PMS3 fridge connection - why is it wired to the car battery?

[charlie2]

Hi, I've just bought a second hand T5 campervan, and it's great! However, in trying to work out the electrics, I had a question about the recommended wiring diagram that's in the PMS3 manual. It suggests that the car battery is connected to "FRIDGE IN", via a relay that only closes when the ignition is on.

As I think the fridge can be powered with the leisure battery, and the car battery is also connected to the leisure battery when the ignition is on, I'm trying to work out what the extra connection from the car battery to the fridge is for. Any ideas?

 

[Ads_Essex]

Hi, I've just bought a second hand T5 campervan, and it's great! However, in trying to work out the electrics, I had a question about the recommended wiring diagram that's in the PMS3 manual. It suggests that the car battery is connected to "FRIDGE IN", via a relay that only closes when the ignition is on.

As I think the fridge can be powered with the leisure battery, and the car battery is also connected to the leisure battery when the ignition is on, I'm trying to work out what the extra connection from the car battery to the fridge is for. Any ideas?

Hello and welcome to the forum.

Are you referring to the two diagrams on page 3/3? If so, it looks to me like you should be following the ‘Caravan / Motorhome Wiring’ diagram and not the ‘Car Wiring’.. presuming your T5 has a leisure battery setup

 

[TallPaul_S]

I really dislike these all in one systems, but... After having a quick read I think it's trying to power the fridge from the starter battery when you're driving, and from the Aux battery when not driving. Quite why you'd do that, I'm not sure. Just a quirk of a system which bundles everything (starter battery charging, aux battery charging, 12v supply and 240v supply) into a single box.

 

[TallPaul_S]

Hello and welcome to the forum.

Are you referring to the two diagrams on page 3/3? If so, it looks to me like you should be following the ‘Caravan / Motorhome Wiring’ diagram and not the ‘Car Wiring’.. presuming your T5 has a leisure battery setup

That's one wiring diagram I think - the car wiring side is the starter battery side and the motorhome wiring side is the aux battery side. It's separated in that diagram because it's designed for caravans which have 2 physically separate systems, only connected by a 7 pin plug.

 

[Ads_Essex]

I’d agree, but I’m of the understanding that with no towbar electrics installed, the socket diagram on the right hand side and subsequently the whole diagram on the left hand side can be ignored.
Again this will depend on the OP’s leisure electrics, but with it being common to have a VSR installed on a T5 and earlier, the split-charge will run the PMS from the leisure battery

 

[roadtripper]

It's because the PMS3 was originally designed for caravans and 3 way Gas/240v/12v absorption fridges that work by heat. That's also why one of the 240v breakers is labelled "fridge"

They would usually be powered by 240v or Gas on site but would be connected to the tow vehicle alternator while travelling to try and keep them cool.

Towing electrics have limited current so there is a separate line for charging the caravan battery and running the fridge as an absorption fridge will easily pull 8-10A continuous.

Generally in a small campervan today you use a compressor fridge that's always powered by 12v and draws about 5A intermittently.

The old PMS systems are easier to understand if you have the mindset "what happens if someone keeps using a caravan all in one system design from the 90s in a campervan 20 years later"

 

[charlie2]

Towing electrics have limited current so there is a separate line for charging the caravan battery and running the fridge as an absorption fridge will easily pull 8-10A continuous.

Ah, that makes sense! Thanks so much for the replies everyone. And yes, I think you can ignore the sockets on the diagram and assume the left is directly wired to the right.

Another oddity is that someone has installed an aftermarket switch for the fridge into the PMS3 panel, as they were claiming that the fridge couldn't be turned off, even when the ignition was off and the PMS battery selector was set to off, which was draining the leisure battery when the fridge wasn't needed. I'll test this, it seems odd that the fridge wouldn't just be turned off with the other 12v circuits when the "battery selector" switch was set to off.

 

[charlie2]

Ah, just found out that the fridge switch is for when the battery selector is set to Van, but you don't want to use the fridge, just the lower power appliances / lights.