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MG MGA - wiring woes

Finely finished my wiringof my 1959 MGA. connected the batteries and when turning on the side lamps and headlights, they worked. howeer, 30 seconds later , noticed smoke from under the dash. The red wire connecting to the s1 switch on the lighting switch, was smoking and melted within the fron harness going through the fire wall.There are 4 red wires connected to the s1 switch. It appears that the wire that melted was a thin gauge wire which appears to make no sense. I would have thought that the wire form s1 tp the lights would be one of the thicker caliper wires.I am sure that this wire was the one going to the front and rear lights , since I disconnected the other three wires and the lights still worked but then melted the wire. could it be tht my voltage regulator from the A1 terminal to the A post on the lighting switch is not working to regulate the voltage. the control box is new. I need some guidance. I am severly depressed for have been working on this project for 4 weeks and was sure it would be successful

michael dvorkin
michael dvorkin

Last thing first, this has nothing to do with the voltage regulator.

I hope you didn't connect a thin red wire to the headlights. Red wires are individual circuits for parking lights, dash lights, map light, and a single fog lamp or driving light. If you use dual fog or driving lamps you need a relay to off-load the small red wire.

Headlights are powered from the S2 terminal of the lighting switch via a heavier blue wire running to the dipper switch. See diagram here: http://mgaguru.com/mgtech/electric/circ_d.htm

If you have everything wired correctly, there may be a short to ground at one of the corner lamps. The red wire fromteh lighting switch is the first to suffer, as it carries the load of all corner lamps including the one which may be shorted.
Barney Gaylord

Michael,
I know that is really painful. I hope someone here can help a you find out what went wrong.

The headlights are powered from the thick blue wire on S2. The headlights can draw around 10amps, so they need the heavy wire. The thin red wire from S1 only powers the side lights, tail lights, and license light. Alltogether, these lights only draw around 2.5amps, so the wire doesn't need to be very thick. One thing I can think of, would be the connections to the tail and parking lamp sockets. These bulbs have two connections, one for the side/tail and the other for the turn signals. If you replaced all the pigtails, this would be really easy to mix up. The side/tail connection draws only 0.5amps per bulb, while the turn sig connection draws 1.75amps per bulb. If you had the connections on all 4 bulbs reversed, you would be pulling 7.5amps thru the red wire instead of 2.5amps. I am not sure of the exact size of the red wire so I can't calculate if this would be enough to smoke it, but it probably would.

Jeff

Jeff Schultz

went back out last night and ran another wire to the junction of dash harness to the rear harness and headlight harness. By connecting the s1 post to the front red sidelight harness, the system works and no meltdown. when connecting the s1 post to the rear red harness wire, it shorted out. also no back lights or license plate lights. Short must be in the rear harness delaing with the rear lights.

Michael
michael dvorkin

Micheal, you have my sympathy, it's a pain when that happens. I never, ever hook up a new harness to the battery. I've been doing related stuff all my life and I still don't trust myself that I got it exactly right the first time.

I hook the battery cables to a small power supply (7 amps or less) or else use my small battery charger to power things up for the first check in lieu of the battery itself. The charger has an ammeter so I can see if excessive current is being drawn. I can then check everything except the starter for proper operation.

Kept me from burning up a new harness in the B.
Wray

This is the sort of experience that should - sooner or later - convince everyone that EXTRA FUSES ARE A GOOD THING! It makes no sense to have unfused circuits running around a car, especially multiple circuits from single feed points, Originality be damned.
FRM
FR Millmore

You took the words out of my mouth FRM the very worst part of an MGA are those unfused circuits. To my knowledge this has totally caused the burn out of 2 cars that I know. The very worst thing is the dimmer switch, this is notorious for shorting out to earth and being on an unfused lighting circuit could easily cause a fire.
Bob (robert)

This thread was discussed on 01/09/2005

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