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This question is for owners of modified cars with
ignition or fuel management systems.

Some modern cars have a very heavy fuse in the main power line from the alternator. Apparently this is to reduce the risk of melting the processors/ fire in the engine bay.

Is this necessary or is it over-insurance ? Should we
fit them as part of the FI kit for example ?

... and today's wry smile is a Volvo dead in its tracks outside Buckingham Palace at the end of a long journey - lots of nervous policemen wondering if its about to blow up.
Initial diagnosis is flat battery / alternator failure but the jump start works and the journey is completed with no indication from the charging circuit. Ten minutes later - fullstop again - no power.

Volvo suggest shorting battery plates - new battery-car starts - charging circuit showing failure.
Discuss availability and cost of Valeo alternators with Volvo - decide to look at the small plastic box on the main alternator line and you've guessed the rest.

The box contains an 80 amp mega-fuze which had been worked loose by people 'leaning on it' whilst servicing the car.

Thanks in anticipation.

It would need to protect the whole of the brown circuit, not just the alternator. As such *all* browns would have to be removed from the solenoid stud (or RHS toe-board stud) and the new fuse placed between said browns and the heavy current cable.

Paul Hunt

Isn't there a danger of wrecking the alternator if the engine is running and the fuse blows, thus disconnecting it from the battery?
Chris Betson

Thanks for your response - Volvo put these fuses in the main cable close to the alternator - this allows the vehicle to continue to run off the battery until low voltage cuts out.

If I remember my 'heavy electrics' Chris, an alternator will spin freely until a load is taken - in our case the alternator is still working - but with a new fuse.

What I really don't know is whether this ultra cautious manufacturer is seeking to protect its customers from a real risk or from something which is very unlikely to happen.

It would be easy enough to up-date our MGs if this was worthwhile and would save a computer. The fuses cost a small fortune from Volvo but only half a fortune from Vehicle Wiring Products.

My understanding of the purpose of the fuse is that it is there to prevent the wiring from burning in the event of an accident, as the alternator is in a vulnerable position. The rating of the fuse is much greater than the maximum output of the alternator and is there to prevent the battery discharching through the alternator or adjacent wiring due to accident damage. I fail to see how it could possibly do anything to save the on-board computer, as the available voltage is well below the tolerance of automotive computers.
George B.

Rover have them in the cable that feeds everything else bar the starter, close to the battery, in the Metro for example. Like George I suspect it is a safety thing to prevent loom fires in the event of a major short - in which case any blown alt would be a small price to pay.

Paul Hunt

George - protection in the accident scenario seems much more plausible and since a major short means sparks I am beginning to think that cars with high pressure fuel from FI in the engine bay probably ought to have the fuze protection.

Many thanks for your thoughts everyone !

FI systems should have inertia switches to cut power to the pump in the event of even a minor bump.
Paul Hunt

Check - as well

This thread was discussed between 05/03/2001 and 07/03/2001

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