Fuse Blowing Nightmare

[UKKid35]

Fuse 26 - ABS, ECU, airbag control unit, ZEE unit - 7.5A

This fuse gave me loads of problems several years ago, it would blow apparently at random, but only when I tried to start the car

It seemed to be related to wet weather, but I'm not entirely sure because the car is garaged

It would happen so often that in 2013 (just checked my eBay receipts) I bought a bag of 100 7.5A fuses to keep in the car

Then as suddenly as it stated blowing, it stopped

However today we were in for a new treat

It was my gf's birthday earlier in the week, and I promised her that we would go for a drive this weekend

This was the first time we'd been further than our local Sainsbury's since the lockdown

On the way back from the South Downs the car died, and I assumed the worst fearing total engine failure, as it was running a little hotter than usual

However I decided to check the troublesome fuse before calling my ADAC breakdown, and thankfully it was blown

The fuse has never blown while driving before

The car then blew three more fuses while driving in the following 15 minutes, but the last 90 minutes home was trouble free

What on earth is going on?

[UKKid35]

It's worth adding that when the fuse blows, not only does the engine stop, but all electrical functions stop

So you end up coasting, but you can't indicate, you can't put hazards on, and if you are on a country lane with nowhere to pull over you are royally screwed

Once you do stop, you have to be able to open the drivers door to be able to get in to the footwell to access the fuse box

You can imagine how difficult this can be

[Heinkeljb]

Put a bit of tape on the fuse which blows, that way you can see which fuse to pull out quickly.
As to why it blows, obviously it is getting excess current draw. Usually frayed wire ends or chaffed cable where it goes through a bulkhead. So first thing to do is try and follow where the wires go that relate to the items the fuse supplies.
If no joy doing that, then probably corrosion in the Zee / SAM unit.

John

[UKKid35]

So first thing to do is try and follow where the wires go that relate to the items the fuse supplies.

Thanks

"ABS, ECU, airbag control unit, ZEE unit" - I presume they are all pretty close to the fuse/relay box itself

The one thing I do remember being remote is the box behind the driver's seat

I seem to remember that it got wet in one of the previous 450s I had and that caused the 3 bar error

Perhaps that is worth checking

[pinger]

Thanks

"ABS, ECU, airbag control unit, ZEE unit" - I presume they are all pretty close to the fuse/relay box itself

All I can offer is that the ABS ECU sits atop the ABS pump/valve block unit and is a bit of a sod to get to as it is located (on RHD at least) on the NS and where the front and rear undertrays hinge. Getting them dropped far enough to gain access is awkward and removing the rear undertray for the first time is as awkward as can be.

Mine is a 700cc car so even if I knew on mine where the rest of the modules are, they're possibly in a different place on a 600cc car.

[Mjolinor]

Put a bulb in place of the fuse. If it is a 7.5 amp fuse then I would not expect the steady normal current to be much over 3 amps if that.

If you initially put a 21 watt brake light bulb there then all should work as normal but if whatever causes the fuse to blow happens then the bulb will light instead.

It may be that 21 watts is a bit low so you may need to put two in parallel to have it behave normally.

[tolsen]

P=UI, where P is wattage, U voltage and I amps. A 7.5 amp fuse then will require a 90 Watt lamp. Perhaps a 55 Watt headlamp bulb will work.

[reinaldo_is_spam]

P=UI, where P is wattage, U voltage and I amps. A 7.5 amp fuse then will require a 90 Watt lamp. Perhaps a 55 Watt headlamp bulb will work.

Trouble is incandescent bulbs have a non-ohmic resistance curve, one will have to experiment with different wattage bulbs to get it to ‘trip’ into brightness at the right level of fault current, and yet remain off in the normal current draw band.

P=UI

Why substitute different letters...... P(in watts) = V(olts) times I(amps)

[Mjolinor]

90w is way too high to be useful. 55 is probably too high. Start with 21 and then double it with two in parallel.

You do not want a bulb that will take for ever to light or you will not see the flash that blows a fuse and if a 90 watt bulb stays lit your wiring will go up in smoke.

You need a wattage that is slightly over the normal load for that fuse. At 7.5 amps I would not expect the constant load to be more than two amps, possibly three, hence 24 to 36 watts. Better to start low and increase than to start high because you do not want anything damaged.

[tolsen]

U is voltage whilst V is volume my teacher told me many decades ago. Perhaps someone brighter than me can explain why U is commonly used for voltage.