74 spider horn and cooling fan fuse blows?

[CharlieLong]

I have a 74 spider and have overheated it a couple times when the fuse that the horns compressor and cooling fan are on blows. I've gotten into the practice of testing the horn, like checking the oil, to make sure the cooling fan is working before driving it. Seems rather frequently I find that fuse "a" blown. I know the elec. diagram I have is wrong, lots of circuits wired in this car different from diagram. I have an idea it might have been a California car...used to have lots of air pollution crap on it. I'm definitely no automotive electrician. Any ideas on where to look for why the fuse blows?

[18Fiatsandcounting]

What rating is the fuse that's blowing? I believe it's Fuse 1, and it should be 16 amps. The other fuses are generally 8 amps. If Fuse 1 is 16 amps, then you might have a problem with the relay that controls the cooling fan. Or the fan is gunked up and slowing and drawing too much current.

Also, and this applies to early 70s spiders but I'm not positive on a 1974: Power for the cooling fan comes through Fuse 1, but power for the relay for the fan is coming through Fuse 9.

-Bryan

[CharlieLong]

Good to know...never checked fuse 9. It has had 16amp fuse in 1. I keep a baggie of variety amp fuses in the glove box. Threw in a 25A to get me home then put 16A back in and a few spares in my baggie. I noticed fuse connection across doesn't actually blow...but plastic fuse casing will melt a little then loses connection. Book says clock would be on that circuit but it's not...clock doesn't keep time anymore but keeps ticking with fuse pulled...cigarette lighter never did work...looks like something else might have been "bootlegged" into that circuit but short of tearing dash out to trace it I have no idea what. Would I fry something if I left a 25A fuse in it?

[CharlieLong]

other things book says should be on 9 still work...but, again, book is wrong on a lot of other wiring that exists.

[spider2081]

i have a Clymer Fiat 124 1967-1980 Manual. It shows 1974 Spider fuse A (1) to be a 25 amp fuse. I never had a 1974 Spider so I don't know for sure. Could a 16 amp fuse be too low a value?

[TX82FIAT]

I think both the horn and the fan are constant hot on circuits switched on by the temp switch or the horn itself. Given the frequency of horn use I would think the 16 amp fuse blows because of something on the fan circuit. Fan comes on during cooling periods and either overheats because of resistance or you have a hot to ground. If it is a hot to ground it will trip as soon as the fan kick on and you will know. If it trips because of the resistance ohms you could trace the circuit cleaning connections and grounds to see if that solves the issue. In many cases fiat used subpar metals that tend to corrode more over time creating more resistance. Start with the grounds and cooling fan switch prongs that may have been exposed to coolant at some point accelerating the corrosion/oxidation of the metal. I clean and tighten the fuse box metal prongs every five years or so. You will hear the fan kick on and then off as the circuit blows. Id also check the fan itself is operating within spec by testing it.

[18Fiatsandcounting]

i have a Clymer Fiat 124 1967-1980 Manual. It shows 1974 Spider fuse A (1) to be a 25 amp fuse. I never had a 1974 Spider so I don't know for sure. Could a 16 amp fuse be too low a value?

I don't have an owner's manual for a '74, but I do have one for a '76 and it lists Fuse 1 as being 25 amps, for just the horn and the cooling fan. So, perhaps 16 is too low.

But, if the ends of the fuse are starting to deform from melting, something is drawing way too much current. It's possible that the fan is just super crusty inside and drawing far more current than its nominal 8 or 10 amps. As others have suggested, make sure you don't have a short somewhere and that the fan is OK. Try spinning the blades by hand (engine off) and make sure it's not dragging.

-Bryan

[CharlieLong]

i have a Clymer Fiat 124 1967-1980 Manual. It shows 1974 Spider fuse A (1) to be a 25 amp fuse. I never had a 1974 Spider so I don't know for sure. Could a 16 amp fuse be too low a value?

Possibly...weird part is little metalic fuse strip doesn't fail...plastic fuse frame actually melts a little losing its contact???

[CharlieLong]

i have a Clymer Fiat 124 1967-1980 Manual. It shows 1974 Spider fuse A (1) to be a 25 amp fuse. I never had a 1974 Spider so I don't know for sure. Could a 16 amp fuse be too low a value?

I don't have an owner's manual for a '74, but I do have one for a '76 and it lists Fuse 1 as being 25 amps, for just the horn and the cooling fan. So, perhaps 16 is too low.

But, if the ends of the fuse are starting to deform from melting, something is drawing way too much current. It's possible that the fan is just super crusty inside and drawing far more current than its nominal 8 or 10 amps. As others have suggested, make sure you don't have a short somewhere and that the fan is OK. Try spinning the blades by hand (engine off) and make sure it's not dragging.

-Bryan

Fan seems to spin pretty freely by hand. Might be down to more investigating what else might be "bootlegged" into that circuit??? Thanx all for your input. If I figure anything out, I'll post it for future reference.

[18Fiatsandcounting]

...weird part is little metallic fuse strip doesn't fail...plastic fuse frame actually melts a little losing its contact???

Those fuses with the bent over aluminum strip that forms a cap on each end are well, not a very good design. They tend to lose contact and then start heating up due to the increased resistance. I'd clean up the contacts really well for Fuse 1 (A), and believe it or not, an ordinary pencil eraser at the end of an old school #2 pencil will clean up the copper contacts in the fuse box. You can also use Brasso or your choice of metal polish, but be sure to get it all out when you're done.

Bend the contacts slightly so that they really press on the fuse when it's inserted. As for the fuse, clean up those cone-shaped ends with 600 emery paper or a soft wire brush. Or see if you can find better versions. I like the copper version (rather than aluminum) but those are getting harder to find. Once the fuse is inserted, twirl it with your fingers a bit so it makes a good contact.

-Bryan

[spider2081]

I thought I posted this earlier but I must have forgot to hit the submit button

My original thought was if the coolant fan was running and you attempted to blow the horn maybe the total current of both the fan and the horn was blowing the fuse. Reading your posts and other replies its possible other things could be wired into the fan circuit/horn by previous owners
You might consider using an inline fuse to power the horn relay directly from the cars battery. This would eliminate the horn circuit from the coolant fan circuit. The violet wire on the horn relay is power from fuse 1. Remove the violet wire from thorn relay and tape up is connector so it can't short out. Then connect the inline fuse to the horn relay.
Also dirty or corroded fuse panel terminals can generate heat a tthe fuse connection. Clean tight connections in the fuse panel is a must.

[CharlieLong]

Thanx all. Everything good to know. Good points...should be relatively easy contact cleaning. Other coincidence is after-market stereo installed might just be into that circuit. Last time the fuse blew I WAS listening to Cleveland Indians (Guardians) game. Wouldn't think radio draws that much but just maybe??? All three at the same time??

[18Fiatsandcounting]

Wouldn't think radio draws that much but just maybe??? All three at the same time??

A basic radio that draws only a few watts would probably be fine, but some of the better radios are 50 watts per channel and up. That would equate to many amps of current when cranked up. If it were me, I'd run a separate power wire for the radio with an inline fuse holder.

-Bryan