Dometic fridge 12v supply puzzle

[ecliptic]

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.

 

[RogerL]

Something not working after a change of tow car suggests something wrong with the car wiring

 

[Buckman]

Something not working after a change of tow car suggests something wrong with the car wiring

Would having a SMART alternator in the car have and effect in this case? I know we have issues with fridge cooling on long distances.

 

[RogerL]

Would having a SMART alternator in the car have and effect in this case? I know we have issues with fridge cooling on long distances.

Could well be - they need an ignition-driven relay not a "smart" voltage-sensitive relay - some towbar fitters aren't up to speed despite smart alternators being used for over 20 years on some cars.

 

[Buckman]

Could well be - they need an ignition-driven relay not a "smart" voltage-sensitive relay - some towbar fitters aren't up to speed despite smart alternators being used for over 20 years on some cars.

I am fairly sure that there is a way around this as per Caravan Chronicles, but costs about £200 for the unit.

 

[RogerL]

I am fairly sure that there is a way around this as per Caravan Chronicles, but costs about £200 for the unit.

Just need to ditch the "smart" relay and go back to old-fashioned ignition-on relay - that way the fridge circuit stays on all the time and the car's smart alternator ensures the car battery doesn't discharge.

 

[ProfJohnL]

If the caravan has been in storage, its very unlikely the fridges 12V element will have failed, It's far more likely an issue with the towing electrics fitted to the new tow vehicle.

 

[Dustydog]

“ Had to change the plug setup to a 12 pin flat to match the towcar.” I suspect other than the possible solutions wisely mentioned something has gone wrong in your conversion to 13 pin from 12N&12S. Check the new caravan 13 pin , pin no 11 is correctly wired to the fridge Earth.

 

[ecliptic]

Something not working after a change of tow car suggests something wrong with the car wiring

“ Had to change the plug setup to a 12 pin flat to match the towcar.” I suspect other than the possible solutions wisely mentioned something has gone wrong in your conversion to 13 pin from 12N&12S. Check the new caravan 13 pin , pin no 11 is correctly wired to the fridge Earth.

If the caravan has been in storage, its very unlikely the fridges 12V element will have failed, It's far more likely an issue with the towing electrics fitted to the new tow vehicle.

Just need to ditch the "smart" relay and go back to old-fashioned ignition-on relay - that way the fridge circuit stays on all the time and the car's smart alternator ensures the car battery doesn't discharge.

Could well be - they need an ignition-driven relay not a "smart" voltage-sensitive relay - some towbar fitters aren't up to speed despite smart alternators being used for over 20 years on some cars.

Something not working after a change of tow car suggests something wrong with the car wiring

Thank you all for your input and suggestions.

The tug wiring has been professionally installed/modified.

The +ve wiring runs via the switching relay - including battery charge line. It is appreciated that the smart alternator will reduce the opportunity to charge the van battery but a connection was required.

The Smart alternator is not directly linked, but rather +ve lines are from vehicle battery via the ignition activated relay.

The Fridge -ve line goes to vehicle earth.

The puzzle is that I have the fridge 12v switch indicator light show (when I switch it on) when I connect the tug with motor running.

Of further note is I have had a fan installed to improve extraction of warm air from back of fridge and this is not running (although it did work before this). Perhaps I should discuss that fault with the installer and get a clue to the greater problem.

Your input is appreciated.

 

[Tuningdrew]

Earthing problem most likely. Particularly if your fans aren’t running.
My other query would be the voltage, or more accurately current available on the new 13 pin socket. Too often the presence of battery voltage at the relevant pin is assumed that all is well but if the pin is “loaded” with say a 21watt bulb between it and the earth the voltage disappears due to a corroded wire or bad connection not being able to supply sufficient current.

 

[Dustydog]

Earthing problem most likely. Particularly if your fans aren’t running.
My other query would be the voltage, or more accurately current available on the new 13 pin socket. Too often the presence of battery voltage at the relevant pin is assumed that all is well but if the pin is “loaded” with say a 21watt bulb between it and the earth the voltage disappears due to a corroded wire or bad connection not being able to supply sufficient current.

