Instrument cluster lights. Bulbs or wiring?

[justinae]

Working through the list of fixes for my 96 XLT 2wd. This one is the lights on the cluster. Naturally it's not staightforward.

There were multiple different plastic connector types which is suspect, but multimeter says bulbs and logic board have continuity. The lights on the stereo light up and do in fact dim when I rotate the wheel.

I plan to get new bulbs just because but I want to find the reason they aren't working.

I'd like to test the wiring going to the cluster to see if that's working right. Not sure where to start. I'll try to find a diagram of the constant power but is there anyone who's tested the harness before?

Are there are relays or other stuff I should start with?

thanks!

 

[justinae]

I'm a bit over my pay grade on wiring here but I tested the 12 pin connector for constant power. With keys in RUN position and headlights OFF I get 12.1v (neg on B/W pin 11 and pos on R/Y pin 5). When I turn headlights on the voltage drops to 11.9. Rotating the dimmer wheel doesn't change it.

Also took out by GEM to inspect and everything looked fine.

I remembered that my horn doesn't work either. Connected?

 

[BobSacamano]

On the same connector, there is a red/black wire for illumination. you could check that.

 

[Curious Hound]

Sounds like an issue with the dimmer module.

 

[justinae]

On the same connector, there is a red/black wire for illumination. you could check that.

Thanks. Checking now. Do you have a wiring diagram you pulled from? I'm wondering if any of the wiring harnesses also have illumination pins.

I just noticed that the bulbs behind the heater panel also don't work.

Sounds like an issue with the dimmer module.

You may be right but why would it correctly dim the stereo and turn on the dome light?

 

[justinae]

@BobSacamano the R/B wire gives no reading when keys are at RUN, headlights are on, and dimmer is at full.

 

[Curious Hound]

Thanks. Checking now. Do you have a wiring diagram you pulled from? I'm wondering if any of the wiring harnesses also have illumination pins.

I just noticed that the bulbs behind the heater panel also don't work.


You may be right but why would it correctly dim the stereo and turn on the dome light?

Because those are different outputs from the module.

 

[justinae]

Ok. I found a

of someone repairing their dimmer switch. Might try that.

About to order new bulbs, even if they aren't the problem. I know LED aren't truly dimmable, but are they effectively plug and play "dimmable"?

 

[Curious Hound]

I know LED aren't truly dimmable, but are they effectively plug and play "dimmable"?

No.

LEDs are dimmed by turning them on and off at different speeds. This is done with a special power supply called a PWM supply. This circuit sends pulses of voltage to the LEDs. If you send continuous voltage, the LED stays on. If you turn the voltage on and off real fast, you can vary how long it is on and how long it is off and trick the eyes into thinking the LED gets brighter or dimmer. With longer on periods and shorter off periods, the LED "looks" brighter. With shorter on periods and longer off periods, the LED "looks" dimmer. This is Pulse Width Modulation - PWM.

That's the simple explanation. So, back to your issue. Your dimmer module probably does this for panel lighting. I have wiring diagrams for a 96 Explorer and it has a PWM output for panel lighting and regular DC output for the Dome light and stereo backlighting. That is why I mentioned 2 separate outputs in one of my earlier replies.

 

[justinae]

Super helpful. Thanks. I assume the dimmer wheel I have won't "trick" the led into on/off PWM? I've read conflicting write ups about plug and play LED dash lights working on some vehicles.