1996 Mazda b4000 • 6 cylinder 4WD Automatic • 96,350 miles

Hello, I have a 1996 Mazda B4000 with a 4.0 EFI and I have noticed lately that once the truck has warmed up and the idle drops so does the battery gauge. It drops very low to the point of the headlights start dimming and the fan motor slows down. But, once I start driving, the lights get bright again, and the fan speed goes up too. Does this mean I need a new alternator? Is there some way to confirm that I need a new alternator? Thanks.
January 5, 2011.

If the RPM's are getting low enough, this is normal and the alt is fine. However, what is causing the RPM's to get that low. Check for vacuum leaks, check idle air control valve, check fuel pump pressure.

Thanks Jacobandnickolas, would you know what the proper setting should be for the RPM's after it has warmed up and I'm in drive and sitting idle? I have conflicting answers. It seems that the RPM's are about 600 to maybe 700 or so. I can't tell that well due to the fact that there are only 500 line increments on the RPM indicator. Even when the truck is idleing what looks like to be very low, it seems fine. Meaning there is no shaking or rattling at that low idle. The truck is running smooth. Thanks for all your help. Can't figure this one out.

Feb 14, 2011.
700 sounds right.