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Can Heat blow a fuse?

I have a 2005 Ford Five Hundred. In July, when the temps outside were around 102 degrees, I drove the car 150 miles back to my home, stopping about 45 minutes into the trip and shutting off the car and then stopping again about an hour later and tanking up with gas. At the last stop just a few blocks from home, I got out, shut off the car, went inside, came back out and nothing. No click, no noise. I thought it was the alternator or the starter or the battery. Took it to our mechanic and he said we blew a fuse. So, he replaced the fuse and also the starter relay. I asked him if it would have blown due to the heat, and he said “maybe”. He also said that if it happens again, he’ll mark the fuse so we can replace it and to let him know. Well, today the temp outside was 98 degrees and… it did it again. Fortunately in front of the house. I had driven it around town after work and made a few stops which perhaps put stress on the car. So we’re putting in a new fuse and see if that works.

The question…can high temps and humidity blow a fuse? The same fuse in a one month period?

Stay out of the heat? No.

Fuse #11 in the fuse box next to the battery powers the starter solenoid. If this fuse is blown you likely have a low resistance (shorted) condition in the solenoid when hot.

So I should have my mechanic check the wiring?

What fuse did it blow, then?
Was it indeed #11. It is supposed to be 30A - is that what’s in there now?

If is it #11 and your fuse is rated 30A, it only supplies the current going through the contacts of the starter relay. The lower that resistance, the better - so I doubt the starter relay was bad if 11 is blowing.
If it is #11, I’d look into the starter solenoid, that thing on top of the starter. It may mean that the starter may need to be replaced as that is likely one assembly.

Here’s the wiring diagram of starter related parts.

There are fuses that use a eutectic solder material as the fuse element that will blow due to heat as well as current. They are called dual sensing fuses. The are mainly to blow a circuit in case of fire. These fuses require a temperature of around 145°C or almost 300°F. I don’t know if any of these are used on cars today, but cars of old had a fusable link that was often made from this material.

I think you have an issue inside your starter though.

A mechanical problem is causing the fuse to blow, not the heat itself. The heat causes a mechanical problem that then causes the fuse to blow. Since the relay was replaced, my best guess is a starter motor problem.

Okay, in summary, after a long, long drive home, which included a stop in the middle you turned the key and there was nothing. No click, no nothing.

You took it to the mechanic, and he found a blown fuse for the starter circuit, and changed it and the starter relay.

Here’s what happened: your engine compartment temperature rose (normal, as the heat from the exhaust manifold and around the cylinders, both hundreds aof degrees hotter than the engien compartment, dissipates). The coil winding in your starter solenoid shorted when the heat caused cracks in the insulation to open up. The polyimide-amide coating becomes heat stressed with age. The short caused the fuse to blow, and may have even burned the contacts in the relay.

Since the starter solenoid had cooled when the mechanic looked at it, he was unable to find the cause of the blown fuse. He probably found the starter relay contacts burned and attributed the fuse to the starter solenoid.

In short, the winding in your starter assembly’s solenoid is shorting out when hot.

That’s my theory.

I like it.

Another thing you should check out is to see if any of the wiring to the starter is hitting the exhaust system and has possibly burned the wire insulation and that is causing a short to ground.

A shorted starter solenoid is a reasonable guess but feedback from the OP would be helpful.
Which fuse is blown?

it’s not that fuse. It’s the fuse under the dash on the drivers side. The fuse is only a 7.5 fuse. The fuses in the fuse box next to the battery are fine.

What number?

I’ll be honest, I don’t know which number it is. Being a chick, I am just lucky enough to find the fuse box. I’ll see if I can someone find that out for me.