mystery electrical (burnt fuse) problem

[jwinkler]

Hi all - it's been a while since I've posted. I guess it's time for the car to start challenging me again.

I was driving this weekend, and out of nowhere, I was engulfed in white plasticky-smelling smoke. I pulled over, and noticed the last of the smoke coming out of the main fuse harness inside the firewall (the car is a classic roadsters). Turns out what had burned was a "custom" wire-and-fuse solution I put together years ago to run a heavier gauge wire to the fan power.

The solution was to run a 6-gauge (vs. the original 12-gauge) wire with an inline fuse to provide power to my black magic electric fan. The power is switched with a relay. What I saw when I looked at the problem, the "fire" had happened at the fuse itself (a 30 amp fuse). No other damage to any wires in the area, and everything except the fan works fine. Fortunately, it was cool enough, and without too many stop lights, for me to make it home under the car's own power rather than on the back of a flatbed.

My question is what would make a fuse (blade style) combust like that? Even with the fuse still smoking, the wire itself wasn't hot compared to the normal heat of the engine bay. The fan has been running fine up until this happened, and my "custom" re-wiring has been in place for at least 7 years.

Could water in the fuse housing have caused this problem - the only other unusual thing that happened was that before I took the car out, I washed it (an unfortunately rare occurrence) with a hose, which results in a lot of underhood water on the flat surfaces. I dried what I could see, but didn't check thoroughly.

If I replace the wiring, is there anything I could measure at the fan (amp draw?) that would indicate a problem?

Thanks,

JLW

[Rick Parker]

I am thinking you had some severe resistance in the fuse block. The fan can pull upwards of 25 amps (peak). The single largest item other than the starter. The 6 gauge wire is more than enough (10 gauge would be plenty) crimped, soldered and sealed with shrink tubing makes for a bullet proof electrical connection. Try using a little Dielectric grease on the spade terminals of the relay and the fuse. Be sure all terminls are tight. Moisture is a very bad thing, control it.
One of the connections on that wire run had resistance in it.

[strictlypersonl]

I've seen some fuse blocks whose wire-connection section was riveted to the fuse clamp. Corrosion between the two pieces caused overheating.

[RICK LAKE]

jwinkler #6 gauge wire is too heavy for a coolant Fan #10 is what most specs call for. First see what the amps are for the fan at HI speed. Add 10% fudge factor, Most coolant fans are 18-23 amps hi speed. A 30 Amp slow blow is fine. Don't go any bigger. Next to check is the ground wires to the fan motor. Try and have 1 good CLEAN ground to the fan from the chassic, 2 is better from 2 different spots. Use star washers for the ground connectors. Shrink wrap the contector to water proof. #10 black wire. You have to remember that there is a spike of ampage when the fan runs on, as high as 40-50 amps with a single, duals are higher. Before you do any repair you need to hook up your fan and run it for 10-20 minutes with it hooked to a cheater wiring. Put the fuse in the curcuit. The fan motor my be going bad. after being hot for 10-20 minute it my blow or burn the wiring. If not, look at the contacts in the fuse block. Is the fuse welding into the fuse block? Weatherproof connectors are needed anywhere outside of the car or where water can get too. It may be possible that the relay contacts welded together or have high resistance. again causing the wiring to melt. A picture of the back of the fuse block if open will tell alot. Painless make a nice 12 curcuit kit. You may want to look into this if alot of the wiring looks in poor shape. Painless and rewiring the car may be the cheapest way to go in the long run. Need pictures. Good luck Rick L.

[jwinkler]

Thanks for all the suggestions. I'll check the ground wires, and rewire and run a 20 minute test to see if the wiring gets hot. I'll also post a high-res picture in my gallery to show how the fan gets power (and to show the fuse damage). The fan is powered straight from the main circuit breaker and bypasses the fuse block. I did that when I did my "custom" wiring because the power draw of the fan through the fuse block as originally wired did cause problems in the block.

Thanks,

JLW

Hopefully, this link will embed in the message:

[RICK LAKE]

jwinkler JL how many curcuits are on that breaker? what amperage is it? If you over power that breaker you will have it go on and off from the load (amps) being to much. This will smoke the connections in the fuse holder. When the load to the fan kicks on, first the spikes amperage, then the steady pull on the curcuit, The contacts in the fuse holder get hot and spread apart. More resistance happens and you have melted wiring. I also hope there is a fan relay for turning on and off the fans. 30 amp is ok 40 amp would be better. I have 2 of the fuse holders you are showing. I use one for each of my fuel pumps in the car. Even though the wiring is heavy the contacts are light duty in the plastic. IMO I wouldn't run more than a 20 amp curcuit to one accessory with these holders. Yes I have melted them too. Yes I did stay at a holiday inn and work under the 1/8 moon shining, repairing a wiring harness for a friend. Old days no problem with that fuse holder, today cheap metal for the contacts. Some contacts are only copper coated, not solid. Rick L.

[Ronbo]

I'd be caucious to leave anything exposed outside of the pass compartment, two tricks: spray red battery terminal grease on all outside connectors and seal all crimp terminals with RTV or liquid electrical tape (or dip-grip).

Some electrical components are best obtained from the marine supply store since they're intended to be exposed to water.

One question was the rubber cap on that fuse holder checked recently? Once it's replaced check for heating of any of the wiring to the fans under load. Doesn't sound like anything outside of the fuse holder caused the meltdown. If you can find one get the larger style blade fuse and holder (about an inch long) the blades on the standard fuse are too wimpy on a 30A circuit. If you can dig that fuse out you'll see one of the connectors overheated.

"if it heats up at all, it's too small" this is the golden rule of wiring. Your fans will run faster as well when you eliminate the 2v+ voltage drop the wiring may be causing.