Hello,
I have a 99 5.9L 4X4 auto D with around 102K miles on it. I just recently bought the vehicle and noticed as I was driving around that the A/C would blow harder and simultaneously the voltage meter would increase. It would do this sporadically with no set pattern. Later on I was driving around at night, and while the A/C and voltage meter increased so did the headlights as if turning on the brights. Not sure where to begin if with the alternator or the PCM or ? Any thoughts or suggestions would be great as it's quite annoying having folks flick you off thinking you are flashing your brights at them for no apparent reason!

 

i would begin by cleaning the terminals and connections at the battery, then i would have both the battery and the alternator tested

let us know what they say and we can proceed from there.

 

+1 one on the terminals and battery/alternator test...

also, if it where me and the battery was more than 2-3 years old i would go ahead and replace it with a duralast gold battery anyways even if its just for peace of mind. With a battery of that age chances are there not much life left in it as the average battery usually gets replaced around then...

just my 2 cents!

 

+3

Battery / Alternator.

 

Sounds like you may also have a battery temp sensor going out. It's in the bottom of the battery tray. Has a round spring loaded flat disc shaped top.

 

Hello all! Thanks for all of the input, so here's some additional information...

Battery was replaced late last year, terminals were both replaced less than a month ago and the alternator was tested as was told was charging correctly. I haven't heard about the battery sensor, is there a way to test this to see if it is the culprit causing the issue?

 

The battery sensor works off the heat build up in the battery. If the battery gets to hot from the alternator charging then the sensor tells the PCM to reduce the voltage to the battery until it cools down. If the sensor is bad the voltage is not reduced and you'll get voltage spikes exactly as you're describing. Not much of a way to check it and it don't cost that much so I'd replace it.