Electrical Problem

I replaced the wiring harness because it melted. When I started with the new harness installed the wire from the alternator to the ignition light on the dash and the wires to the inerthia switch melted again. Has anyone had a problem like this?

You have a short circuit in something the wires feed. Go to this link, under stock schematics, find MGB and get the correct wiring diagram for your car.
http://www.advanceautowire.com/

Anytime you have melted wiring you must find the reason the wiring melted and repair the problem. When you are rewiring a car that had melted wiring connect a fuse (10-15 amps) between the battery ground cable and the battery. The fuse will blow if you still have a problem, but the new wiring will not melt. Start removing circuits one at a time and testing again until you find the cause of the short circuit. In your case the problem will probably be in the alternator or the inertia switch, but don't just change them, find the problem rather than throwing new parts on.

Clifton

If the problem is the inertia swich can I connect the wires directly together until I get a new switch ?

Yes, you can just connect the two wires together for the inertia switch. Use a meter to determine if the switch is bad. The switch should not have any grounds, so I don't know how it's failing would cause a meltdown. Verify that the wiring is correctly connected. Check the white wire circuit for damage.

I installed a fuse on my ground terminal on the battery. I checked all the wiring out, it all seams to be OK, but as soon as I try to start the engine the fuse blows every time.

I installed a fuse on my ground terminal on the battery. I checked all the wiring out, it all seams to be OK, but as soon as I try to start the engine the fuse blows every time.

What's the rating of the fuse?

The fuse or small bulb is only a temporary test to check that new wiring or circuits aren't short circuited, if there is a problem the fuse blows before the wiring is damaged. Once the inital checks are done the fuse MUST be removed and the battery cable connected normally before attempting to start the sngine. I guess I assumed you would know it's a temporary test and not a repair.

Maybe I didn't understand when your wiring burns out, is it only after you start the engine?

Clifton

I was using a 7.5 amp fuse when starting the car

The wires get hot and start to melt after the car starts.

Forget my suggestions to use a fuse or light for testing. Did you replace the complete wiring system in your car? I assume you have connected the white/yellow wire from the alternator to the ignition light and the alternator plug brown wires are in the correct position. I would remove the alternator and have it tested. Did you try it with the white inertia switch wires disconnected and tied together?

Clifton

Rick-
After rereading your post above, the only difference I can come up with between the car being on and the car running is the alternator. Have you had your alternator tested?