The MGA With An Attitude
SEQUENCE OF WIRING - Don't Burn the Harness - ET-100H

At 08:21 AM 9/4/2007 -0600, Mac Askari wrote:
.... "When the dash was installed and power was applied to the dash harness I had a big melt down in the dash lighting system. I had to cut the wires in the harness and redo with new wiring for all the lamps."

This is a series of notes on a recommended sequence for making the end connections after the harness is laid in place and all of the electrical devices are in place (but not yet connected). In general, wire the dash panel separately before installation in the car, as was done at the factory. Then make all ground connections (black wires) throughout the car. Then connect power to the lighting switch, followed by connecting all red wires (parking lights, tail lights, license lamps, map light, supply to the dash dimmer switch). That takes care of more than half the wiring circuits in the whole car with very little confusion. The remaining wires are all color coded with unique colors, usually only a few at a time in remote locations, so not much left to cause confusion. Do as subsets the headlamps, fuse block, brake light and turn signals, generator and control box, ignition system and fuel pump, horns and heater blower.

Do individual circuit testing with almost every wire being connected. I would never recommend connecting the entire harness without testing, and then connect the battery and expect things to work. There is far too much risk in that approach. One of the irrefutable laws of electricity should fall under common knowledge (and common sense). If you connect battery power to a ground contact (with no resistive load) you get a welding current that can burn wires. This does not necessarily require an error in wire connections, as a ground fault in any electrical device can do the same thing.

For testing of the circuits it is appropriate to use a power supply that may be limited to 10 amps or less (like a small battery charger for instance), so you are far less likely to fry wires if you happen to encounter a short to ground. Another alternative to limiting the current is to connect one brake light bulb (21 watts = 1-3/4 amp limit, 7.2 ohms) in series with the power supply. You could use two bulbs in parallel (42 watts = 3-1/2 amp limit, 3.6 ohms) for less dimming effect on vehicle lamps. You might also use a 3.2 ohm 50 watt power resistor (3.9 amp limit at 12.6 volts = 50 watts) for current limiting during testing. When each circuit is tested and known to work, then move on to the next circuit.

The following pages give a wire by wire sequence for hooking up the harness without blowing fuses or burning wires.

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