The smell of electrical fire in the morning

[DirtDonk]

...IDK why Ford put such a big fuse on it - it would run with a 3A.

Does seem strange that. Could that circuit be shared with other things, even on the later model systems like yours? Or is there some set of circumstances where some sort of spike could be encountered you think?

Or maybe it's just an industry standard that, if the wire size is capable of handling it, they worry more about that than protecting any supposedly delicate component. So just fuse it for the wire gauge.
That way they don't blow a fuse and disable a critical system such as the ignition system until something else in it fails first and the engine would have died anyway?

Paul

 

[Steve83]

...terminal failures in our forced failure lab.

AAaaahhh... So you're FORCING these failures! That explains how you can get those numbers.

Have you ever personally found a naturally-occurring failure like that? Particularly: on a vehicle - particularly: on a Ford? I haven't, working at several dealerships and independent shops, and browsing junkyards for decades. The closest I've seen is the fuse pic I posted before, and even though that's relatively common, I've never heard of anyone having a fire from that - the blower just slows down. The SCDS recall is sort of the same process, but that's still a VERY different failure mechanism.


But back to the original topic...
Do you think THIS fire could not have been prevented by an appropriate fuse? IOW: do you think the current necessary to set the box on fire was LESS (by at least 10% ) than the current it normally passes to run the engine?

Could that circuit be shared with other things, even on the later model systems like yours?

I measured a jumper wire in place of the fuse, so I was measuring everything. And on '92-96 trucks (like mine), that fuse only feeds the ICM, coil, PIP, RFI cap, and the TRIGGER side of the EEC PWR relay (thru a diode). Other than the diode (a common 1/4W) & relay (a common Bosch/ISO/Tyco cube), the whole circuit is shown in these diagrams:

.

Or is there some set of circumstances where some sort of spike could be encountered you think?

Not a spike that would 1) blow a 3A fuse but not a 20A, and 2) be desirable to allow. At least, not that I can imagine...

Or maybe it's just an industry standard... So just fuse it for the wire gauge.

Not that I've ever read, heard of, or been taught. Even if the factory oversized the wire for future expansion (which is incredibly rare), it still wouldn't benefit Ford to install the oversized fuse to match loads that might never be installed. It would still be prudent to install the correct fuse for the existing load, and put a note in the owner's manual or body-builder's book that upsizing the fuse is OK.

 

[Jdgephar]

AAaaahhh... So you're FORCING these failures! That explains how you can get those numbers.

Have you ever personally found a naturally-occurring failure like that? Particularly: on a vehicle - particularly: on a Ford?

I have a couple examples. The first is the issue with the ford alternators (the ones with the three 1/4 spade terminals. They are notorious for overheating. My personal example is the fan switch that melted on my 89 pickup. no fire, but melted the connector. your fuse is also a great example of where it overheated. thanks to forced testing of this, the industries are using plastics that just melt instead of catching fire. The board that we have in our lab is actual field returns. (Again mostly just melted parts). That's why we force the failure in the lab to make sure nothing catches on fire.

Now as far the issue here, the MSD box got hot. Depends on what current it was drawing and what size fuse in line.

 

[Steve83]

You're right - I totally forgot about the 2G flamethrower! :-[ But that's a voltage source; not a load, so it presents a different problem. And that connector was just a bad design for the NORMAL operating conditions. I'm trying to think of a load fault that would cause a fire (similar to this thread's topic) without blowing a properly-sized fuse.

So what about this particular fire?

...do you think the current necessary to set the box on fire was LESS (by at least 10% ) than the current it normally passes to run the engine?

If not, then a fuse would have prevented this fire, which is how this whole tangent got started.