RPM reading drops when lights are on

[xtreem3d]

Happy Holidays guys,
I have a machine that just started to show the rpms and battery volatge drop when i turn on my lights. The RPM 's don;t actually go down just the readout. I seem to remember that the alternator is what guages the RPM;s. Any idea if that could be it or more like a battery? Sounds like a no brainer to check it out but it's at a high end business for snow and I don't want to turn it into a repair shop. I am hoping I can identify the problem and take what I need there to fix it quick..
Thanks
Steve

 

[Mike10]

Could be the alternator but check your belt tension first.

 

[xtreem3d]

Could be the alternator but check your belt tension first.

well that would be a quick fix, huh? thanks mike i will check it out