Headlights causing engine to lower idle...

[jspurl3]

I am working on a 77 halfcab for a friend, fixing a few little issues. I noticed that every time you turn the headlights on it seems to put a slight load on the engine, before I start throwing parts at it, I was seeing if anyone has experienced this?

Stock motor, factory wiring harness with new H4 headlights and aftermarket heavy duty headlight upgrade harness that routes straight to the battery with relays (looks like the one from Wildhorses).

 

[JAFO]

So the alternator kicks in when you turn on the headlights? Weak battery maybe? Or wimpy alternator? Or normal operation for the load of the headlights maybe?

 

[nvrstuk]

He needs a bigger engine!!

His alternator is working harder to generate enough amperage to illuminate the lights and it creates more load on the engine causing it to slow down a very small amount. Almost all 4 cylinders do this before computers were programmed to sense the load and increase the idle speed.

 

[smokinjoe]

bump up the alt, get a Taurus alt or aftermarket 1 wire.

 

[blubuckaroo]

There's no fix for this. A bigger alternator or even that you've wired the lights to the battery won't matter. Any load applied to the electrical system is going to put a load on the alternator and slow the engine down.

I learned about this the hard way.
When I was in high school auto shop, I got to compete in the National Plymouth Troubleshooting Contest. You were to trouble shoot an engine problem, adjust it to spec, make a lap around a track, and have it tested by the judges.
Our team got dinged because our idle speed was a bit too high. We had done a considerable amount of cranking for the troubleshooting then had adjusted the idle speed. When we made the lap around the track, the battery's charge was back up, so the alternator load was down, and the idle was too fast.

One thing you might try however is to make sure your idle mixture and timing are correct. A poor idle won't support the extra load as well.

 

[bronconut73]

One thing you might try however is to make sure your idle mixture and timing are correct. A poor idle won't support the extra load as well.



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[patterdale]

Yup. Normal.