Headlights effect my idle, whats up?
 

I noticed lately that when I turn on my headlights it effects my idle. Headlights on my idle goes down, headlights off my idle goes back to normal. What should I check?
Thanks,
Bev

Hi Bev,

What is the model and year of the car?

dd

Does turning on the AC have the same effect?

It is a 1977 300D with 127,000 miles

Have you made any mods to the lights, like adding 80/100 watt type? A big draw on current, makes the alternator harder to turn, resulting in lower idle speed from the resistance of the alt. Running the a/c causes this also.

turning on your headlights will increase the load on the alternator. i'd start with the charging system...check all connections and the voltage regulator first. what's the voltage at the battery terminals with the car idling? and with it idling with the lights on?

I haven't changed anything related to the lights, I put a new sanden compressor in last fall. That is the only changes made. When I turn on the air it changes the idle, but that happens with my diesel suburban also and with other cars I have had. My headlights seem to change the idle much more than the air.
Thanks for your help guys!

How bright oar the lights? Does this happen with the parking lights also?

If you have a small short some where, that increases heat on the wire and amperage pull, which then makes the alt harder to turn.

If no short, resistance also creates heat and increases the call for amps to push the same voltage, resulting in the same harder to spin alt.

Get the charging system checked, and check all connections, making sure they are not getting hot.