Battery or Not?

[no1plantman]

My 99 4.0 SOHC Sport is acting funny, started surging, like flipping a light switch off and on and the led display and speedo were whack. I get 12.35 with the truck off and 12.35 with the truck running and lights and radio on. The minute I turn the AC/heater on, she dies. any takers?

 

[SoNic67]

The alternator is bad.
Soon you won't be able to start and if you keep trying you will kill the battery too (if is old enough)...

 

[no1plantman]

what should the alt be reading tyo determine that it is the culprit?

 

[pcam13]

13.5-14.5 running. Check the AC output for a bad diode. Ac ripple waves send all that stuff whacky when a diode fails.

 

[SoNic67]

14.0-14.4V with engine running at 1000 rpm. High beams on/off should not affect voltage out of this range.

 

[no1plantman]

14.37 at battery and alt when running. Battery at 12.34 off and alt at 13.26 off

 

[J_C]

You shouldn't read higher at the alternator than the battery with it off. Clean the battery terminals including their rear where you're taking the measurement, then remeasure, BUT I also think it's probably a bad alternator. Low battery that's still high enough to (start) run the engine shouldn't mess with the speedo.