Was driving the roadster yesterday and I honked the horn at friends. It honked once then it stopped working along with my electric fan and turn signals. Drove home and found a blown fuse. I replaced the 20a fuse with another and the fuse immediately blew again. I replaced that with a 30a fuse and everything seems to be working.....for now. The fuse that blew fuses a distribution block that powers my relays and gauges. I have relays for electric fan/horn/hi & low beam headlights/fuel pump/and all 4 turn signals. Be advised, my electrical skills are basic, at best. All component feed wires are 10ga while trigger and ground wires are 18ga. Now the original 20a fuse had over 1000 miles on it. Is it possible I had too low an amperage fuse inline and I needed at least a 30a or higher? Or, is there a short in the wiring which would entail me removing the body from the frame to access every inch of wiring, which I really don't wanna do! Any help is greatly appreciated!
Horns can be a fairly large current draw. Best bet is to measure that draw with a meter and fuse accordingly.
Do you have a horn relay? Can you duplicate the issue by blowing the horn? Was the fan running when the fuse blew? Report back if you can duplicate the issue. Personally I would run a separate circuit for the horn, but you absolutely need a horn relay and you can put an in-line fuse in the power line for the horn. Sent from my iPad using Tapatalk
I am no expert but maybe your headlights are taking a lot of power and when adding another draw it is burning the fues? I had fog lights that cooked their harnesses on me one time *Edit looks like some other are faster than me.
If that block only feeds relay coils and gauges, 20 amp should be plenty big enough. If you're feeding any of the relay-controlled stuff out of it also, it would be iffy. It may be a relay failing. Try pulling all the relays and try a 20 amp fuse again. If the fuse holds, plug the relays in one at a time until you find the offending relay. If that's not it, then the problem is in the wire feeding the block and you'll just have to trace it out to find the problem.
There are many ways to troubleshoot. So much easier if you have a helper. One way is remove fuse, replace with an ammeter. 30amp ammeter would be ideal. +positive side to source, ---negative side to loads. Now you can turn those things on-off seeing if current draw is normal for each one. Another way is if you have (or get a pack...Harbor Freight) clip leads. If you have "alligator clips" you can make your own, 10ga wire, long as you want them (36" is good). Instead of fuse clip in old style incandescent headlight. It's now current limited by headlight. Now you can move wires (wiggle)...if headlight turns on, you found short to ground. Another way is disconnect your battery negative lead. Now using an ohm meter,or multimeter on ohms, lowest (Rx1) scale. Zero the meter (leads together...if no zero adjustment just note reading...say it's 0.3 ohms, test lead resistance). Now one lead on good ground, other on load side of fuse (fuse removed). Again, turn things on watching resistance. A 12.5 volt battery, 1.25 ohms=10 amp load (ohms law). So with helper watching meter, move wires, etc, see if it shows short to ground. Turning things on-off resistance gives you idea of current draw. Yet another. Reconnect battery. A 20 amp circuit breaker you can sub in for the fuse. Try things on-off until breaker trips. Also wiggle wires seeing if breaker trips. Sent from my SM-S320VL using Tapatalk
Sometimes in a hurried install a wire gets pinched, beneath a mount. Do first check component draw, later chase out 'shorts'.
To save blowing fuses put a light bulb across the fuse contacts. When the bulb lights you have a short to ground. Disconnect a circuit at a time until you find the circuit with the short.
When time is money I have dealership mechanics run a short jumper across the fuse and have someone with a pair of pliers to pull the wire when it starts to get hot or you let the smoke out. Not my recommendation ,but one way to do it faster. Do you have a fuse for each component ? One fuse for multiple components makes it harder to diagnose. As simple as your harness should be for your roadster it only makes sense to isolate each function with it's own fuse. Do you have good relays or the cheapest ones you could find ?
Good advice all! I've got easy access to most of the relays. I think I'll just replace the fan and horn relays and see if that helps. Question: I've got a SPAL electric fan and a pair of Cadillac horns. What amperage Bosch relays should I get?
Looking on the Spal website, it looks like different fans of the same size have different amp draws so you will need to check your fan model number against their suggested fuse rating. I just checked two 16 inch fans and one takes a 15 amp fuse and one takes a 40 amp fuse. That is dependent on air flow rating. One being 1103 and something cfm and one being 1918 CFM. Meaning that not all 16 inch Spal fans are created equal. https://www.spalusa.com/spal-automotive-usa I'm with the guys who say, what unit on that circuit isn't running though relays or what unit is powered by that fuse directly even if it has a relay? The Spal fan can have up to a 40 amp draw. Halogen headlights can draw big amps especially the high wattage H-4 ones. The horn could have been the killer if it was wired direct without a relay. Wire running to a relay getting shorted?
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