridge, then the habitation lights dip in brightness. This shouldn't be the case if the batteries are seperate, with the habitation battery supplying the lights and vehicle battery supplying the fridge.

The input to the control box which should be hot only when the alternator is running is marked D+. I am seeing 10.5v on here with just the ignition on so looks like I will need to get my head under the bonnet.

Are there any links to typical wiring for alternator and 2 batteries which might help me ?

Thanks,

andytw


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## andytw (Sep 30, 2009)

Just a final update should anyone comes here for a read.

There are a couple of relays under the bonnet but disconnecting both has no impact on the batteries getting joined at "ignition on" time.
As yet, I have not been able to identify what these relays actually do.

Anyway, I spoke with the service manager at Hayes and explained that I was looking for why the fridge gets power and the batteries are getting joined at "ignition on" time rather than at "engine started" time and he said "Yes that is correct, that is how they used to be wired and they used to have problems if the engine battery was flat then the leisure battery would jump the flat battery and cause wires to burn out, can be remedied by fitting an additional relay", so it does sound as if mine is truly correctly wired for that model.

Will now put everything back as it was and move on. 

Cheers

andytw


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