It does seem earthing is the problem at the caravan side of things. You mentioned a voltage reduction using your multimeter on the earth return. It does seem a poor connection on the earth side? It

 

[RogerL]

The tug wiring has been professionally installed/modified.

The +ve wiring runs via the switching relay - including battery charge line. It is appreciated that the smart alternator will reduce the opportunity to charge the van battery but a connection was required.

Get the car back to the installer for checking - professionals have been known to make mistakes.

Find out if the relay is a voltage-sensitive relay (VSR) or not - VSR's get confused by smart alternators so a basic relay switched by ignition on is more appropriate.

 

[Ferrodo]

My fridge wouldn’t run on 12 volt either. I could switch to 12 volt okay but after a few seconds the fridge alarm would sound off. My VSR was one of these Halfords 30amp jobs about the size and weight of a box of matches. I replaced that with a 140amp DURITE unit and never had a problem since.
Regards
R

 

[Beachball]

Welcome to the Forum Ferrodo

 

[Ferrodo]

Thank you Beach Ball, happy to be part of.👍😄

 

[Buckman]

I have been told tha with a SMART alternator the 12v will probably not work with the engine running i.e. idling as the SMART alternator will only kick in when the car's voltage drops below a certain level. As suggested the 12v for the caravan needs to come from a different source. We now prefer to load the fridge with ice blocks to get around the problem and the fridge will keep cool for the long trip from Portsmouth to Zaragoza via Santander in Spain as that is the first opportunity we have to switch the fridge back onto mains.

 

[Tuningdrew]

Measuring the voltage at the pin with the ignition switched on may well show battery voltage at the pin but if the smart relay isn’t switched in there may only be a few milliamperes of current. Quick test there is to start the engine and load up the electrical system by turning on a few consumers, lights, heated window, heated seats etc and see if that brings the fridge on.

 

[Tuningdrew]

Duplicated

 

[ecliptic]

Earthing problem most likely. Particularly if your fans aren’t running.
My other query would be the voltage, or more accurately current available on the new 13 pin socket. Too often the presence of battery voltage at the relevant pin is assumed that all is well but if the pin is “loaded” with say a 21watt bulb between it and the earth the voltage disappears due to a corroded wire or bad connection not being able to supply sufficient current.

Get the car back to the installer for checking - professionals have been known to make mistakes.

Find out if the relay is a voltage-sensitive relay (VSR) or not - VSR's get confused by smart alternators so a basic relay switched by ignition on is more appropriate.

I have been told tha with a SMART alternator the 12v will probably not work with the engine running i.e. idling as the SMART alternator will only kick in when the car's voltage drops below a certain level. As suggested the 12v for the caravan needs to come from a different source. We now prefer to load the fridge with ice blocks to get around the problem and the fridge will keep cool for the long trip from Portsmouth to Zaragoza via Santander in Spain as that is the first opportunity we have to switch the fridge back onto mains.

Measuring the voltage at the pin with the ignition switched on may well show battery voltage at the pin but if the smart relay isn’t switched in there may only be a few milliamperes of current. Quick test there is to start the engine and load up the electrical system by turning on a few consumers, lights, heated window, heated seats etc and see if that brings the fridge on.

Thank you everyone. I now have several channels to sail down. Definitely need to check the type of ignition activated relay, and the earthing .
Suggestion to load systems on the tug and see if there is a change in the fridge will be interesting.
So, back to the wiring installer for a check and confirmation, then if all confirmed OK, shall need to check the fan installer for suggestions as to why his fan is not working.

Shall be back in a couple of days with progress report.

Cheers

 

[Dustydog]

Good luck mate. We can’t wait to hear the final solution.

 

[ProfJohnL]

...The puzzle is that I have the fridge 12v switch indicator light show (when I switch it on) when I connect the tug with motor running.
...

