Volt Drop At Idle With Blower Motor on High and Brake Lights

[Diesel]

My truck was dropping voltage at idle tonight. I could bring the voltage back up by lowering the fan speed when running the AC. Taking my foot off the brake pedal so the taillights would turn off would also bring up a little voltage. What could this be?

[PJ 3]

Could be your positive connections on your battery terminal. I know that my truck did the same thing. My headlights and blinkers also affected it the same way

[Diesel]

Terminals are tight. I have a 91.5 so I don't know if that means anything with my charging system. I get a feeling its the battery in my truck that is no good even though its pretty new (Walmart 2010 battery...)

[BC847]

Both the AC compressor and the AC blower on HI are a pretty good load on the alternator.

What was the engine RPM when this occurred? Is your idle rpm high enough? IIRC 850ish in drive with AC on high.

[Diesel]

Im not sure what my idle is but I am guessing then that there isn't a problem if that is a high load. I just figured a working charging system shouldn't lose volts because of a blower fan especially when these trucks have no electronics.

[BC847]

I should reword my response above.

- The blower on high is a good load on the alternator.
- The AC compressor is a good load on the engine.

The compressor can pull the idle rpm down enough that the alternator's output will dip a little. Seems like all alternator's output ratings are presented at a given minimum rpm.

[Diesel]

Ahhhh yes now i got you. Ok that makes sense. I think you are absolutely right. I will also swap in a known good battery to see what happens.