Testing the headlight switch / rheostat

[Gregg]

Just recently my gauge cluster bulbs are not dimming. Everything else is working fine: headlights, indicators, hazards… Even when I turn the headlight knob CC past the click, everything illuminates. There just isn’t any light before the click. Simple answer, bad rheostat in the switch?

Also is there any reason why I can drive at night with the gauge lights on past the “click”?

The HL switch is less than 10 yrs.

Thanks guy!

 

[DirtDonk]

Age doesn’t matter anymore. A 10-year-old switch is Jesse such a crapshoot as a new one. Might work fine for a while, might not. Might work forever, might not…
No reason at all, you can’t run it past the click. Since our Broncos don’t have dome lights for the most part, there’s no power going anywhere that it shouldn’t. Even if yours has a wire somewhere, it should be terminated safely from the factory.
What year is your bronco?

One last thing to check before you get a new switch. If you haven’t done this recently, pull your fuses out, wipe them clean, then put them back in.
A layer of oxidation often builds up and creates resistance. It doesn’t sound like this is the issue in your case, but it’s a good habit to get into every couple of years anyway.
Possibly more often depending upon where you live.
The instrument cluster illumination fuse is the tiny one that’s hard to get in and out.

 

[Gregg]

Age doesn’t matter anymore. A 10-year-old switch is Jesse such a crapshoot as a new one. Might work fine for a while, might not. Might work forever, might not…
No reason at all, you can’t run it past the click. Since our Broncos don’t have dome lights for the most part, there’s no power going anywhere that it shouldn’t. Even if yours has a wire somewhere, it should be terminated safely from the factory.
What year is your bronco?

One last thing to check before you get a new switch. If you haven’t done this recently, pull your fuses out, wipe them clean, then put them back in.
A layer of oxidation often builds up and creates resistance. It doesn’t sound like this is the issue in your case, but it’s a good habit to get into every couple of years anyway.
Possibly more often depending upon where you live.
The instrument cluster illumination fuse is the tiny one that’s hard to get in and out.

Thanks for the reply. I have a 71. We put in a AAW harness within the last 10 years. I pulled and checked all the fuses and they all tested good. No tiny one anymore. I’ll order another switch and post up if that was the culprit. Thanks again!

 

[DirtDonk]

And while you were messing about with the switch, did you rotate it back-and-forth multiple times in succession?
Doesn’t sound like yours is acting like the typical “getting tired and dirty over time” type of situation, but sometimes just moving it back-and-forth on the rheostat will make a change.
In those cases, you usually see some activity right away, which it sounds like yours has not. So it may still require a new switch.
But I thought I’d ask if you had rotated it many times already.

 

[Steve83]

Even if your headlight switch isn't exactly identical to this diagram, you can still test each circuit the way it explains:

(click this text)


You should put ALL your truck's details & history (as much as you know) into your signature(<-click that) so it shows with each post. Phone apps don't always show signatures, so you may need to switch to a real browser in desktop mode on your phone, or just use a desktop/laptop computer. Put your location (nearest city) in your profile &/or signature, & upload an avatar of the truck. Not all its details are relevant to these issues, but you don't necessarily know which ones are relevant, so just put everything in now. The more pics you post (NOT in your sig) of the truck, engine, wiring, labels, & undercarriage, the more likely we can help you.

 

[Gregg]

I finally got around to put in the new light switch. Worked great until something shorted in turn signal switch (again). The horn wires were shorting with the gear indicator light. I'm guessing thats what cooked my first rheostat (and second....). After disconnecting and moving my horn switch *, I fixed both light switches with a little copper jumper and I'm back in business with my old switch and a spare in my parts box.

* I'm over the stock horn set up. I get it working for 6 mo to a year and then it starts honking when I turn (usually when my boss is in front of me....) or shorting out. So I finally moved it off the column and have it under my dash.