MG Car Repair: 71 Midget electrical problem, chassis frame, bad earth

Question
I was troubleshooting an electrical problem on my '71 Midget, and I've caused more problems than I started with. I turned the ignition on after working on non-functioning turn signals(the ignition light is on). When I turn the key to start it, or turn on the head lights, the electric goes dead, and the ignition light goes off. I cannot get the ignition light to come back on unless I disconnect and reconnect the battery. Any suggestions?

Thanks,
Chris

Answer
Hi Chris.  This is a classic electrical problem.  Either you have a bad earth (ground) circuit, or you have a bad battery.  

If you have an electrical multi-meter, turn the headlights on.  With headlights on, the battery should still be able to produce more than 12.5 volts.

Next, check the voltage across every connection.  You may find that you are losing 12 volts across the bolt which connects the battery earth strap to the frame of the car, or across the positive battery terminal, or across the earth strap that attaches the gearbox to the chassis frame underneath the car.  One corroded terminal could cause all this trouble.  

Finally, try a set of jump leads across the battery.  If everything works OK, then your battery is poor (unable to generate enough current to power the starter) so it needs to be replaced.