Issues with Fridge on 12v and system quirks. Advice appreciated.

[Wraithwrider]

Hello all

I've just put a post in the 'introduce yourself' section.

Our van is a Mk 6 Transit that's had a high quality conversion done by persons unknown. I have no wiring diagram.

We've owned it for 4 1/2 years and 10,000 miles.

3 fuses in the fuse box have been failing and this area of the box has been suffering a lot of heat damage. The centre fuse of the 3 is I have discovered the fuse for the fridge from the front 'CAR' battery. I've sort of worked out that this 10 amp fuse wasn't blowing straight away but it was running hot and damaging the 10 amp fuse to it's left and the 30 amp fuse to it's right.

The fridge is a Dometic 3 way unit rated at 120W. Simple maths tells me that the fuse should have been a 15 amp one (my local van electrical expert confirmed this) and I replaced it this last short trip to Kielder (we got back today) with a 15 amp fuse hoping that all would be well.

Previously, when this 10 amp fridge fuse blows I lose voltage indication when the switch on the panel is selected to 'CAR". Oddly however the fridge still works on 12V and the Leisure battery drains. I have been told that the fridge isn't supposed to operate from the leisure battery so this seems odd.

When I fitted the 15 amp fuse I ran the engine and switched on the fridge with the voltage meter selected to first the CAR battery and then the VAN one. Both voltage levels dropped a fair bit (surprisingly so) when the fridge was switched on and the engine note changed too.

Yesterday when we stopped the 15 amp fuse had blown itself apart! The adjacent fuses are fine. The fridge wasn't working on DC from any source now. When hooked up the fridge worked fine on 230v.

This morning I put a 10 amp fuse back in the fridge socked and didn't turn the fridge on, to check out a possible earth short. The fuse is good and for a change the leisure battery kept it's charge.

So there seems there was a connection between the fridge and the leisure battery.

I'm wondering if the fuse blew because the low charge leisure battery was taking current from the CAR battery as well as the fridge current?

An option is to try replacing both relays I guess, if they could cause an odd fault like this.

Any ideas?

Peter

 

[mitzimad]

Check wiring sizes as to small cables could be the problem

 

[pappajohn]

Oddly however the fridge still works on 12V and the Leisure battery drains. I have been told that the fridge isn't supposed to operate from the leisure battery so this seems odd.

You will probably find it's not the fridge fuse blowing but its the split charge to the leisure battery instead as the leisure battery isn't getting a charge from the engine when running.

 

[Wraithwrider]

I'm fairly certain I've sussed it.

I've disconnected the batteries and with a generic 12v wiring diagram in hand I've traced the circuit from batteries through the 2 relays and the 6 gang fuse box.

The fuse that's blowing is a 10 amp fuse, fuse 2. Fuse 1 is 10 amp and is the relay energising supply from the ignition/alternator. Fuse 2 is the +ve supply to both charge and fridge relays! The charge circuit and the fridge circuit both have 30 amp fuses. So, covering the input circuit is 10 amp protection for both fridge and charging which have 30 amp fuses a piece!

It should obviously be 30 amps.....

I suspect that a previous owner got it wrong.

I'm going to check all continuity and relay function before installing the correct fuse and trying again.

I can't find any isolation relay. Is it possible there isn't one or is it perhaps in the charger?

Many thanks.

Peter

 

[autorouter]

So there seems there was a connection between the fridge and the leisure battery.

If for some reason there is a connection between the starter battery and the leisure battery, that could be why the 15A fuse blew apart. When the engine starts, the starter motor takes a very high current for a few seconds. Maybe several hundred amps. If the leisure battery is connected to the starter battery while this happens, the leisure battery will attempt to contribute to this high current, and could for example be trying to send 100A or more. That would account for the fuse being totally wrecked.

The leisure battery is normally charged from the alternator/starter battery while the engine is running and the alternator is producing power. The split charge relay is triggered by the D+ (engine running) signal from the alternator. The relay connects the leisure battery to the starter battery once the engine has started, and disconnects when the engine stops. One of the reasons why there is a relay and not a simple direct wire connection is to avoid the leisure battery being connected while the starter motor is working.

Edit: I wrote this before your post #4 showed up on my screen, so maybe it's not relevant.

