Headlight switch functions

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Mar 23, 2021 | 11:39 AM

Anatta

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Mountain Pass

Joined: Jan 2020

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Headlight switch functions

I had read in an older thread and confirmed elsewhere on the interwebs that the D1 and D2 terminals were for the dome light. I have a brand new headlight switch, and everything works (i.e: headlights, parking lights, high beams, etc) as it should, but I get no voltage off of D1 or D2 in any position. Can anyone explain to me the correct function for these terminals or how I should wire my interior lights?

A little background, I just completed a wiring job on my 66 Custom Cab from scratch using an aftermarket custom harness. Everything works as it should, no electrical issues whatsoever now. I don’t have a dome light, so my plan was to install two courtesy bulbs under the dash that I could turn on with the headlight switch, since I don’t have a factory dome light, and there are no switches in the door jambs and I have no plans to add any just yet. My thinking was that when you turn the **** counter clockwise all the way past the detent, then you would have 12v at one of these D terminals to power a dome/interior light, but apparently that may not be the case?

Please enlighten me (pun intended). Thanks!

Mar 23, 2021 | 11:52 AM

53deere

Fleet Mechanic

Joined: Nov 2005

Posts: 1,838

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From: Lehigh Valley, Penna.

Those two terminals are a switch, one goes to the light, the other to the fuse block for power. It does not get power from the rest of the switch.

Mar 23, 2021 | 11:58 AM

Anatta

Thread Starter

Mountain Pass

Joined: Jan 2020

Posts: 198

Likes: 11

Quote:

Originally Posted by 53deere

Those two terminals are a switch, one goes to the light, the other to the fuse block for power. It does not get power from the rest of the switch.

Interesting. Why would Ford design it this way when the switch is getting 12v and is itself basically a power supply for all of the other lights?

So what you’re saying is, I would need to run a wire from some other 12v source to D1 and then connect my bulbs to D2? How then does the switch work, by turning past the detent?

And is there any reason I couldn’t jump a wire from any 12v constant terminal on the headlight switch to D2?

Mar 23, 2021 | 03:26 PM

66v8baby

Fleet Mechanic

Joined: Sep 2016

Posts: 1,459

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From: Southern Maryland

Quote:

Originally Posted by Anatta

You can't jump wires on the switch unless you destroy the connector because the connector covers all the terminals. Not a good idea to have a bunch of unprotected connectors with power running through them that close together.

I don't have the wiring diagram in front of me, but I don't think you need to add anything. You either have a bad switch, a bad harness or you don't have a good ground. Take the switch out and perform the following test:.

1. Remove the switch and check for 12V at the connector to D1. If no, trace back to find where the circuit is open. If yes, go to step 2.
2. With the switch removed, check for continuity between D1 and D2 when the switch is rotated to the left detent. If no continuity, replace switch, if there is continuity, go to step 3.
3. Install switch, rotate to turn on dome light and test for 12 volts at the dome light connector. If no, trace back to find where the circuit is open. If yes, go to step 4.
4. Drill a new small hole for the ground at the dome light and re-attach ground.