How to install a 12 volt automotive relay to power a 3 way camper refrigerator.

In this post I don’t just want to show you how to hook up a relay I will also show you how to make it turn on and off with your Ignition switch.  Absorption type or 3 way fridges usually work poorly when run on 12 volt while driving and the most common reason for this is voltage drop, caused by inadequate sized cabling and loose connections and long cable runs. You need to run 8 or 6 awg from your starting battery through a relay then to your fridge so that the 12 volt heating coil at the rear of the refrigerator receives a high enough voltage.

In this post I’ll show you how to wire a 4 pin 12 volt  relay with a resistor to power a 3 way RV fridge in a camper. Relay wiring can be confusing for most people so  I’ll try to simplify it for you.  You should use a relay with a built in resistor because it helps prevent voltage spikes that are created by the relay from damaging other electrical components in your electrical system.

To complete this project I’ll be using :

  • A length of red and a length of black 8 awg cable.

  • A Hella 50 or 70 amp 4 pin relay with built in resistor.

  • An add a circuit fuse tap

  • Cable ties

  • A length of red and a length of black 15 or 17 awg cable.

  • Some crimp on female spade terminals.

  • Some crimp on eye terminals

  • A test light or a multi meter.

  • A crimping tool

  • Screwdrivers.


Wire strippers, Test Light, Volt Meter, Relay, Cable, Fuse,

Items needed to install a Relay



Depending on their size these  refrigerators  use between 10 to 25 amps when running on 12 volts.  It isn’t practical to run them from solar.  They run well on on gas or ac voltage when parked however when driving they need to run on 12 volts and the best source of power is the the vehicles starting battery and alternator. These refrigerators draw so much power that if you forget to turn them off when you turn your engine off, you’re likely to kill your starting battery. Nobody wants that.

To avoid this problem you can tap into a circuit through the vehicles  fuse panel that is  ONLY LIVE WHEN THE IGNITION IS TURNED ON.  The circuit must shut off when the ignition is off or you’ll flatten your battery and you can use a test light or multi meter to find the circuit .   When you find the circuit you can use some power from the circuit with an “Add a Circuit Fuse Tap”  (pictured below). You can then run a wire from the fuse tap to your relay (terminal 86).


Fuse Tap.

Add a Circuit Fuse Tap (standard size blade)


Add a circuit fuse taps come in a few different sizes to suit different size blade fuses. There are 6 different types of blade fuse most vehicles use the mini  fuse or the standard. you need to figure out which size blades your vehicle uses before you order a fuse tap. If your vehicle doesn’t have blade type fuses you won’t be able to use this type of fuse tap.

Test Light

Test Light.


To find a suitable circuit locate the fuse panel in the vehicle and remove cover so you can see the fuses.  Using a test light (above) find a good ground or battery negative and attach the alligator clip.  The top of the spade fuses (pictured below) have two little holes that you can poke the pointy end of the test light into.


Blade Fuse

Standard Blade Fuse


If the test light lights up, the circuit has power.  (Useful tip: Check both holes in the fuse, if only one makes the light work then the fuse is blown and is unusable)

Remember the light must not work when the ignition is turned off.

Showing how to test a blade Fuse with a Test Light.

You can poke a test light into the top of a blade fuse to see if it has power or not.


Testing fuses in a fuse box with a test Light

using a test light to check for live fuses with ignition turned on.


When you decide on a circuit to use, remove the fuse from fuse panel and fit that same  fuse to your “add a circuit fuse tap”  You’ll notice that there are 2 slots for a fuse you have to put it into the correct slot . The fuse tap will come with instructions telling you which slot to use. The other slot is for your new fridge circuit and you will need to buy a 1  or 2 amp blade fuse for this slot, as the relay shown at the top of the page only draws  .16 amps for it’s low amp circuit.

With both fuses in place push the fuse tap into the slot where you removed the original fuse from, and check to see if you have power at the end of the short wire coming from the fuse tap. You can do this using your test light, and you should only have power there when the ignition is on.

Now that your fuse tap is in place you need to find a spot to mount the relay somewhere in between  your starting  battery and your fridge. Keep it away from hot areas especially the  exhaust system.


Diagram of internal workings of a relay

The internal view of a relay in the off position.