I can't find any isolation relay. Is it possible there isn't one or is it perhaps in the charger?

What do you mean by an isolation relay? If you mean the relay that isolates the habitation circuits when the engine starts, then it's quite possible there isn't one. Apparently only UK MH/vans have them, Continental ones don't have them.

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[autorouter]

The fuse that's blowing is a 10 amp fuse, fuse 2. Fuse 1 is 10 amp and is the relay energising supply from the ignition/alternator. Fuse 2 is the +ve supply to both charge and fridge relays! The charge circuit and the fridge circuit both have 30 amp fuses. So, covering the input circuit is 10 amp protection for both fridge and charging which have 30 amp fuses a piece!

It should obviously be 30 amps.....

If it's as you say then it should be bigger, I'd suggest at least 50A. On my Hymer there's a 50A fuse just on the split charge relay, and the fridge is on separate wiring all the way back to the starter battery, with a 20A fuse.

 

[Wraithwrider]

There is a charge relay. So the car battery is isolated from the van battery until the engine is running.
One fuseway is used for the alternator supply to energise the pair of relays, both charge and fridge. I feel this is unnecessary as the current draw is 300 to 400 mA. I can always use a small inline fuse here if it needs a fuse at all.
I’ll make this fuseway redundant and split the feed to the relays using the now spare fuse. I’ll use 15 amp fuses in each and the same the other side.

I’m going to replace the fuse box as it’s suffered heat damage and I’ll label all the fuses. I’ll update.

I suspect that someone has interpreted the wiring very wrong at some point and fitted completely unsuitable fuses.

The 12 volt system in our van is quite basic. Like the van itself really. We don’t have huge funds and other pastimes eat into our resources.

 

[autorouter]

Previously, when this 10 amp fridge fuse blows I lose voltage indication when the switch on the panel is selected to 'CAR". Oddly however the fridge still works on 12V and the Leisure battery drains. I have been told that the fridge isn't supposed to operate from the leisure battery so this seems odd.

Maybe the fridge relay is a changeover type, which is has 5 contacts: 2 for the coil, 1 Common (COM), 1 Normally Open (NO) and one Normally Closed (NC). Most relays only have 4 contacts, and don't have the NC contact.

The fridge is connected to the COM, Alternator/starter battery connected to NO. The difference is the leisure battery may be connected to the NC. If that's done the fridge will always work from 12V if it's set to 12V. From the alternator/starter battery when the engine is running, and switching to the leisure battery when the engine stops. There should be a fuse in the wire from the leisure battery, so you could remove that if you don't want it to run from 12V when the engine isn't running.

One fuseway is used for the alternator supply to energise the pair of relays, both charge and fridge. I feel this is unnecessary as the current draw is 300 to 400 mA. I can always use a small inline fuse here if it needs a fuse at all.

It definitely needs a fuse, to protect the wiring in case of a short circuit, but fuses on the D+ (engine running) signal wire are usually small, maybe 5A or even 2A.

If there's a supply wire from the starter battery/alternator that's feeding both the fridge relay and the split charge relay then it's going to need a big fuse on it. I think the biggest standard blade fuse is 40A, and even that might not be big enough. You may have to go for a larger Maxi blade fuse, which you could fit inline if you don't want to modify the existing setup too much.

 

[Wraithwrider]

If there's a supply wire from the starter battery/alternator that's feeding both the fridge relay and the split charge relay then it's going to need a big fuse on it. I think the biggest standard blade fuse is 40A, and even that might not be big enough. You may have to go for a larger Maxi blade fuse, which you could fit inline if you don't want to modify the existing setup too much.

I intend splitting the supplies to each relay and using a 30 amp fuse in each. Both relays are 4 pole. I'll fit an inline fuse to the signal path with a 2 amp fuse.

To think that both supplies were combined and a 10 amp fuse was fitted......

 

[autorouter]

I intend splitting the supplies to each relay and using a 30 amp fuse in each. Both relays are 4 pole. I'll fit an inline fuse to the signal path with a 2 amp fuse.

The usual fuse size for a wire with just the fridge supply is 20A.

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[Nedge68]

Have you checked that the relays are OK and are sufficiently rated?
If the wrong fuses have been fitted it is possible that the relays are also not correct, I would expect that a high power relay be fitted for the split charge.