Many modern fridges require two 12V feeds. One is low current that powers the control circuit and that would almost certainly be the one that powers the fridges indicator light. It usually requires an independent high current 12V feed to run the fridge when towing. and the symptoms suggest this is the current feed that's not working

If you simply asked your fitting company to fit a tow bar, without mentioning it was for a caravan, then they may have only fitted the standard trailer wiring, which does not include power for the fridge. If you asked them to fit a tow bar and electrics so you can "tow your caravan", they should have checked if you wanted fridge and battery charging circuits. in which case they may not have completed the contract.

Its worthy of note that several contributors over the years have had to take a car back to the tow bar installer, and to get them to rewire for the fridge.

I don't care how professional a company may seem to be, mistakes can and do still happen. Talk to your fitters. Tell them what is (or what is not ) happening, they should at least have the opportunity to check it out and even put it right.

 

[Dustydog]

Prof .
That’s interesting but where through the 13 Euro pin connection does the second power source come from? Looking at the Dometic wiring diagram the OP quoted there is only one power feed.
The key pins are 10 and 11.
Note an extra cooling fan has been fitted.
For me the two focal problem points are the replacement13 pin caravan plug and the fitting of the cooling fan. I assume he’s tested the continuity of the 12 v fridge circuit?

 

[RogerL]

Prof .
That’s interesting but where through the 13 Euro pin connection does the second power source come from? Looking at the Dometic wiring diagram the OP quoted there is only one power feed.
The key pins are 10 and 11.
Note an extra cooling fan has been fitted.
For me the two focal problem points are the replacement13 pin caravan plug and the fitting of the cooling fan. I assume he’s tested the continuity of the 12 v fridge circuit?

The low current control circuit comes from the caravan's leisure battery - only the high current cooling circuit comes from the car.

 

[Dustydog]

The low current control circuit comes from the caravan's leisure battery - only the high current cooling circuit comes from the car.

Roger,
I thought that was only the internal light? I stand corrected. However if this aspect was a crucial factor for operation you start to wonder why the manufacturers don’t fit a battery at point of sale , nor do they say it is mandatory / essential. Still a very valid point 👍

 

[ProfJohnL]

Prof .
That’s interesting but where through the 13 Euro pin connection does the second power source come from? Looking at the Dometic wiring diagram the OP quoted there is only one power feed.
The key pins are 10 and 11.
Note an extra cooling fan has been fitted.
For me the two focal problem points are the replacement13 pin caravan plug and the fitting of the cooling fan. I assume he’s tested the continuity of the 12 v fridge circuit?

Hello Dusty. I'm sorry I did not make it clear . The low current supply, (just as RogerL points out) comes from the caravans own 12V battery supply, not directly from the cars connection.

The low current system is indeed crucial for controlling the ignition of the gas and signalling how the fridge is configured. All it requires is a 12v supply which could be derived from the caravans 230V ac Power supply when its connected to mains. Modern caravans do not strictly require a 12V battery becasue the modern power supplies which has adequate smoothing, and capacity to run most items in a caravan. They will not run a mover!

The often quoted "need" for a caravan battery derives from the time when caravans did not have "power supplies" but just had chargers, and the old crude chargers did not produce smotthed 12V power but very peaky outputs, often reaching as much as 21V and falling back to zero V, 50 or 100 times a second depending on the type of rectifier used. Electronics like televisions and water heaters cold not use such unregulated power. They needed a battery to absorb the peaks and to fill in the troughs of the of the charger's power delivery.

It was also quite common for the chargers and early smoothed power supplies to have very limited current capacities, so items like water pumps might not start properly unless unless the battery was connected.

Most modern caravans are now fitted with switched mode power supplies (SMPS's) and these are available with current outputs of up to 20A, and these will run a caravan (except a mover) without a battery, but of course mains power is required, so a battery is more of a convenience to allow off grid operation.