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MG MGB Technical - Why In-line fuse for parking lights?

Anyone know why the parking lights (red wires) are fused? My 68 B had an in-line fuse in the red circuit but after I restored the car I left it out. Should it be put back? Should there be a fuse for front and back parking lights? What about the turn signals and brakes? I don't understand why MG chose to fuse parking lights when there are so many other wires not fused. Any responses appreciated.
Samuel Sullivan

Just put in a new wiring harness. Both the front and rear red circuits should be fused in-line right there where the knot of snap-connectors is in the engine bay where the main harness ties in to the rear harness. There is a hole with captive nut that you screw two fuse brackets into to hold the fuses there. Be interesting to see why. Only thing I can think of at the moment is that it is an un-switched circuit, unlike the brake lights and turn signals, and can't be shut down by turning off the igntion key but only by turning off the dash light switch. Perhaps it is some protection from the dash switch frying. A lot of power goes through that dash switch (headlights and tail lights) which is why some people do relays for the head lights to increase the power to the lights and incidentally reduce the amount of power going through that switch. I am just guessing and don't know for sure and am no electrical expert but I do know a little bit about my own wiring diagram...forced knowledge over the last several years. Haven't touched it since the new harness, however. HIGHLY recommend it.

JTB
John Bamford

Hi all.

This is a bit generalised, but I hope it might help :

If an unintentional short occurs in an unfused circuit a large current will flow, limited only by the resistance of the wiring / connectors / switches and the current capacity of the voltage source (ie the battery and alternator / generator).

The initial result of such a current is usually melting of the wiring insulation followed by melting of the copper wire, which breaks the circuit.

The melting of the insulation may lead to other shorts, and / or a fire.
I suspect that many car fires are started by this mechanism, often as a direct result of a minor accident that causes a short.

Before the wire melts the voltage supplied to other circuits may drop substantially, possibly resulting in loss of the function of the other circuits, for example the ignition might cut out, maybe at a very inconvenient time.

A fuse is a deliberate weak link in a circuit.
If an excessive current flows in the circuit the wire in the fuse will melt before the wiring gets hot enough to start melting the insulation.

Modern cars have many fuses with good reason !.
I am fitting more fuses (modern blade types) to my BGT, not less.

Don
Don

Sam, Brake lights, turn signals and parking lights were all fused in the original wiring. The main circuits that are not fused are head lighs, fuel pump, distributor and overdrive, if installed. If you can be sure you will never have a short circuit you will not need fuses. If you do have a short circuit the damage can be rather extensive as Don said, if the circuit is fused the only damage is a blown fuse. FWIW, Clifton
Clifton Gordon

Samuel

Many years ago I was headed down through the Pali tunnel from Honolulu to Kailua with the lights on (not an MG) when I lost the tread on a front tire. Impact of that section of the tread on the wire leading to the side light removed the insulation and provided a dead short against the now bare fender steel resulting in LOTS of smoke inside the driver's compartment as the main wiring harness fried. Seeing was a greater problem that any loss of control due to the tire shedding its tread.

I now believe strongly in fuses on sidelight wires.

FWIW

Larry
Larry Hallanger

Samuel, if the car maker fitted a fuse then it was for a good reason. MG didn't expend money on non-essential parts, they prefered to give good performance at a reasonable cost.

Modern manufacturers are a different story. If you can save $1.00 on a run of 1 million cars, you earn your extravagent salary.

Good "OLD" British cars are built for long term reliabilty and don't skimp on quality for the sake of a few dollars! That's why they all went out of business and "Jap Crap" has ruled the world!

Oh for the good old days.

Cheers

Tony
A G Oliver

This thread was discussed between 09/10/2004 and 13/10/2004

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