Expert: Brad Sears - 6/6/2009

Question
QUESTION: i have a 31 model A tudor sedan. recently the ammeter went from a solid +6 to a steady -2 on the ammeter when driving. have had the generator tested, new cutout, new ammeter, and rechecked all the wiring.  battery at rest is 6.2v.  if i disconnect the front wires of the cutout, the generator will put out 7+ volts depending on the RPM/movement of the brushes.  however when i reconnect the horn/headlight wire to the front of the cutout, the generator output drops to barely 6.  had the vehicle in a classic car shop, they can't find a thing wrong with wiring.  could this be a ground problem with the lights/horn/generator?  thanks

ANSWER: The wire that you disconnect that you say is the horn and light wire, is it also the live battery wire?
Let me know.
Brad

---------- FOLLOW-UP ----------

QUESTION: brad -
 update, picked up the car this morning from the classic car place.  there are 3 wires at the front of the cutout.  two wires are joined to a single eyelet, these are horn/headlight.  the other eyelet is the live battery wire. when the live battery wire is attached to the cutout, that is when the generator output drops. it was suggested that perhaps the battery need replacing. i do not know how old it is, but then again, it starts the car right up. comment was made that sulfide buildup or a dying battery might prevent a charge.  appreciate any assitance, this electrical stuff is like finding a single tree in a forest.  R/ Joe  

Answer
OK, I got the picture now. If the battery has at rest 6.2 volts and it will hold that overnight it appears that the battery is ok. I would look to the generator as the source of the problem. It appears to produce voltage when under no load but as soon as you load it then it refuses to charge. There are several connections in the generator case that could be faulty and test under no load conditions as OK. To make sure that the battery is OK you can load test it. A simple load test is to disable the ignition system. Put a volt meter across the battery and crank the engine for 15 seconds. The voltage while cranking should not drop below 5 volts. If it does charge the battery and test again. If it still drops below 5 volts the battery is weak, but still would not cause the problem you have.
Brad