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MG MGA - Holy Smoke!

Hey all, Disaster! I had reached the stage of the restoration where I was ready to start the rebuilt engine.

After finally putting the car in fourth and pushing it backwards, I finally got oil pressure when spinning the starter.

I turned on the ignition switch but got no joy. I tried the map lights and wiper but they were also dead. Next, I removed the regulator box cover and found no 12 volts at terminal "A1".

Looking closely, with the meter probe in hand, I apparently shorted the contact on the upper relay. By the time I pulled the battery cable off, the black leads in the harness were fried. Even the ground lead to the W/W motor was cooked.

I am now in the process of replacing the main harness, Now, the question. What can one do to prevent this from re-occurring? More fuses?

Oh yeah, it also appears that the W/W motor is gone. What else could be damaged? Generator?

Thanks, Tom, angry with himself.
Thomas McNamara

On my web site there is wire by wire instructions for installing a wiring harness. It is a long and boring article, several pages, but it will get you there in one piece. The important part is to test every circuit as it is being connected so you don't run into any nasty surprises when you switch on power at the end. Start here: http://mgaguru.com/mgtech/electric/et100h.htm
Barney Gaylord

Been there, done that Tom.

I had a perfectly good wiring loom that worked just fine, until I tried to connect up the non-operational original sidelights.

I followed what I thought was the correct colour wiring diagram, switched on the sidelights and watched the front part of the loom quickly go into meltdown!

It cost me the UK equivalent of $1000 to get the wiring loom replaced!

Now, whenever I am brave enough to fit something new to the electrical system, I disconnect the battery, double check everything, then I connect a low output battery charger as a power supply.
That way, if anything is wrongly wired, the battery charger would just trip with no damage to the loom and give me a second chance to do the job.

So far, I havent made the same mistake again but the thought of burning up another $1000 certainly aids the concentration!

Colyn
Colyn Firth

Hard to figure what you did, but it sounds like you fed power directly into the earth side of the system. That should not damage anything except the black wires, plus whatever wire was supplying the power.
Fuses in the car system won't help, as they go in the hot side of circuits.
Fuses on both legs of a power supply would prevent this, or more careful work!

FRM
FR Millmore

Trouble is, the black wires are surrounded by lots of others that can heve the insulation melted off them.
Art Pearse

Hey all, whew! I have replaced, wire for wire, the melted wiring loom. Yes Art, you are correct, the black wires, (earth), melted insulation anywhere they touched.

I now have all of the dash lights, heater fan, W/W motor ignition circuits "hot". Everything works after connecting the battery through a 5 amp fuse.

The big mystery is; what the heck happened? How could/would +12 Volts from the battery appear on the ground lead?

Perhaps, tomorrow, after a 3 year wait, I will start the motor! Hooray, I hope. :)

More to follow, Tom
Thomas McNamara

This thread was discussed between 06/10/2012 and 12/10/2012

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