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Old 21st May 2021, 21:20   #1
Antisyzygy
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Default 12s Electrics.

Hi guys, new member here, have been perusing some of the old threads in search of info.

I’m having problems with the 12s socket on my 2001 75, specifically the pin that powers the fridge whilst driving. As an aside, the plug pretty much disinteregrated when I took it apart to check the wires so I will probably upgrade the two 20 year old plugs to a 13pin - so if anyone has any info on that it would be more than welcome.

At present, the relay is the main suspect and a new one is on the way from
Lithuania and will be here in a week or two- in the meantime, I’m trying to rule out other possibilities and I have checked all the fuses that seem likely - but does anyone know specifically, what fuse would power pin
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Old 22nd May 2021, 08:49   #2
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Just curious about the possible 20 amps to the fridge.

It seems a lot ( Like 4 headlamp bulbs. ) A fair old additional load on the alternator if the rest of the cars electrical items are in action.

An old battery would die in seconds without the engine running.---
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Old 22nd May 2021, 09:07   #3
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Are you testing at the pin with the engine running ?


There is, should only be, a supply to the fridge pin and the caravan battery charge pin, only when the car engine is running and the alt has reached a specific voltage.


This is to ensure that with the engine stationary supplies to both are cut off to prevent the car battery being discharged.


However there is a permanent supply to the van, engine off, this goes to the habitation relay in the van, or on older ones, the van-off-car switch, this is to allow the 12v lights, water pump and toilet pump to be used on comfort stops, but has no connection to the van battery.
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Old 22nd May 2021, 09:09   #4
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Quote:
Originally Posted by COLVERT View Post
Just curious about the possible 20 amps to the fridge.

It seems a lot ( Like 4 headlamp bulbs. ) A fair old additional load on the alternator if the rest of the cars electrical items are in action.

An old battery would die in seconds without the engine running.---
The fridge should only be powered when the engine is running.
I have mislaid my two handbooks but I think my two cars are wired differently
as I once left the caravan connected to the car overnight and had a flat battery in the morning.
something to do with pins 9 and 10 on the 13 pin socket, not sure what the equivalent is on the ops socket. Modern caravans apparently have a relay that will disconnect the fridge from the cars battery if the engine is not running, not sure if my caravan has this relay or if it works

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Old 22nd May 2021, 09:24   #5
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Quote:
Originally Posted by Antisyzygy View Post
I’m having problems with the 12s socket on my 2001 75, specifically the pin that powers the fridge whilst driving.
Reading other members' replies I've deleted my first response because I now understand that Andy is asking about a towing socket!

Simon
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Last edited by SD1too; 22nd May 2021 at 10:02.. Reason: Realisation of the truth!
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Old 22nd May 2021, 09:47   #6
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I guess we could do with a bit more input from the OP to fine tune our replies to him.---
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Old 22nd May 2021, 10:50   #7
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Though this is not directly to do with wiring a 12 and 12s into a 13, simple if you take your time, are methodical and get a diagram.


Some things to be aware of, some modern vans have ATC so to future proof, wire up for this also.


Many van owners complain of the fridge not cooling sufficiently on 12v, nor the van battery not being kept charged on long journeys.


The usual cause is volt drop between the car and van due to thin cabling, in extreme instances, around 14v can be seen at the battery, engine running, but only 10-11v at the fridge terminal block itself and at these voltages the fridge won't cool nor the battery charge.


If you tow a van and find this, there are a couple of work rounds, one is to utilise spare pins in the 13 pin socket to double up the + and - cables, or wire a dedicated supply back to the battery + and a good car chassis - ,you should find details for these on the net and possibly caravan forums.


One other one thing that won't apply to 75's though, but will to many new cars, I've put this in because there may be 75 owners who tow with their 75 and a newer car, is the problem with smart alternators in that some, if not most, reduce the charge rate from the normal approx 14v to around 11v when the car battery is fully charged, to save fuel, kicking back in when the car battery voltage drops to a certain limit.


The outcome of this is that when the voltage is at 11v or thereabouts, the fridge doesn't cool, nor the van battery charge, there are work rounds to this, one simply disconnecting the alternator sensing wire at the battery where fitted, the other is by getting the dealer to alter the alternator control criteria, for a fee.
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Old 22nd May 2021, 12:04   #8
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Although now not a caravanner any more, I used to be and have wired the sockets on a number of cars.
The fridge supply should only be live with the engine running, so as not to give excess battery drain. You can buy, as I originally dig, special relay boxes for the job. However, as an electronics engineer I baulked at the prices charged, and subsequently fitted a normal car rely (I have plenty spare from scrapped acars). This is wired so that the coil energises only when the ignition is live.
The wiring to the contacts of the relay have a feed from the normal +12v (ie live battery) via a fuse - as you suggest 20A or maybe 30A, to protect the load on that circuit. So having now got a switched (via ignition) feed, this is then wired to the 12S socket.

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Old 23rd May 2021, 08:25   #9
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Thanks guys, have been testing with engine running but only idling, will try with revs up to see if that makes a difference, but light on fridge definitely used to come on at idle speed so would have thought there should be some power there, at least.

My understanding is that the 12s won’t make a warm fridge cold, but it will keep an already cool fridge at temperature.
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Old 23rd May 2021, 09:03   #10
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My understanding is that the 12s won’t make a warm fridge cold, but it will keep an already cool fridge at temperature.


It should, but it depends on the voltage at the fridge heater element terminals, to see if you have excess volt drop, due to cable length, cable too thin, poor connections, get the car running at a fast idle, prop the accelerator pedal slightly or get some to hold it at approx 2,000rpm, measure the voltage at the battery terminals, say 14v, then measure the voltage at the fridge 12v heater element terminals, if all's well you should get at least 12.5v to 13v, anything less and the fridge will barely cool or maintain a low temperature, if at all.


It's not unknown due to length of cable, inadequate size, corroded connections to see 10v at the fridge, obviously it wont cool, but folk say it should do because there is 14.5v at the battery, its whats at the fridge that counts.







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