Alternative to using a Voltage Sensing Relay
Modern Caravans require two 12 volt power supplies from the drawing vehicle to supply their auxiliaries, one is a permanent supply (constant live) and the other only becoming live then the drawing vehicles engine is actually running, terminating immediately
the engine stops.
Historically this controlled power supply was controlled by a standard automotive relay being signalled from the drawing vehicles alternator via the IND output terminal. Most modern vehicle alternators are controlled through the vehicle ECU as so with the
alternator warning light (ignition light) so no IND terminal exists upon these modern alternators.
Some modern vehicles are designed by the manufacturer to control this power supply through the vehicles ECU or Tow-Module when a franchise towbar and tow-electric kit is fitted to a vehicle.
There appears to be only one alternative option for modern vehicles to control this power supply if they are unable to use any of the above methods, this is with the use of a Voltage Sensing Relay (VSR). These VSR's are often referred to as a Split Charge
Relay, although calling them by this name is incorrect terminology, a true split charge relay is something entirely different. These VSR's work quite well as long as they are adjusted correctly and checked annually as sometimes periodic adjustment is needed
due to vehicle battery deterioration which can affect their trigger point.
We now get down to the nitty-gritty;
Some modern vehicles have what is called a Smart Alternator, VSR's are not compatible to be fitted to a vehicle which has a smart alternator due to the alternators voltage regulation can drop off so low at times that any VSR would cut-out and not function
correctly making the caravans controlled power supply unreliable. Someone recently said to me that it will probably take a few decades before tow-bar fitting companies realise that smart alternators and VSR's don't mix.
So what if a tow vehicle is unable to use any of the above methods to control the controlled power supply to a modern caravan ie, Euro Plug (pin 10) or 12s Plug (pin 6)? Well one thing is for sure, a relay cannot be connected to a vehicles standard ignition
switched supply as the caravans' fridge would put a 10 Amp loading on the vehicles battery during cranking and the vehicle would be able to draw additional cranking power from the caravans leisure battery, but in theory a fuse should blow-out within the caravan
before any damage to wiring was likly to happen.
HERE IS THE ANSWER;
This is not an official method, it is not in any books, it is not used by tow-bar fitting companies or vehicle manufactures, this is my own idea/design/invention whatever you wish to call it;
This method is quite simple, using the tow-cars engine oil pressure switch upon the sump to signal a standard automotive relay in a similar manner to which this was achived historically when a relay was connected to the IND terminal of an alternator.
To achieve this the relay must be controlled by a Transistor which will allow the relay to be switched upon the negitive side and also allow it to energise when the negitive side of the relay coil is broken (cut) by the oil pressure switch when the vehicle
engine is started.
My following post will explain how to do this, it is not difficult and well within the scope of most within our Caravan Club as most are very skillful DIY persons.
Snowy