90 Pickup - No Low Beams, Only High Beams Work
 

Hey Folks, so I've read through everything I can find on this no-low-beams headlight issue and here's what I've tried so far:

-Tested headlight fuses in underhood box
-Tested headlight relay in underhood box
-Tested 6504 bulbs—they're old but both filaments light up fine on the bench
-Pulled combination switch and did complete FSM test of Light Control Switch and Dimmer Switch (with multimeter)

Couple of side-notes to remind myself (and anybody else struggling with similar)... the headlight relay delivers 12V+ to the headlights (after passing through 10A fuses) whenever Light Switch is rotated all the way forward, but the grounds of the low beam filament (red-green wire) and the high beam filament (red-yellow wire) are switched/activated through the Dimmer Switch. And, when looking at the three pin-sockets on a disconnected headlight plug: 12V+ is the RIGHT, Low-Ground is TOP, and High-Ground is LEFT.

https://cimg4.ibsrv.net/gimg/www.yot...51edbf329e.jpg

So everything looks fine/good in all my testing EXCEPT that when I connect my multimeter from the red-green wire on the large combi-switch connector (B-6) to the red-green wire on either of my headlight plugs (diagram below), there is no continuity. Those red-green wires at the plugs have continuity with each other, left vs right headlight, but there appears to be a break in the red-green between the big connector and the actual headlight plugs—either in the actual wire inside the harness at some point or in some kind of add'l connector that's not shown on the diagram (?). (B-5 however, the high-beam ground connection, does have continuity to the expected pin on the headlight plug, which explains why those are operational.)


https://cimg5.ibsrv.net/gimg/www.yot...d1a4e5736a.jpg

Is there something else I'm missing here? I'm thinking about just running some new 14ga between B-6 and the headlight plugs (probably replace the plugs too) since it could be very tough to pinpoint exactly where the red-green stuff failed... but open to any comments/suggestions, thanks!!!

I think I found the culprit: dug around more under the dash and discovered that both headlight grounds (red-green and red-yellow) go through this big connector on the top of "Junction Block 1" behind driver's kick panel. Hard to see back in there, but the red-green pin is noticeably blackened compared to others. Guess I need to clean, replace, or bypass that connection (also, I could be wrong but I think a small piece of metal fell out of there, when I initially pulled the big plug out, geez... who knows).

https://cimg8.ibsrv.net/gimg/www.yot...9025df6356.jpg

this issue is usually cause by the contacts in the headlight hi/low switch in the steering column "dimmer switch"

Yeah, I’ve seen that in reading many other threads, but I’ve already pulled all that out, examined, and tested, and the dimmer switch and light control switch look great and work well. Regardless, if red-green doesn’t have continuity out to the headlight plugs, low beams will never function…

Truck is 34 years old but has just 98K.

without having the wiring diagram in front of me I can't offer much more help.
I would have to agree that if there's no continuity they won't work, but just for giggles manually ground that B-6 pin in the drawing you shared and see.
It sounds like you know what you are doing, but without being there I can't verify that you are correctly testing for continuity or even if you have the correct pin in the correct plug. (trust but verify)
good luck.

Thanks for weighing in, @akwheeler — I sorta know what I'm doing, but it's always good to have a second opinion. I've done a lot of work on vehicles over the years, and I'm now in that stage where I can still (barely) crawl around under a car's dash... but need reading glasses to make sense of it all!

Anyway, I got the problem fixed and she's back on the road. The red-green connection at Junction Box 1 was definitely bad/smoked, so I bypassed it and added a little bullet connector: problem fixed and I've got low and high beams again.

https://cimg2.ibsrv.net/gimg/www.yot...13ca76712a.jpg

whatever works! congrats.