Tach Spikes with Acc use??

[Yukon6.2]

Hi All
Yesterday i was out with the 98 k3500 tow truck.It was a little hot so i switched on the AC and i noticed the tach hit the 5000 mark.So i tried a few more switches and every power switch i turned on would spike the tach and stay there.Headlights,blinkers,and even the aftermarket stuff like the light bar,backup lights ect.The battery guage showed a steady 14 volts at all times.Dosn't seem to be any charging problem.
Do'es anyone know what might be the conection between these?
Thanks Thomas

[JohnC]

Tach runs off alternator. Accessories load alternator...

[trbankii]

I was going to say similar to John, but then saw you said it spikes to 5000rpm. That's not right. As John says, accessories will put a load on the engine. But something else is wrong there. I think I'd start by checking grounds.

[Yukon6.2]

Hi All
Reviving an old post.
A couple of days ago i noticed the voltage was low in the tow truck,the alt wasn't charging.
Replaced it with a used one,it charges good,and the tach spiking when lights or heater or anything else gets turned on is gone.Went for a short tow and the tach showed steady with heater and flashing lights on.
Now i have to get another alt,or try to fix the old one,for a backup.
Thomas