MG Car Repair: Battery problems, overdrive switch, 12 volt battery

Question
I have a 1966 MGB.  It has been converted to neg ground and has a 12 volt battery.  I have experienced dead batteries several times in the last few weeks.  The battery seems to be ok, and im wondering where the draw on the battery is coming from.  Ive noticed that the overdrive switch has been left on.  When it is on does it provide a draw on the battery??

Answer
Hi Dave,

There is a procedure to go through to test for that.

First you must test the voltage across the battery (12v +) Then with the voltmeter still on the battery start the engine and set the RPM at about 1500 RPM and read the battery voltage. It should read 13.8v to 14.5v. If it is any less or more then that you have a charging system problem.

Question!! When you changed polarity, did you polarize the generator? and did you switch the small leads on the coil? and switch the two leads on the heater fan motor? Also did you switch the red and black leads inside the tach? The leads in the wiper motor should be switched also. Permanent magnet motors run backwards when the polarity if reversed.

The test for a draw on the battery is to turn everything off and remove either one of the battery posts and put a 12v test light between the cable end and the battery post. If it lights the light, you have a draw that will run the battery down. Then the procedure is to leave the test light connected and start by removing fuses from the fuse box. If the generator has been replaced with an alternator, unplug the wires from the alternator to see if the test light goes out.

That should get you started on finding the problem.

Howard