Fuse 24 failing for No Discernable Reason... :)

Taz x said:
what size fuse is fitted? could some 1 have fitted a smaller fuse at some point?

it would be far easier to diagnose if it wasn't intermittent
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It's the right size (5A). Kind of makes me want to stick a 7.5 in it, but I won't :)

It's not intermittent exactly - the fuse WILL always blow - it's just hard to know how long before it does! But you're right, that's what makes it annoying.

I wish that BMW would let me drive around with their diagnostic laptop plugged in while I go about my a day, rather than give them the car to burn around in. But I guess they don't keep spares of that kind of kit.
 
As an aside, it does make me wonder what the dealership hope to learn from their diagnostic laptop - given that it's the diagnostic circuit that is failing, and the diag kit is pretty simple from what I'm told. "Reading the codes" won't do a whole heap, as we've tried that in the past of course.

Trapped in a world of "we'll just plug it in and see what the computer says"... :)
 
its dangerous but if you do put a big fuse in, the fault will show up very quiclky

if its under warranty you can blow something up then jut put back a fault 5A fuse and admit to nothing, naughty
 
You are into auto electrician territory. Sounds to me like there is a wiring loom fault somewhere but it will be a nightmare to diagnose unless you know what you're doing.
 
What exactly are the components covered on this fuse? Some are more prone to going open and should be eliminated before starting on the looms, If any sensors are involved on this circuit they are always a good bet, along with anything with a relay.

Unfortunately stealership technicians rely on computers and wholesale component replacement rather than basic principles. Methodical metering and disconnecting should be a first step, a diagnostic will just pull up a load of false codes.
Good luck, be interested to see what it was.
 
Ewazix gives good advise, you could disconnect every device that draws current through that fuse. Put the fuse back in and see what happens, if it is still blowing then it indicates a wiring fault, if not introduce each item at a time, this may help determine what is causing the problem
 
Had a quick look at your wiring diagram. I saw that an aux pump is connected to the circuit. Maybe the bearing in the pump has failed. This may give higher electrical load to the motor and the fuse might blow.... :|
 
Cloudberry said:
Had a quick look at your wiring diagram. I saw that an aux pump is connected to the circuit. Maybe the bearing in the pump has failed. This may give higher electrical load to the motor and the fuse might blow.... :|
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not sure what pump that is, what controls it? if its automatic control i.e. coming on via some sensors then yes it could blow as it energises, its the most likely item of the 3
 
Thanks guys!

They say that the pump is the "cold start pump" and used to help the engine start when it's very very cold. I don't know what "very very" means exactly, but it's hardly been frosty in Hertfordshire recently! :)

Dealership called again today to confirm my appt tomorrow. They seem pretty clueless about what process they will follow to diagnose beyond "plug the computer in and road test it". I've got a fair background in vehicle mechanics (to the point of having wired my own ECU up two years ago) so I'm really wanting to know how they are going to approach this - will have a chat with the workshop manager tomorrow morning.
 
Spoke with the lead tech there. He said they wanted it for a few days to test out, so I will be leaving it there next week. Joy.
 
Update - they've tried unplugging the Cold Start Pump and reproducing the fault... nada. But then they've also been unable to reproduce the fault at all, apparently. Whereas I can always reproduce it within a day or so.

Feels a bit like they are waiting until the warranty runs out on April 11th... :(
 
Got the car back, unplugged the cold start aux pump. Done 200 miles in it since and the fuse hasn't failed yet!

I will give it until the end of the week before declaring victory...
 
fingers crossed that is the fault - least it is diagnosed, then you can get it repaired
 
I've done 200 miles now over two weeks and the fault hasn't recurred. I am therefore declaring

:driving: VICTORY!!!! :driving:

Thanks to all for their advice/support.
 
Alastair said:
I've done 200 miles now over two weeks and the fault hasn't recurred. I am therefore declaring

:driving: VICTORY!!!! :driving:

Thanks to all for their advice/support.
Click to expand...

Sounds like you have it then, nice one :thumbsup:

It never ceases to amaze me how useless most main dealer technicians are these days, isolating the possible culprits on a circuit and twenty minutes with a meter should find most faults - unfortunately unless the "computer says no" (apologies to non-little Britain residents who might not get that one) they rely on fault codes and systematic replacement.

I've just got my Ford TDCi back following a week in the sick bay for a mystery power loss. Having called in a specialist and updated several software packages (£160) they couldn't diagnose the problem. They finally examined and failed the EGR valve which I had suggested on day one ("oh no it can't be that sir, no fault codes showing"). I wasn't being clever I had just used Google! Red faced service manager and £160 refunded!

As a matter of interest what exactly does this cold start pump, pump?
 
When you start a car, it needs to run very rich - i.e. running with the choke. The pump increases the pressure differential on the intake manifold so that the engine 'sucks' in more air (oxygen) for the cold start combustion. More fuel needs more air. After a little while it switches off automatically. Hope that explains it :)

What worries me is WHY its blowing the fuse!! Glad you isolated it though mate :D
 
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