Thread: Hot fuse

Sounds like a hot joint.
Are the wiring connections to the fuse tight and secure?
The heat is a product of current and resistance.A poor connection can create a lot of heat well before the fuse blows.
If you can can your hands on a DC clamp on ammeter you can easily measure the current flow through the circuit.
Fix the poor earth on the A/C compressor.It will eliminate it as a part of the problem.

Hope this helps.
Cheers Garry.

I would also have thought the poor earth would give you the heat in the fuse so check that first... Other thing to check is the quality of the fuses. I had a batch of el cheapo's that were presumably correctly rated but the plastic sheath was noticeably cheaper than better quality fuses. They ended up melted into lots of interesting shapes. Fuse got hot did not blow but (if I remember correctly) problem went away when I used better quality fuses.

the loose earth can definitely cause overheating in the wiring......
it creates resistance.....which causes more current to flow through the wiring.....

it may not be enough to blow the fuse.......but i dont think it will be far off either.....
or......more likely......you have a loose connection at the fuse itself......where it plugs into the fuse box.......
if thats loose....it will be arcing.....and getting hot.....the heat will travel up the terminal and melt the fuse.......

Originally Posted by flagg

Hey guys,

The short version:

The fuse for my A/C gets hot to the point of melting the plastic housing.. but the fuse (20a) doesn't blow. Why? If there was too much current wouldn't it blow?

The long version:

I have two A/C units in my 110, one front (std for 1986) and Yokohama (old 3rd party) rear. It works well, and recently I had the compressor replaced.

This was 'professionally' done.. and I had to take it back twice already because they wired it all up together ('it all' being both A/C units and the front fan) on the same switch. (yes, this means that I can't have the heater on without the A/C being on as well........)

Basically I'm sick of these idiots and don't want their incompetent fingers on my pride and joy any more - so I need to fix this one by my self now.

I'm confused as there is a 20a fuse and it /should/ blow if there is enough current to melt plastic... how to I find out if it is pulling current from elsewhere? (if I take out the fuse the A/C doesn't work)

Also, the earth for the A/C is a little loose (thread on the screw is a little stripped, so dosn't always make a good connection - would this cause anything other than the A/C stopping?

Cheers,
Dec

You need to check that the wiring is capable of supplying the current need in the circuit. If the wiring is not heavy enough 20A is more than enough to overheat it. Check the connections to the fuseholder as it is more than likely (if this is where the heat is) that one is loose, and will cause local overheating without blowing the fuse.

Fix the loose earth connection. It will NOT cause MORE current to flow through the circuit, but could cause local heating there as well, and any discontinuity in the circuit will stop the system from running properly.

Cheers,

Lionel

Fuse holders melt on many different cars. VK Commodores spring to mind. What a joke their fuse boxes were.

The fuse contacts MUST be clean and tight on the fuse. Any fuse that has melted will cause oxidation on the fuse clips. A sliver of fine wet-and-dry, work it with a small bladed screwdriver to clean the contacts. Blow clean. Bend the clips if possible to grip the fuse better. Buy brand-name fuses only, Narva have been good for me for the last few years.

The problems with loose fuses/earth points come under my definition of "terminal disease" Tight metal to metal contact flows electrickery best, rust/paint/corrosion/too small x-section area cause a flowing current to develop a voltage drop: amps x volts = watts. For a terminal carrying 10 amps to drop 0.5 volt causes 5 watts of heat to be generated. That is quite enough to melt plastic fuses.

Fixed the earth - still hot.

The wires are all of a reasonable gauge.. (although the earth is thinner than the positive - but it isn't hot at all).

Tried swapping the fuses, and also plugging the wires into the fuse next to old one - again still hot. Interestingly, one side of the (glass) fuse seems to heat up first, but they do both heat up rather fast).

Going to try a completely new inline fuse.. other than that I'm out of ideas. Its only the fuse and the terminals that are getting hot..

Originally Posted by flagg

Fixed the earth - still hot.

The wires are all of a reasonable gauge.. (although the earth is thinner than the positive - but it isn't hot at all).

Tried swapping the fuses, and also plugging the wires into the fuse next to old one - again still hot. Interestingly, one side of the (glass) fuse seems to heat up first, but they do both heat up rather fast).

Going to try a completely new inline fuse.. other than that I'm out of ideas. Its only the fuse and the terminals that are getting hot..

Aah, glass fuses. Ya really need to tell a bloke all the info right up. There are fuses and fuses. Cheapie ones have glued-on end caps, the fuse wire is just jammed in. When the glue fails the end caps fall off. You need externally soldered fuses, or a new blade fuse holder. Narva makes externally soldered fuses with brass end caps, but they come in packs of 50. Go visit a competent sparkie and get something of good quality.