Console Battery Gauge Reading/Alternator Charging Question on a 1969
 

My alternator went bad on my 69 so I took it out and had it tested to be 100% positive and they did confirm it was bad. I replaced it with a brand new NAPA Power Master 63 amp unit. I have electric Holley fuel pump and run 2 spal 13 inch fans and that is it for electrical consumption unless I have the lights on. Of these 2 fans one keeps the engine cool which gets triggered off a sending unit and the other I have toggeled just incase I need some extra cooling. I have confirmed that I'm getting 14.75 volts at the battery now when I start the car and when I look at my battery gauge in the console it reads to the right side or positive side of the gauge showing that I'm getting a charge at the battery. As soon as my one fan comes on the console gauge jumps up so its right between the 0 and the +40 on the gauge and stays there while the fan runs. At the same time I have a meter on the battery and I'm getting about 13.98 volts to the battery now. What does this tell me? I would of thought the gauge would have read the other way (towards the negative side) showing less of a charge to the battery when the fan is running. Do I need a bigger alternator or do you think a 63 amp unit is ok? Do I have a problem somewhere still? Is my gauge messed up? Any thoughts?

Re: Console Battery Gauge Reading/Alternator Charging Question on a 1969 (James)
 

The 69 ammeter is actually a differential voltmeter wired between the battery and alternator. When alternator output is greater than battery output the needle deflects to the positive side. If it goes to the negative side when the fan turns on this would indicate a problem. Deflecting to positive should not be a concern, especially if your independent voltmeter shows 14 volts.

Re: Console Battery Gauge Reading/Alternator Charging Question on a 1969 (Missileman)
 

Thanks for the explanation and help!!!!
Could I be correct in saying that since my car does not have air conditioning and had a 42 amp alternator from the factory is why the battery gauge has a range of -40 to 0 to +40? The reason I ask is I found out that my one fan pulls 20 amps when it is running. My gauage reads right between 0 and 40 on the plus side when it is running. Is this showing me basically that the alternator is having to generate 20 amps of juice to run that device?