D+ supply

Interesting, we have the Fiat automatic with a smart alternator and don’t have the fridge characteristic you mention. Can you quantify the time delay in the fridge switching off?

The supply to the fridge in its 12V mode comes from the vehicle battery but is only used when the fridge senses a D+ signal. Whether or not the vehicle has a smart alternator is immaterial in this context, though it does affect leisure battery charging which is another matter entirely.

In answer to your question the 12V supply comes from the converters socket which is in the base of the B pillar behind the driver’s seat. This connects into the Sargent EM40 panel, usually located nearby, which is the interface unit between vehicle and habitation electrical systems that AS fit. You’ll most likely find it underneath the bench seat at the cab end, see picture below. The D+ signal and fridge 12V supply are derived from this panel, via their respective fuses.

In trying to understand why your fridge is slow to switch off once the engine has stopped, you could pull the D+ fuse upon stopping and see if it makes a difference. That will then tell you whether the characteristic is due to the fridge electronics or due to an anomaly in the D+ signal to shut down. 

If the latter then I would suspect a fault in the EM40, which has to invert a D- signal (a so-called “active ground”) that Fiat provide at the converters socket. Maybe a relay that’s slow to respond. You could seek advice from Sargent, who I’ve found generally helpful in answering queries.

A circuit diagram of the EM40 is shown below.


Tim

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We don’t use the fridge on auto, having been caught out by an undetected interruption in 240V supply resulting in the gas tank being emptied. Consequently I’m not familiar with the changeover process, but I thought the 12V mode would still cease on engine shutdown and that the gas ignition was delayed by 15 mins. But I’m not certain.

Tim

That’s all very odd. We don’t operate the fridge on its auto changeover setting and when the engine is turned off the fridge immediately stops operating on 12V, as it’s designed to do. On starting the engine we never get a low battery warning.

What sometimes does happen is the engine cooling fan comes on when stopping and turning off the ignition, even in cold weather. That will then trigger a low battery alert.

As for start/stop, it doesn’t operate unless the vehicle battery is fully charged.

Tim

Ah, thank you, I didn’t realise the regeneration would work after engine shutdown. I’ll do some homework on it.

Your experience at Sargent in having a B2B fitted will be of interest to several of us on here. Hope it’s a positive one.

Tim

There are TWO 12v supplies to the fridge.  

One is a permanent feed from the leisure battery that is there whether or not the PSU is switched on or the engine running.  This supplies the fridge control panel so that the fridge can be operated on gas without EHU, habitation 12v or engine run.  When the fridge has automatic fuel selection invoked, the control panel will manage a delay between losing the 12v engine supply and enabling gas.

The second 12v supply is from the starter battery/alternator (and with a split charge rather than B2B charge, also the leisure battery).  This supply energises the 12v elements that cool the fridge (careful choice of words here) and is switched on/off by a relay whose operation is controlled by the D+ output from the alternator.

The D+ also tells the habitation electrical control system that the engine is running to disconnect that system from the habitation load, retract the step etc. (in some MES systems its a motion detector that controls fridge and step supplies).

I don't know is their is a difference in operation between auto fridge operation between Thetford and Dometic and the first auto fridge I've had is the current Thetford - another of several facilities that I swore I would never use.... until I found out how useful it is, particularly on foreign sites with dodgy electric supply.

Dbvwt wrote:Not all vans have a 12v LC feed that is direct from the leisure battery. Mine was fed from the EC500 and was disconnected if the EC500 was switched off. I ran a permanent 12v feed myself to the fridge control panel to overcome this.

I was certain with previous vans but have never switched my EC500 off. Not messing with anything at the moment as need to resolve an airbag fault indication before next week.

I had to go and try and found that when my EC500 is shutdown it does remove the permanent supply from the fridge AND from the Truma wiping all stored settings and the time!

None of my previous van Sargent systems had a shutdown facility and permanent supplies were permanent (I teed off the fridge to give a permanent supply to motorhome wifi) but I had an excellent dual channel solar system maintaining the batteries. I leave this one hooked up on the drive as the smart charging ensures neither battery gets overcharged and I'm in the van most days for one thing or another.

I find the real beauty of the auto fridge is that when in a hot europe, visiting a town, chateaux, etc. I don't have to remember to do anything with the fridge (or gas bottles, having a fixed lpg tank)