Wiring Up Your Tow Vehicle for a 12 Volt Fridge Supply

This is an ambitious DIY project, but when broken down into small chunks it is not too hard and these instructions are deliberately verbose to give you lots of in depth information so you don't worry tooooo much! A top auto electrician will charge around $180.00 for this job. We really aren't doing it to save money, but to get it done 100% correctly as not all auto electricians do "Virgo Quality" work (probable because they have to earn a living!). The big plus is you will learn new skills along the way. We will be running a wire directly from the battery to a self resetting overload relay, then to the rear of the car to a relay (an electrical switch) that is only "on" when the ignition is switched on - this way it is impossible to flatten your battery by forgetting to switch the fridge off battery. Your vehicle already has quite a few of these with the headlights being a good example - listen for the "click".

We have to go to all this trouble because fridges draw from 8 to 10 amps which is a very high current. Electrolux will be releasing models with a HIGHER current draw in 2002 - up to 15 amps in the larger "tropical" models.

Pretty ambitious stuff, but if you allow half a day and potter along you will end up with a top job and your fridge will work as perfectly as others we have helped and ours does. Step one is to go down to your auto electrician and get the following:-

•  6mm red cable - allow 3 metres longer than your vehicle - allow $17.00 - $25.00
•  6mm black cable - allow 3 metres longer than your vehicle - allow $17.00 - $25.00
•  1 X 20 amp self resetting relay - allow $6.00 - $10.00
•  1 X Bosch/Hella Relay NORMALLY open and single pole - allow $6.00
•  1 X Bosch/Hella relay plug in base - allow $3.00 (if fitting under the dash only)
•  1 X Hella socket 10 amp 2 pole
•  1 X In-line 3AG fuse holder or similar
•  1 X packet cable ties - the ones about 4" long are easiest to use
•  1X one metre length of split looming conduit - 12 mm is easiest to use
•  Assorted terminals from a kit or you can buy small packs for $2.75
•  2 X 15amp spade terminals (non-insulated type)

A combined brake and fridge overload

Neat and safe spot on a Forester

Before we start a few explanations and don'ts! 

Most of you know that the return wire or earth is actually the frame of the vehicle. We don't rely on this with fridges or brakes (due to the coupling) so two cables MUST be used. If you go on and do your brakes the same logic applies. There is a slight disadvantage with this system in that you do need larger cable to compensate for the voltage drop in the return cable run. Unlike 240 volt, direct current (DC) suffers loses directly proportional to the length of the cable. The advantage of course is absolute circuit integrity. To guard against electrical fire hazard plus protect the appliance and wiring, a self resetting overload and an in-line fuse MUST be used. An overload is a relay/fuse that cuts out when a set amperage is exceeded or there is a short circuit. It MUST be located as close to the battery as you can physically get to prevent fires etc. You MUST keep it away from hot areas such as exhaust manifolds otherwise you will get false triggering. There is heaps of info in PDF format (215K) at www.colehersee.com.  You can use either the self resetting type or the plain vanilla simple overload. These are dearer and fairly uncommon for automotive use but VERY common in the marine area - Whitworth Marine is a good source.

Diac protected relay in box!

Hella Relay socket 4973

A relay plug in base is just a set of sockets suitable for easier mounting and painless replacement, particularly useful in hard to get at places like up under the dash etc. They are also good for easy trouble shooting as you can quickly test by simply plugging in another spare relay. By using insulated terminals or intelligent mounting you can leave it out. We don't use one on Foresters as the location behind the rear number plate is super accessible, but suggest you do as you could live to regret it further down the track if something goes haywire. 

Most 4WD's have an auxiliary 12 volt socket located in the boot area. No matter what any "expert" tells you please don't use this wiring as it has ZERO chance of being a single dedicated circuit with sufficient amperage available. Just because your fridge pilot light comes on when you plug into this doesn't mean it's OK. Try measuring the voltage when operating and you will find it way down. Low voltage causes wires to overheat. Eventually you will MELT associated wiring in the loom of your car. If you don't believe us give Electrolux a quick ring for their advice. By the same token you can easily run an Engel fridge off rear accessory sockets as this is more or less their intended use.

Why are we using a Hella fridge line socket instead of the good looking A'van Narva one? The Narva is aluminium and if you look at yours chances are it is covered in a white oxide coating similar to verdigris - not good! It also uses blade terminals. We try to keep the number of blade terminals to the absolute minimum - best practice. Both Hella products use tight forever, screw terminals. Remember you MUST protect any cable passing through a metal hole. Avoid anything rubbing against a cable particularly clutch and brake pedals.  

Under NO CIRCUMSTANCES leave the ignition on whilst stopped for lunch as this defeats the whole purpose of using the relay - unbelievably most very expensive motorhomes don't even have a relay. Use the gas or if you must leave the motor running. Be aware that you can cause an explosion if your fridge happens to be running when you refuel. It is also illegal. Electrolux suggest you always travel with the gas turned off at the LP bottles. You must NEVER have the gas and 12 volt running together - this CAN happen easily on the small fridges with electronic ignition. Be very aware of this if you have the small fridge as it is not IDIOT PROOF! The RM2330 90 litre models as fitted to most high sided units, are totally idiot proof, as it is impossible to run two power sources simultaneously. Never use 240 volt cable for automotive use. Sorry about all this info, but the background is important. You are now hopefully a fridge wiring expert!

