Oh, the saga. Here's the long version. You know this stuff, but I'll just mansplain it.
You know the Mud people that switch their instrument cluster gauge bulbs to LEDs? Yeah, that's me, and no one on Mud actually talks about this as far as I know.
The old dimmer is a rheostat; it's a variable resistor sitting in the ground path of the dash lights. It dimmed incandescent bulbs by wasting power as heat, no problem. Incandescents pull real current, so adding resistance meaningfully dropped what reached them, and they dimmed smoothly.
Then I converted the dash to LEDs.
LEDs use milliamps instead of tenths of amps. Running that tiny current through the rheostat's resistance causes almost no change because there is not enough current for the resistor to drop meaningful voltage. So the rheostat barely dimmed them. LEDs also don't fade gracefully with voltage; they hold brightness then cut out. This happened repeatedly until the rheostat died. I even replaced it with another working unit costing $50.
The truck's wiring made the dimmer load-bearing. FJ60 dash lights ground through the dimmer with an isolated ground and no direct chassis path. When the dimmer circuit's connection failed, likely after the paint shop, the bulbs lost their only ground, and the whole dash went dark. That was the original gremlin. I took time to rule out all wiring, fuses, and so on. The DIM12N fixes both problems at once:
- It dims by PWM, switching the ground path on/off thousands of times a second and varying the on-time, which is how I dim LEDs properly: full current when on, adjustable duty cycle, smooth range.
- It's negative-output (low-side), so it sits exactly where the rheostat sat: in the ground leg, using my three existing wires (#1 feed powers it, #2 return switches through it, #3 ground). No harness rewiring was my goal.
- The only remaining task was to retrofit: find a remote pot to hide the module in the dash, with the factory knob location driving it to preserve the stock look.
Resistors dim by starving current, but LEDs don't draw enough current to starve. PWM dims by chopping time instead, so it works regardless of load. A low-side PWM fits directly into the rig's ground-side dimmer position.
If you ever feel brave, you should take the old units and do the same thing you did with the flasher unit. I'd buy one from you.
