3 way fridge question

Depends on how the original tug was set up.
It would be wise to run a separate heavy permanent supply to the rear Anderson plug bypassing the bcdc under the bonnet and ignition feed to the van through the trailer plug to turn the fridge on and off with the engine fed through a relay off the direct battery supply, don't run the fridge through the bcdc in the van.

If you feel the need to run off batteries during short stops then an over ride switch to run the fridge off the van batteries, but this is inviting human error.

Is it a compressor fridge or peltier?

As above, it depends on how the fridge is wired up. When you select 12V as the energy source, it connects up to a 12V heating element, and it will pull about 10 amps whenever you have 12V selected. On my Dometic fridge, the 12V doesn't even go through the thermostat and it runs all the time. This will flatten your battery quickly, so I fitted a Fridge Switch, which shuts the 12 V power down unless there is movement in the van. My van does not have batteries, so the only time the fridge can run on 12V is when it is connected to the car. If you have batteries in the van I suspect that the fridge is supplied via the batteries and the batteries are charged by the car when you are driving. Hopefully the fridge thermostat is also being used otherwise it is going to be a big drain on your batteries.

One thing to check that might save you a lot of trouble.

Has the 12 volt element been fitted, I have seen a few that just aren't there, never been fitted, then if so has it ever been wired in, many people that have had them before don't think they re worth the expense for their intended usage, and if it is there and connected to anything, is it working, again I have seen a few that have blown elements and the owners decide it isn't worth the expense to fit a new one.

Those fridges are designed to run on gas, which they do extremely well, and work wellish on 240, but 12 volts is just there to slow the rate of warming when in transit, not very efficient at all.

Odog
First of all, if you have a DC 1240 under the bonnet AND it gets hot from the engine bay heat via radiated heat, then the DC1240 will have to derate itself to save it's own skin and not deliver 40 amp to anything while driving. I have seen a DC unit 25amp which deliver less than 6 amps due to the heat load forced upon it.

AS I read it you have a a 1240 delivering to a battery which has an Anderson plug attached and it feeds to a 1220 and two batteries in the van. Is this correct?

I don't think a big DC unit running a small DC is correct. You didn't explain exactly the wiring setup though.

If the 1220 sees it still has a sufficient input voltage it will continue to run unless voltage sensing within itself. With a multi meter you can check what voltages are happening when stopped and/or engine running, that will give a clearer picture of it all.
The fridge will take the top off one or two van batteries fairly quickly, 10 amps draw of the fridge is optimistic, my 3 way is more than that..

IF you wish to use the fridge on battery it may be advisable to upgrade the wiring from the batteries to the fridge. Unless the wiring is heavy enough the fridge will still draw lot of amps but less than the heater indicates, that means although you are drawing current, it isn't hot enough in the heater to really do anything so the energy is being wasted and fridge isn't even trying to cool.
I recently rewired, made a shorter circuit and heavy wiring in my van and circumvented the long run to the fridge. With good positive AND negative supply I got 2v increase at the fridge over the original wiring performance while fridge is on 12v. When on road alt directly charging, the voltage at the fridge is 13v.

Hi Odog,
Our caravan fridge has a relay that is connected via the seven pin plug to the tow vehicle and the fridge will only work on 12v if the vehicle is running. A good safety precaution regardless of how the main 12v to the fridge is obtained.
Maybe yours is too. Some of the newer tow vehicles seven pin plugs don't have aux. anymore instead they have a reverse wire. It pay to check it out, Cheers

Odog
Nearly all cvan manufacturers do not add insulation at the sides and/or top of the fridge and so the area gets very warm and the fridge simply soaks in the heat the condensor has just got rid of.
With insulation it is far more efficient and cools down quicker too.
The addition of two 75mm or 100mm computer fans wired in series works well. With one blowing fresh cool air into the bottom region of the rear of fridge and the other fan sucking/expelling hot air out of the enclosure will allow the condenser to get rid of heat much quicker

I have insulated and added fans to a few and they cool much faster and in hot weather the fridge can actually operate properly. Without the fans pretty useless when hot.
The two fans wired in series, each running on 6v, only draw about 80ma to 100ma because they are in series and are also quiet.
I run mine all the time in hot weather. Solar assist can be used too if required.

hi
Unless the wiring can carry around an extra 15amps ,you will need an extra cable .
Always pick up power on the supply side of any charger if needed.
The acceptable size would be at least 25mmsq / 3b&s

OR

Run an extra fused cable 13mmsq/6b&s runs from aux batt in car then onto relay then onto fridge