Best to tie the new cables to existing wiring

Passing through the firewall

•  Mount the overload - site it as close to the battery as possible. If a brand new vehicle and you are doing the brakes as well it makes good sense to mount the brake overload adjacent to the fridge. We did this and it looks really neat. Look at our pictures for ideas. Our spot on the Forester between the washer bottle is perfect and very safe. Put the in-line fuse in the same line after the overload. If using a marine overload you may have to make a small bracket. Similarly if you use the screw in fuse holder you may also have to make a bracket.
•  Locate a spot to go through the firewall. This is a bit daunting the first time you do it. Please DON'T drill holes. Look for large rubber grommets and gently poke a pointed piece of fencing wire through. A helper is essential for this. Attach string to your wire and gently pull through. Attach your red and black  wires to the string and pull through.
•  Next feed the wire down to floor level trying to avoid anything that moves or rubs. Remove the trim from the bottom door sills and feed the cable along here. This is preferable to running along the centre tunnel if your vehicle is constructed this way. NEVER run cables under the vehicle.
•  Continue right to the rear of the vehicle with the black and red cables.
•  Get your meter out and set to DC volts. Get a helper to turn the ignition on and off whilst you try to locate a circuit that is operated from the ignition. Look behind the covers of taillights. We need this to turn the relay on and off. This is only a small load so it can be any circuit. If you have a cigarette lighter socket this circuit is ideal. We had one in the Forester, but couldn't get at it. 
•  Find somewhere to mount the relay. Ours is in a brilliant spot - in a lovely little well directly behind the number plate. Heaven help the next guy that buys this vehicle if it fails as it could be a real head scratcher! Nice people leave a note in the log book!
•  If you can't find a live wire connected to the ignition at the back you will have to mount the relay under the dash using the plug in base. Hint: look near the cigarette lighter socket or the radio.
•  With the ignition off run a piece of 4mm auto cable from the point you just found (use a piggy back spade adaptor or cut the wire and use a crimp joiner) and put a spade terminal on the relay end and connect to the COIL terminal. The other side of the coil goes to frame (earth). Anywhere on the vehicle. Again it only needs to be 4mm cable.
•  Turn the ignition on and off - you should hear a faint click come from the relay.
•  Carefully route both the red and black cables from the firewall back to the overload. Tie it frequently using cable ties and use the split loom cover wherever you think necessary. On some jobs we went the whole way and it looks better.
•  The red wire goes to the overload. Crimp on a terminal and attach to the overload and tighten. The black goes to the battery earth.
•  Put a terminal on the earth wire and attach it directly to the battery. Again use ties to neaten things up.
•  Run a 6mm red cable from the overload to the battery positive. Crimp on terminals. Connect to overload but DO NOT connect to battery at this point.
•  Using a 15amp push on spade connect the red hot wire to the Normally open relay contact.
•  Drill hole in bumper and roughly fit Hella socket.
•  Connect the earth wire to the socket. Run a short length of 6mm red from the relay N.O. terminal to the socket and do everything up tight and neaten with ties and clips.
•  Connect battery wire.
•  Attach A'van, plug in  and test. Turn on the switch on the fridge (red - or turn to DC on the 90 litre model). Start motor and the switch should illuminate BUT nothing happens on the large fridge!!!! You must feel for heat on the outside vent (poor design folks!)

The Hella socket on a Forester

Neat spot for "hiding" the relay on a Forester

Doing anything for the first time usually throws up a few problems and we always tell you about the stupid errors/oversights we make. This installation was done in a Forester so your vehicle may need minor changes to these instructions. We had zero problems on our vehicle and many others, but in helping Siggy Nowak (fellow A'vanner) on his Forester the thing would just not work and it ended up being finished under floodlights! Somehow we had been given a relay with different pin outs. We never suspected this, as that's why we use a plug in base to make the job idiot proof. This one came in a plain wrap so it was obviously mixed up back down the track. So the trick is to make sure the relay "ticks" when you turn the ignition on.

TIP: Place a piece of dowel on top of it and in your ear, and get a helper to turn the ignition on and off.

If you are reading this with the intention of buying an A'van you can save yourself a lot of problems down the track by requesting that they fit a LARGE UTILUX or preferably a 12 pin Brylite flat plug. A'van are extremely obliging in this area and will fit anything you ask for if it is being made at the factory. Obviously you must tell them what you want at the time of ordering. The Brylite has seven normal trailer pins BUT it then has an extra five heavy duty pins located in a row under the normal set. These are rated at 20-25 amps. This is the better option as it is future proof.

Beautiful example of using an Anderson plug

Lots of interest in fridge electrics at club meets!

The 15 amp spades only come non insulated

Relay mounted without a base

Very neat tow sockets on a Forester

Anyway, if you successfully complete this project you can truly be proud of yourself, as it is a cut above your normal DIY job and the knowledge gained will remove most of the mystery out of 12 volt electrics and set you up for life in this area. Don't forget to share the knowledge around with your friends though. Please don't hesitate to email us if you have the slightest problem. We are here to help! If you do a terrific job and have any tips to smooth the way for others please send photos and details. Please oh please, let us know of any errors in the text! We would like to thank all those who have written in with suggestions for making these instructions clearer. We have taken them all on board and this page has has 11 updates! Thanks particularly to those in the "trade"!

Bob Eustace

Refrigerators
Message 2 on this topic
From: Geoff Mower [2002-03-21]

I suspect that you will get a swag of messages advising John Ludviksen not to run his fridge on gas while travelling. At best it will almost certainly go out, and his food will spoil. At worst, of course, he risks blowing himself up! (I’m sure I recall a fairly recent report of an explosion caused by a camper-trailer owner pulling into a service station with his gas fridge running.) The gas supply should always be turned off at the bottle while travelling.

I recently wired my VX Berlina with a brake controller and 12 V hotline with relay. It is a bit fiddly, but well within the capabilities of the average handyperson(!). I was guided by the comprehensive instructions (fridge and brakes) and illustrations given on the A-vanners’ website, Around Oz. This is an excellent site, and well worth visiting, even for non-A-van owners.