Marker lights vs headlights .....

[6600DURAMAX]

Having small issue - probably simple fix but wanted opinions on where to start looking.....

When I turn on my headlights my parking lights (only in the front bumper) go off?? Side marker lights stay on just fine......

[PbFut]

Do you have an original cluster and do the turn signals act up as well. If so it is a cluster ground issue. Easy fix by simply adding a good ground wire to back of cluster. Easier than attempting to trace the actual cold joint.

[dmjlambert]

I agree. Check fuses. Also you can try grounding the front parking light housings better, use a test wire attached to a good ground and touch it to the housing and see if the light comes on. Or use a lighted test probe to check for voltage on the wire that is entering the front parking light housings. Parking light wire should be purple. Some year trucks, probably earlier than your truck, the parking light turns off with the headlights on. So, you didn't do anything such as swap out the headlight switch for one that may be for an earlier year did you?

[6600DURAMAX]

I did install a new headlight switch ..... was suppose to be for 67-72...

Wonder if its that? I think I still have the old headlight switch, hate to pull the dash back out but I might have to.

Turn signals all work - I am running an aftermarket gauge cluster but it is grounded.

[dmjlambert]

You don't need to pull the dash for headlight switch. Just reach up in there and press the knob release, pull knob off, and unscrew the nut that holds it, and remove it from behind the dash. Since you changed it once, you probably already know the knob release button usually takes some finesse to convince it to release.

Then look at it and compare with your old one.

You can use a multimeter to test when the switch is in the various positions you get continuity on the right terminals. Search the forum here for info about which terminal is for which function, and ask questions and show pictures if you get stuck.

[GMCPaul]

Sounds like you have a 67-68 switch.
1967-68 =8-Terminal headlight switch
1969-72 =7-terminal headlight switch
1967-68 the front park lights go off when headlights turn on using original style switch, if you want the front park lights to stay on with headlights use the 7 terminal 69-72 headlight switch as they are interchangeable.
Paul Jr @ GMCPauls