Volvo towbar wiring for ignition-on circuit

Before voltage sensitive relays became the norm that was how they were wired till about 10 years ago.If I remember correctly power is cut from the charging and fridge circuits whilst the engine is cranking.

Not a lot of help to you....But....I have an XC90 (2017) with factory fitted towball and electrics. I have never experienced any problem with fridge etc. I have parked up at service stations for about 45 mins and never had any problem on return. Never checked to see if fridge remains on when just plugged into car, though. I wouldn't worry about it.

This seems to be how it is with most modern vehicles, and definitely with Volvos.  I had a Jag X-type new in 2004 and had just this issue, so I made the dealer fit a voltage sensor type relay so the fridge feed only came live when the charging voltage when over 13.5V.  My next car was a Volvo XC70 with dealer fitted towbar and I had the same thing that you are describing so I investigated a bit further and found thet the Caravan Club had updated their info regarding towing electrics to say that this is now the norm, so I left it and never had a problem.  I now have a 2017 XC60 and have the same set up, again with no problems.  So, I wouldn't worry about it.  Quote from the CAMC  guide on towbar wiring:

"At one time, access to an appropriate connection on the vehicle alternator made it straightforward to ensure this point was only once the engine was actually running, and not just when the ignition was turned to on. Modern engine bay and alternator designs limit such access considerably, often making a direct connection to the alternator impractical. Some wiring kits just use a connection to an ‘ignition-on only’ power supply. These successfully prevent the fridge flattening the car battery while parked, but do not protect against the situation where the car battery is already flat and the engine attempts to start from the caravan one. However, the Club has no reports from members of such problems occurring in practice."

I would guess that the circuitry in the caravan has diode's fitted so the current only goes one way from car to caravan and stops it going from caravan to car.

Thank you all for your comments and suggestions.

@Vulcan

I should have said that I checked to see if power was cut while cranking the engine, as it is for the radio for example, but it isn't: pin 10 remains live while starting the engine.

@NevChap

Pin 10 is only live when the ignition is on; it's pin 9 that is constantly live so a properly connected fridge, i.e. to pin 10, wouldn't get any power from the tow car while parked up with the ignition off.

@meridienmick

I have the Caravan Club document and had noticed the very point you make, i.e. this seems to be the norm but no one seems to have a problem with it.

@lornalou

Diodes in the circuitry would certainly prevent the starter motor drawing on the caravan battery via pin 9, but there is significant voltage drop when using diodes which could impact adversely on the charging rate although using Schottky diodes apparently largely overcomes this.

It would seem that this setup is the norm and in ordinary day to day use is only going to cause a problem if the tow car battery is in poor condition.

Thanks again.