voltage guage jumps

[dalez]

Very strange problem just started this week. I start the truck and take off down the road and with the heater fan on high and the voltage guage starts jumping between 16 volts and 15 volts. when I turn the heater fan down to 4 it quits jumping around when it does jump around I can tell that the voltage is actually dropping because the fan does slow down with out touching the switch. After the truck warms up it quits doing it. Before this happened I noticed that the voltage when I first start the truck was at 16 volts that seems very High could it mean that I have a bad battery. I am tempted to put a new battery and alternator on but don't want to spend the money on both if I don't have to. I did check the cables between the alternator and the battery and cleaned them even though they were clean. 2001 2500HD 8.1 alley 50000 miles. just got it 6 weeks ago. any help would be great.

[facn8me]

sounds like a volage regulator problem. You can check the cables for resistance with an ohm meter and rule them out first.

[BBSilverado]

I agree with facn8me, sounds like the voltage reg in the alternator. I would replace that soon before you cook your battery.

[71GMC_3/4T]

Another thing to check is the grounding of the battery (-) to the chassis. Use a voltmeter and measure the voltage between the battery (-) terminal and a good clean spot on the chassis. It should be very near zero volts.

 

The heater motor is probably the second largest current draw on the system, with the starter motor obviously being first. I've noticed my heater motor slows down when the engine is idling and the fan is on high.

 

I agree with above folks though, sounds more like a regulator problem.