Charging System Voltage

[MACH1MUSTANG]

On my 69 351W I have charging problem or maybe I don't. Anyway, at idle I am getting 12.15 volts at the battery but when I shut it off the battery voltage goes up to 12.43 when I am checking it with a multimeter. The battery is fairly new and I just replaced the voltage regulator. The battery doesn't get low or cause had starting I just thought it was kind of strange to see a higher voltage reading at the battery with the car off. Any advice or suggestions?

Thanks,
Jim

 

[JB66]

With engine running, you should have 13.5-14.5 volts going to the battery. What aftermarket equipment are you running?

 

[MACH1MUSTANG]

Only a Sport Comp Tach.

 

[JB66]

check the voltage at 1500/ If it's not as I posted, you have problem.

 

[GrandeStang73]

Does the car seem to have any other symptoms of a bad voltage? examples: weak acceleration, headlights dim, battery ever going dead. and is there any other accesories? air conditioning, heater, high beams on?

 

[krash kendall]

Your alternator isn't chargeing. The higher voltage with the car off is because you are not loading the battery with the ignition. The battery may not be dead now, but it will be after some driving, especialy with the lights on.

 

[10secgoal]

Yep krash has it. Lower voltage when started means a drain. Unless higher at 1500 or so like JB66 said. You should see about 1.5v higher when the car is running than when off.

 

[MACH1MUSTANG]

O.K. so far I've changed the voltage reulator and now the entire alternator and still I'm getting the same result. Anything else electronic that could be bad other than a broken wire? There is a 3 prong connector coming from the alternator and going to the voltage regulator. With the car running how do I hook up a volt meter to those wires coming from the alternator to see of there is power at that point? I don't want to just start touching wire for fear for frying something. If I have a bad wire, I want to try and isolate it to a specific run of wire.

 

[GrandeStang73]

take youyr voltmeter and take the negative and ground it on your chassie, then the red prong on your alternator, with it still on, touch it with the red end on your voltmeter, thats where you get your charging number

 

[Max Power]

This sympton can happen when you have an open in the wire to the field terminal on the back of the alternater. The wire, if I am not mistaken, is the orange wire from the regulator on non-tach cars. I cen't remember on tach cars. Get a wiring diagram to see for sure.

 

[JB66]

On a factory alternator/regulator system, to check the maximum alternator output, you need to pull the connector off the regulator and jumper the 'A' and 'F' terminals on the connector to each other. Read the voltage at the battery terminals. It should steadily increase, but don't let it go over 14.7 or so. Or you can take it to one of the auto stores for free testing. Other than that, check for continuity or shorts on each wire (disconnected, of course).