Classic/Antique Car Repair: 1950 Ford F-1 6 volt

Question
Have Ford 6 volt problem.  Battery has 6.2 to 7.4 volts when not running.  Positive ground to engine block.  Took positive cable off battery after cranking truck (to see if engine would quit) and checked voltage shows 18-22 volts at battery.  Has brand new voltage regulator (as was having the same problem with prior regulator) and flashed the regulator to polarize it before cranking.  With both positive and negative posts connected to battery goes as high as 40 plus volts at posts!  Original wiring still on truck.  Any ideas what's wrong?  Generator has also been rebuilt recently.

Answer
Well let s what we have here. First if the voltage at the battery when the engine is not running reads 6.4 to 6.5 volts it indicates a fully charged battery. The 7.4 volt reading that you are getting is a fully charged battery plus a bit of surface charge. To get an accurate reading on the battery once you have shut the engine down is to turn on the headlights for 15 to 39 seconds. This will remove the surface charge and give you a true what is called open cell voltage. Now the reading on the battery cables with the battery disconnected is not a valid test of anything other than your voltmeter works. The charging system needs to work up against a load like the battery. The voltage that you look for is with the engine running about 1500 RPM and no accessories turned on. The charging voltage should be about 7.4. If it is higher then the voltage regulator is out of adjustment or defective, or there could be a short in the wiring harness. With the engine running at fast idle and a volt meter across the battery, you should see up to about 7.5 volts. If the voltage is higher disconnect the field wire from the generator. The voltmeter should drop to battery voltage. If the voltage does not drop then the problem is in the generator. Reconnect the field wire and the voltage should go back up again. Now disconnect the field wire at the voltage regulator. The voltage should drop. If it does not drop the problem is in the wiring harness and if it does drop the problem is the voltage regulator. Make sure that the voltage regulator has a good ground as well. Good luck, keep me posted.