Headlights Surging

[69time]

Not sure why... (that's why I'm here) but when I have my headlights on they seem to surge from dim to bright while at idle. New alternator and headlights. Could this have something to do with my voltage regulator?

[mvfd70]

Probably. My regulator went out on my dodge and was blowing high beams. Replaced it and that problem went away but now my headlights surge at idle. Long story short, get a good regulator. Not the cheepo from Auto Zone

[VetteVet]

This is pretty common on the stock headlights on our trucks. The reason is mostly because of the low amperage output of the alternator at idle. A new regulator might help but I doubt it, especially if it charges OK at 1000 rpms and higher. It 's a shame you didn't come on the forums before you bought the new alternator. We would have advised you to convert to a newer internally regulated version.
There are some things you can do that might help. The first one is to check your grounds and headlight connections. Then a headlight relay mod will get more voltage to the headlights. I can post a nice diagram for the GMC trucks and you should be able to do the mod for less than $30.00. It is quite simple to do. What it does is power the lights directly from the battery without using up voltage by powering them through the headlight switch, the dimmer switch and all the included wiring. Even if that isn't the cure your headlight switch will last longer and your lights will be brighter at speed.
You can also run an under drive pulley on the alternator and it will spin faster at idle but be careful not to run the alternator too fast at rpms. Check your fan belt tension as well.
It is possible to adjust the external regulators by increasing the spring tension on the coils but I don't usually recommend that because it can lead to overcharging.
Check the electrical forum for some posts on alternators and you will get a lot of good ideas.

[mr48chev]

vettvet, I'd like to see that drawing as I run headlights with h-4 bulbs 55/100 and they are a bit hard on dimmer switches.

And the lights doing their thing at an idle is something that we who still run the external regulators have to deal with or change alternators. I'll upgrade when mine finally dies but if it isn't broke I don't see a current need to fix it. With that said, tomorrow I'll probably be hunting the diagram to hook up the 110 amp alternator that I have stashed in the garage for the occasion.

[VetteVet]

Originally Posted by mr48chev

vettvet, I'd like to see that drawing as I run headlights with h-4 bulbs 55/100 and they are a bit hard on dimmer switches.

And the lights doing their thing at an idle is something that we who still run the external regulators have to deal with or change alternators. I'll upgrade when mine finally dies but if it isn't broke I don't see a current need to fix it. With that said, tomorrow I'll probably be hunting the diagram to hook up the 110 amp alternator that I have stashed in the garage for the occasion.

I'll post both the two and four headlight diagrams. The top one is for the chevy 2 headlights with bright and dim in the same light and the second one is for the 4 light system that GMCs use.