Another lighting debacle

[js1732]

1990 5.7 TBI
-Both front parking/side blinkers work until headlights come on.
-Once headlights on, only side blinkers work.
-Changed/inspected all grounds, fuses, bulbs, and connections.
-Tried searching as much threads as possible and still no luck.
Before throwing anymore money and time at this, figure I ask the community. Thank you in advance!

 

[GoToGuy]

Have the " OE service manuals " with diagnostics and wire digrams?

 

[95burban]

Have you changed the multifunction switch?

 

[someotherguy]

1990 5.7 TBI
-Both front parking/side blinkers work until headlights come on.
-Once headlights on, only side blinkers work.
-Changed/inspected all grounds, fuses, bulbs, and connections.
-Tried searching as much threads as possible and still no luck.
Before throwing anymore money and time at this, figure I ask the community. Thank you in advance!

Just on a hunch, disconnect the rear lamp harness for test purposes. It's got a connector near the end of the driver side frame rail. That way you're ruling out problems with the taillight circuit boards and any trailer harness add-on you may have.

Have the " OE service manuals " with diagnostics and wire digrams?

You could be a welcoming sort of person by pointing out to the new guy with only 1 post where those manuals are linked, for free, right here on the forums: https://www.gmt400.com/threads/88-98-service-manuals.43575/

Have you changed the multifunction switch?

He's got a '90, all the switches are separate entities.

Richard