can someone answer this for me? in the morning when my car is cold, the alternator puts out about 13.5 volts. As the car/alternator warms up, it slowly starts putting out less voltage. By the time the car is fully warmed up... I'd say after about 20-30 minutes of driving, the car puts out no more than 12.5 volts when driving. When I'm idling it will drop to 10.5 volts.. at which my radar detector resets itself... When it's cold, I get 13.5 no matter what I do, revving doesn't affect it. After it warms up and I'm idling, it will go from 10.5 to 12.5 from revving the motor... I'd say on average when driving after the car warms up, it will stay around 11.8 volts, with the heater on... if that matters. I'd say its the alternator, but its only 1.5 years old, and the battery is only slightly older. What do you guys think? I wonder if its possible that the alternator wiring is bad and it thinks the battery is fully charged, so it shuts off... and then my voltage drops... I'm puzzled at this one. Thanks in advance, -Steven

Check the voltage at the alternator output terminal for an accurate reading.

Nope, the Alt is always suppling power to the car when the engine is running.

Don't trust the dash VOLTS gauge, it lies. Use a handheld voltmeter to check the voltage at the battery and/or Alt. Should read 13.5-14.5 volts.

Might have a dirty (high resistance) contact in the ignition switch, or bad power or ground connection in the dash wiring.

I will go out and check the actual battery, since I guess I could be getting a bad connection at the cigarette lighter (where my radar detector in voltage readout mode is located)

however, the guage in the dash is relatively close to the digital readout, and its 100 relative, one never goes up and the other down, or one up and the other stays the same.

Also EVEN if I had bad connections on the voltage readers, would they not be proportional to the real voltage? If my car reads 13.5ish at both voltmeters, and once it warms up and they both read about 11... that would mean that the voltage input to my dash, and my detector were not proportional.

Something to add onto this... the temperature dropped significantly here in Texas the last few days, and my voltage is staying at a healthy 13+ right now! Very strange, but when the weather gets warmer, my volts go down...

at about 40F outside my volts stayed at about 13.2 for a while, then later it got to about 50-60F and my volts dropped to 12.8... its almost completely linear with the temperature-- and it's got me stumped.

I will go outside with a voltmeter however, and take a true test, BEFORE and AFTER I start the car

what should the voltage read after the car sits for 10 or so hours?

also my brother's car reads 13.5 when the car is on, regardless, and off he gets like 12.5 and it slowly drops to 12...

when I turn my car off its always lower than 12