Hey Guys,

I just replaced my alternator today for the reason that when I drive with my a/c on, my voltage drops at idle and I have to bring the rpm's up in order to get a charge in and bring the voltage up. So, that is why I replaced my alternator. However, with the new one in, I am still having the problem. Any ideas? My battery is roughly about 3-4 years old if that helps.

 

#1 What voltage do you get at idle at the battery? (with A/C on and with A/C off)

#2 What output voltage do you get on the alternator's B+ post? (withe A/C on/off)

#3 What RPM are you running at idle with A/C on?

#4 Is it a Monteo or Monteo Sport and what year?

#5 Are you using a gauge on the dash or are you using a multimeter to check the voltage?

#6 Is the battery actually dying or is the car stalling?

 

For some reason my vehicle didn't want to replicate the problem but here's my best.

1.A/C off 14.14, A/C on 14.09
2.A/C off 14.18, A/C on 14.07 (12.80 with A/C on and Headlights on)
3. A/C off idle is 700ish, A/C idle is 980ish
4. 1999 Montero Sport
5. Gauge when driving but multimeter produced these results.
6. No

The biggest drop I have noticed is when I have the A/C on and the headlights on. The voltage drops to 12.30 when I put the car into drive.

Maybe I just might be paranoid or maybe not.

 

Those are good voltage numbers. What you have is simply your RPMs varying due to AC load. Don't sweat it.

 

Most all vehicles have lower voltage at idle even lower with many loads on. As long as it goes up with RPM don't worry about it. Factory voltage meters are not always accurate.