why would the 40 amp fuse blow

[Blaydon]

Hi there,
Question: We have a 2018 Laika on a Fiat. We have a b2b system, two leisure batteries and the usual stuff, fridge lights etc.
The 40amp fuse next to the batteries has blown - a few times. Never straight after replacement but we have now had three go in the last few months. My question is simple - the answer may be not quite so simple - What would make this fuse blow and how to I go about testing or checking what is going on?
Any clues from people far smarter than me would be most appreciated. I can use a spanner and screw drive and with a bit of direction I can use a multi meter.

Cheers
D

 

[Lenny HB]

What size is your B2B as if its a 60 amp one and wired to that fuse it could be your answer.

 

[Insider]

What size is your B2B as if its a 60 amp one and wired to that fuse it could be your answer.

Lenny - would think you’re right but……wouldn’t the B2B be direct into the leisure battery as opposed to the 40A fuse which would be between the leisure battery and control block.

OP - a rough circuit diagram would be ideal

 

[Stealaway]

Is the fuse wire being blown to bits. It's a short.
Is the fuse wire just slightly melted, it's an overload

 

[dabhand]

Check the manual to ensure it should be 40 amps.
40 amp fuse holder damaged, melted inside or badly wired.
Water in the engine bay causing a short circuit
power spikes
Replacing a wrong fuse
faulty or failed wiring

theres a few to be going on with, good luck

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[dabhand]

I shouldn’t think in a 2018 van it will be this, but in my first van (1983) I had the same problem, it turned out to be the fridge 12v sending a spike to a shunt in the electroblock which couldn’t cope with the load, don’t ask me how, but i exchanged the shunt for a 60 amp fuse and it didnt happy again

 

[Realist]

Buy a clamp meter and see what’s putting load on the circuit and then switch off an item at a time until it’s found.

 

[autorouter]

wouldn’t the B2B be direct into the leisure battery as opposed to the 40A fuse which would be between the leisure battery and control block.

There's more than one way to wire a B2B. Smaller B2Bs are often wired through the control box. Maybe the fridge is wired through there too. Which battery fuse blows, starter battery or leisure battery? If the B2B is 30A and the fridge amps is somehow going through the same fuse then that will take it over 40A.

 

[Blaydon]

Hi there - thanks for the clues. The B2B was put in by the guys at Vanbiz. I will drop them a line and see what they say. We have been going to them for years and I am sure they can work some magic, even if we need to take a trip back to see them - we like Taunton and another visit would not be too much of a hardship though diesel prices mean we may need to sell a kidney to cover the costs.

Cheers