I just put an aftermarket tachometer on my 88 pickup, so i have never really seen the actual idle. Today i noticed the RPMs drop about 2 to 300 when i turned the headlights on. It happened every time for me today. As soon as i turned the headights off, it would go back to a little higher idle. I kinda doubt this is normal, i could be wrong. But anyone have an idea how or why this could be happening?

 

Could be your battery is going, but most likely it's caused by bad connections. Clean both battery terminals & the connections, & ALL grounding connections from body to frame to engine to alternator. 9 times out of 10 this will solve electrical issues. I'm certain someone will detail "the big 3" for you, too....

 

I know turning on the headlights create a surge on the system. The alternator is pulling power off of the motor to recharge the battery. The RPM's should go back up after the battery is charged again. Does your truck do this or does it stay at the low RPM's? If it does I would say your alternator is needing new brushes...

 

mine does the same thing....surge, but then it comes back up to my rpm range 700-800...did it do it before you put the aftermarket tach on?

 

im really not sure, because my idle has been a little high and after i put the tach on i adjusted it a little lower. So i may have never noticed the difference since my idle was at like 1200 before i adjusted it.

What would be the best way of grounding my truck?

 

I would suggest set your rpm level to spec with no load on the battery or alternator... Everything off. Take the truck to autozone or what ever you have and have them test the battery and alternator. If it isn't those two, then start checking ground points. Eliminate the two easiest first and go from there