A relay has two circuits inside it, one is a high amp circuit that allows current to pass through heavy cables from your battery to your fridge.  The other is a low amp circuit that turns the relay on and off by using a little electro magnet to open and close the high amp circuit.  “Figure A” shows the insides of a relay. The red circuit is the high amp circuit and the circuit is (off or open) because the contacts are open. The blue coil in the middle is the low amp electro magnet circuit

In the diagram below the electromagnet is activated which pulls the red contacts together, completing the high amp circuit and allowing current to flow to the fridge.


diagram of inside of relay turned on.

The internal view of a relay in the closed circuit position, or (turned on)


You now need enough heavy cable to run from your battery positive+ to your relay, then from your relay to your fridge this cable should be red. I’m using 8AWG cable because with smaller cable you get too much voltage drop, especially over a long distance,  then the fridge doesn’t work properly, and that means warm beer.  Many 3 way fridges work poorly on 12 volt, and voltage drop is the most common problem. If your fridge is more than 15 feet from your battery you might consider using 6AWG  cable.

8AWG cable with fuse holder and eyelet.

heavy 8AWG cable with fuse holder and eye


When running your cable, crimp an eye to one end, big enough to attach to your battery+. Then no more than about 4 inches (100mm) away you need to attach a fuse holder that can accommodate a fuse that’s rated for your fridge. THIS IS VERY IMPORTANT you must have a fuse here or your vehicle may catch fire. Your fridge will have a label  stating the fuse amperage, mine has a 15 amp heater element and calls for a 20 amp fuse.

Remove the fuse from the holder until the wiring job is complete in case you short out your wires and blow your brand new fuse.  From your fuse holder run cable to the relay ,then crimp a female spade terminal to the end of the cable big enough to fit terminal 30 on the relay.  If you bought a 50 or 70  amp relay the terminals should come in the packaging, if they weren’t supplied you’ll have to buy a couple of 3/8 wide uninsulated female spade terminals.  If you want to make a good reliable crimp job you could solder your terminals to your cable but it’s not necessary .   In the diagram below notice the terminals numbered 30 and 87 are wider than the others.  That’s because they are for the high amp circuit.  The other numbers on a 4 pin 12 volt relay are 85 and 86, they power the electromagnetic switch inside.


Numbered terminals on a 4 pin relay

The spade terminals on a 70 amp relay numbered 85,86,30 and 87.


Now you need to run the same size 8 awg cable from terminal 87 on the relay to the positive + connection on the back of the refrigerator.  This should also be red colored cable .  If the cable is too big to fit the terminals on back of fridge you may have to trim a few strands from the end.  Now you run black colored 8 awg cable from the negative – terminal on back of refrigerator back to the negative –  side of your battery you’ll probably need crimp an eye onto the cable to attach to the battery.  That is your high amp circuit complete.  Don’t install the fuse yet.

You could use a 30 amp relay, but I’ve found in the past that they get a bit warm and heat causes voltage drop. They also have small spade terminals and it’s difficult to purchase small female spade terminals that fit 8 AWG  cable. If you buy a 50 or 70 amp relay they have larger terminals and they usually  come supplied with larger female spade terminals that will fit the larger cable . They also stay cooler.

Now you need to complete the low amp circuit , you can use 15 or 17 awg cable for this as your drawing less than 1 amp for this circuit.  Run a length of black  cable from terminal 85 on the relay using a smaller female spade terminal, to the negative –  terminal on your battery  or a good ground .  Now run red cable from your add a circuit fuse tap in your fuse panel ,to terminal 86 on the relay . Tie your wires so they don’t rub and short out anywhere.   Refit the fuse to the high amp circuit turn on your ignition switch and check to see if you have voltage at the back of your fridge using your test light  or a multi meter.

With this relay wiring setup there is no other switch except your ignition switch, so all you have to do is adjust your rv fridge switch to battery and start your engine. If you wanted to add a manual switch (just to make things more complicated) you could add it in series to the low amp red cable that you ran from the fuse tap to terminal 86 of the relay but there’s no need it just makes the system more complicated

If you have a multi meter  and you want to know how much voltage drop your getting from your battery to your camper refrigerator, you can start your engine and turn on the fridge set the multi meter to 12 volt and place the probes on the positive  and negative battery terminals and make a note of the voltage.  Now go to the back of the camper refrigerator where the two cables attach and check the voltage there and note the difference from the battery voltage. Hopefully this has been helpful and happy camping.


Checking voltage at fridge with volt-meter.

Checking the voltage at the cable connection at the rear of the 3 way fridge.