Dome Light LED Trouble Shooting

[bronco t]

I recently installed a BC Bronco soft top (love it) and then installed a WH wiper cover which required adding a dome light. I added a surface mount LED light thinking it would be much better than the factory dome light. Not so! The problem is that the LED will only light up if I twist the headlight switch full right and the back to the dome light detent. If I do not make a full sweep the LED light will not light up. Checking the schematic the dome light detent is just an on/off switch and is not wired to the dimmer portion of the headlight switch that dims the dash lights. I bench tested the Led on a spare headlight switch that I have and got the same results. If it matters, I do have the WH LED gauge cluster lights installed and they dim as expected.
What am I missing?

Any help would be appreciated.

 

[73azbronco]

I do know the dome switch has to be grounded through the harness, the body at the window won't cut it. Not sure of routing,

 

[Broncobowsher]

Your description of the dome light is correct for how it should work. It is not dimmed. It is full brightness at the detent at the end of the dimmer on the headlight switch. That is just how old car headlight switches worked. From the 50s through the 90s.

In a normal vintage car, the light comes on when the door is opened. The twisting of the knob was an over-ride to turn the light on while the doors remained closed. The Bronco lacks the door jam switches and the headlight switch is the only switch to turn the dome light on.

 

[DirtDonk]

Even to this day dome lights don’t usually dim.
The switching methods vary, but they are almost always full brightness when you turn them on.
Except perhaps under the control of the computer where it might start off low and gain brightness over a period of time.
But all you’re doing is turning it on and off.

Think about it. If it was working like you were expecting, the dome light would be illuminated at all times when the headlight switch was pulled one much.
Dimming and brightening the dash illumination, would dim and brighten the dome light. But it would never go off until you turn the lights off.

 

[Slowleak]

I’m interpreting this to mean that the dome light will not come on simply by turning the knob fully to the left (counter-clockwise). He has to turn it all the way to the right as if dimming the dash lights, and then back all the way to the left. That’s not the way it normally operates.

 

[DirtDonk]

That would be interesting then. I would initially say that it’s a problem with the switch itself. But they go on to say that a spare switch did the same thing, and that the other LEDs dim like normal. Which is pretty good, since many LEDs don’t dim consistently anyway!
I can’t initially see any reason why you would have to go full dim, then full bright to get the dome light to work on its click.

Is that what you’re saying bronco t?

 

[Lawndart]

Your description of the dome light is correct for how it should work. It is not dimmed. It is full brightness at the detent at the end of the dimmer on the headlight switch. That is just how old car headlight switches worked. From the 50s through the 90s.

^^^^ as BB stated -
I also have a BC soft top. I do not run a wiper cover, however I have a small LED light strip wired into the normal dome light socket.
There is no dimming. The strip comes "full on" just as the normal light would have - turned full CCW.

 

[Steve83]

... It is not dimmed. ...

^^^^ as BB stated -...
There is no dimming.

He's not trying to dim it - re-read the original post.

The Bronco lacks the door jam switches and the headlight switch is the only switch to turn the dome light on.

...but it's pretty easy to add jamb switches.

(click this text)


(click this text)

...the LED will only light up if I twist the headlight switch full right and the back to the dome light detent. If I do not make a full sweep the LED light will not light up.

Sounds like the dome switch inside the headlight switch is dirty - probably due to lack of use. The more you use it, the better it should work.

If not, turn it to detent (ON) without the full sweep (so the LED doesn't come on) and then measure voltage across the LED's wires as close to the LED as you can get (use pierce probes). If it's ~12 (very close to battery voltage) and the LED isn't lit up full-brightness, then the LED is defective. If it's substantially lower (the exact threshold depends on exactly how the LED is built), measure voltage from the LED's ground/return wire to the cowl; and from the cowl to the B- post (not the clamp around the post). If either is NOT zero, you have a poor grounding path that needs to be fixed.

If all of that is good, but the LED isn't on, measure voltage from the LED positive wire to the cigar socket positive (or its Bu/Wh wire hanging under the dash, or fuse 4). If that's NOT zero, try to move the meter probe from the LED to the LG/Y wire going into the headlight switch; if that's not zero, replace the headlight switch; if it is ~zero, find the fault in the Bk/Bu wire (probably at C313, or where the harness bends in the w/s hinge area).

Refer to zone B34 in this diagram:
(click this text)

 

[bronco t]

Thanks for the responses. As you all have suggested and corrected my post - the dome light portion of the headlight switch is the problem and only works after the turning the dimmer switch full clockwise the counter-clockwise. I did plug in the factory incandescent dome light and got the same results, which eliminated my suspicion that the problem was the LED. Odd that the spare new switch I have has the same issue! I purchased a an Echlin HL6541 headlight switch (made in Taiwan) and will get around to installing it next week. Again, thank you for your replies!

 

[bronco t]

Update, the new headlight switch is installed and the dome light is now working! The metal flap on/off tab was bent and was slpiiing off the wheel. Unfortunately the headlights stopped working and after further review the Nightlighter T4 relays had lost ground. This sparked another squirrel chase of installing ground wires from the engine to the frame and the frame to the body And core support Of which I should have done years ago!