MG Car Repair: 1979 MGB Electrical Problems, fuse panel, headlamp switch

Question
Mr. Fitzcharles,
   I need help.  I recently purchased a 1979 MGB and it has suddenly developed serious electrical problems.  While uninstalling the radio/heater control console to remove a faulty radio, a short occurred and the bottom fuse in the fender fuse panel was blown.  The car started fine and I replaced the fuse, but since then things have rapidly gone downhill.  Initial symptom was a failure to start which I believed to be a dead battery.  The starter would engage the engine and make a weak attempt at starting and then cease until the next time the key was again turned and again the same weak attempt.  I pulled the battery and took it to a service center where it tested ok.  Back again in the car, the next attempts were even worse than before, now the starter would no longer engage the engine.  No solenoid click either.  Also, a clicking sound which had previously occurred behind the drivers seat whenever the ignition switch was moved to the acc position had also disappeared.  A voltage tester to the solenoid terminals shows good voltage to the solenoid, but that it drops from 12-14V to less than 1V upon turning the key to the acc or start position.  I would have concluded a solenoid problem, but the same voltage drop  occurs when the horn lever is pushed or if the headlamp switch is flipped.  With any of these actions, voltage disappears system wide, yet remains a constant 12V at the battery terminals.  I’m out of my element and out of ideas.  I would greatly appreciate any kind of suggestions you could provide.

Thank you,
Travis

Answer
Hi Travis,
Your symptoms only narrow the problem to only one thousand things. The ONLY way I was ever able to diagnose massif electrical problems is with an wiring diagram in one hand and a 12v test light in the other.
However, one of your symptoms does point to a very high resistance in a main cable (positive or negative) so you must start by doing voltage drop tests on the cables on both power and grounds.
If you are not familiar with cable drop tests you have basically done one already when you seen the 1v. Start at the battery and put the volt meter on a low volt scale and put the test leads on either end of the same cable being tested. For example, put the positive test lead on a clean piece of metal on the engine and the negative test lead on the negative POST of the battery. (not the cable end) and try to start the engine. You should see less than .5v. If you see a lot higher voltage than you have a bad ground between the engine and the negative POST of the battery. Now put the positive test lead on the positive POST of the battery and the negative test lead on the positive POST on the starter solenoid and try to start the engine again. Again you should see very low voltage. This is a voltage drop test of a cable.

Any time you know that you have a battery with 12v and under load you see only 1v, it means there is a point in the circuit that has very high resistance (bad connection)

An old mechanic's trick is to hold the starter in the start position for about 10 seconds and then quickly feel all connection ends, positive and negative for heat. The cable end will get hot. It can be a battery cable end or on the solenoid or a ground strap ends between the engine and frame or a negative cable end on the battery or on the frame.

If all that fails to ID the fault you need to take your volt meter or test light and test each joint starting at the battery and ending at the battery.

If you need a wiring diagram and don't have one let me know and I will put one up on my web site for you to copy.
Howard