12V

[john_5063641]

Please can some kind person help. We have been away touring Spain for the last 6 weeks but had problems with the 12V with the car running. It seems that when we turn the fridge on the light keeps flashing on and off. If we put the caravan lights on at the same time they flash on and off so that when the fridge is on the lights go out then the lights come on the fridge light goes out. John.

 

[rayc]

John, with post 98 vans there is a habitation relay in the van. It is operated by the fridge supply from the car which is pin 6(+) and pin 7 (-) of the 12S cable. When the relay operates it switches the 12v system in the van off ( causing the lights to go off ) and swithes the permanent feed on pin 4 (+) and 3 (-) to charge the van battery.

The reason your lights are flashing on and off is because there is a problem with the fridge supply, propably the live on pin 6. When the engine starts the supply is there which switches the lights off but the fridge load causes the voltage to drop which releases the relay which allows the lights back on. This is caused by either a high resistance in the supply to the voltage sensitive car relay, or if the relay has an adjuster it needs to be set. To do this start the car engine and with the van connected adjust it so that the van lights go out and stay out but not tooo far. The relays are also notorious for hanging on when the engine is stopped preventing the van lights from coming on and the 12S cable has to be unplugged.

Hope this helps, any questions then please come back to me. Ray

 

[WatsonJohnG]

Ray is spot on in my opinion.

The voltage sensing relay is used as it avoids the need to have a wire from an ignition controlled source and thus saves the fitters time.

I have helped lots of people on rallies who have had a problem with voltage sensing relays and when I fitted one to my car the fridge was still on half an hour after the engine was switched off .

I would either replace the voltage sensing relay with a new adjustable one (I hear that they are better now) or as I have on my car replace the voltage sensing relay altogether with a normal relay of the type used for spot lamps etc.

 

[BigAl1]

I don't think you have a problem at all. I assume that the towing vehicle's engine is running at idle speed when you experience this 'problem'. When you switch on the fridge to draw its 12volt supply the overall battery voltage drops a little which will trip the relay that sends the car's battery 12v supply to the fridge and so the fridge stops to operate and the green lamp goes out (if there is one). Also when this relay trips it stops charging the leisure battery and instead powers the interior lamps of the caravan(via the switching relay within the caravan).

When travelling at road speed, the engine is running much faster so the alternator is supplying a higher rate of charge and a voltage of about 14v, therefore the fridge runs constantly and the leisure battery receivs a charge, that is to say until the engine revs fall and the switching relays do their bit again.

I am sure any 'sparks will either explain this better or say I am totally wrong.

 

[rayc]

John, further to my earlier reply there are 3 previous postings in this section of the forum that are relevant to your problem.

'fridge not working on 12v' 3rd July , '12v fridge problem' 26th September and 'fridge 12v intermittent' 26 August.

John (G): its amazing the number of reported problems associated with the 12v fridge supply. There must be a lack of training or knowlege with towbar fitters and the operation of the voltage sensitive car relay - but I guess it gives the fitters repeat work. Nice to see your still posting. Ray

 

[rayc]

Alan B, I will not argue your point but alternator output is fairly stable across the rev range compared to generators of old. If there is a volts drop at low revs the last thing you want is for the fridge supply to drop out everytime you stop at traffic lights or are in a queus of traffic etc.

The relay is normally set to operate at tickover with the fridge switched on and car headlights on.

I think that is enough technical ttalkj for me on a Saturday night.

Ray