The MGA With An Attitude
ADDING FUSES, good and bad applications. - ET-201A

At 07:00 AM 6/7/04 -0400, Walter Tange wrote:
>I've read your article ET-200 ....
>1. On my MGA1500, I added one relay and fuse for the compressor horn by manufacturer's advise. The original horn still works also, but separate and with the original switch.

Very good advice by the horn manufacturer. The instant-on air horn compressor draws VERY HIGH current, more current than anything alse in the car (except for the starter motor). The compressor draws more current than the original horn (even twin electric horns). I presume the air horn instructions also specify or provide a large gauge wire for power supply to the compressor (by way of the relay). The relay is used to avoid the requirement to run this high current feed wire behind the dash, and to avoid need for a high current manual switch.

>2. I added one fuse for the extra cooling fan, without relay.

Also a good idea. Fuses are seldom bad, as long as they are properly applied. If the fan motor might some day short out, or the new feed wire might short to grouhd near the motor, it should blow the fuse rather than burning the wire. This is more important when the wire is boumd in a harness with other wires, where a burned wire might take out other adjacent wires when it burns.

>3. I used a spare MGA fuses-holder, mounted near the starter switch, to give two times the 12 Volt supply to the motors.

Huh? This I do not understand. It is generally taboo to use two fuses in parallel in the same circuit. Please clarify.

Note: This is a small matter of semantics. Walter does not actually have two fuses in parallel. He has an additional original type MGA fuse block (for vintage appearance) mounted on the inner fender, supplying two additional fused circuits for the new horn and electric cooling fan. A heavy wire runs from the battery cable connector on the starter switch to the new fuse block to supply the high current needed for the instant-on compressor for the air horn.

But the momentary misinderstanding above does emphasize the point that you should never install fuses in parallel. If you install a device which draws 10 amps, you need wires large enough to carry 10 amps, and you might use a 15 amp fuse. In case of a short circuit, the 10 amp wiring can carry more than 15 amps long enough to blow the fuse.

To have any increase in power you need larger wires. But the fan motors only draw a certain amount of power when running. Putting two fuses of the same size in parallel doubles the current capacity before a fuse would blow. Two 15 amp fuses in parallel might supply nearly 30 amps continuously without blowing. This may be enough current to burn your 10 amp wiring without blowing a fuse, same as if you had no fuse at all. To protect the 10 amp wiring you need to limit total fuse capacity to about 15 amps. That would imply installing two 7.5 amp fuses in parallel. This has exactly the same effect as using a single 15 amp fuse, just another device and more contacts. If the circuit should ever draw enough power to blow a fuse, the absence of one fuse with that current flow is guaranteed to blow the second fuse as well. So there is never any reason to install two fuses in parallel.

>6. The only thing I did change behind the dash is the connection to the terminal of the first step of the light switch. On one terminal there are 4 wires: parking lights, dash lights, read lamp and fog lamp on the rear (an obligation in Belgium). Lying on my back, under the dash of my MGA and looking into the half dark to have 4 wires in 1 terminal of the switch is not done (well mannered sayed). Now I have 1 wire in the terminal of the switch going to a row of 4 screwed terminals, mounted on the firewall were it is better accessible.

Be sure that one wire is larger than the original wires, as that new single wire has to carry the combined current of all four circuits. On the other hand, maybe the original wire feeding the switch was no larger than the rest. For structural reasons the minimum wire size in the harness is generally 18 gauge, even though that's larger than is necessary for many of the lower current curcuits.

>But, should I still fused this one wire?

The fuse itself is not a bad idea, as a matter of electrical function. But when you place additional fuses in various "unexpected" new locations around the car, future maintenance gets a bit tricky. You need to fully document any change in circuitry so a future mechanic has a chance to find the new fuse if it blows, and not have to spend time hunting for a problem that may look more like a faulty connector or broken wire. It is best to position all fuses in one or two discrete locations, or to position any individual fuse (or relay) very close to the connected device where it will be quickly found.

Locating a new fuse behind the dash could be confusing. If I was an unknowing and unsuspecting mechanic, and all of the red wire circuits stopped working at the same time, I might think the lighting switch went bad. If you hide fuses where an uninformed mechanic can't find them, you might eventually get larger repair bills to cover the extra time the mechanic spends searching for the unknown problem. Adding fuses is the subject of the next page.

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