2001 OBW odo/clock not lit. Other illum OK. Bulbs good.

[brycarp]

Hi All,

On a recently-acquired 2001 Outback Wagon, I got into the various buttons, the HVAC control and instrument cluster to replace all of the burned out bulbs. So that's all good now and the illumination brightness ring on the turn signal lever works fine. I have good bulbs behind the LCD odometer and clock readouts. But still those displays don't get lit. If I take the bulbs out and shine light through the hole, that light is able to backlight each display just fine. I already got a junkyard illumination control module (mounts behind the stereo on passenger side, part number 83023AE01A) and swapped it in with no change to the problem.

In the Factory Service Manual "Wiring System" section for "Combination Meter", it shows that the odometer and clock backlight bulbs are driven by a block labeled "Dimmer Circuit" that gets fed from the same brightness controlled power circuit that drives the other illumination bulbs. Does anyone know why the odometer and clock backlight bulbs are treated "specially" rather than just being driven from the very same brightness-controlled voltage that comes in from the illum controller?

I suppose something is burned out in that "Dimmer Circuit" that lives somewhere in the instrument cluster assembly.

I'm posting in case someone else has been through this problem and can give me further info.

Thanks!

Bry

[brycarp]

Just to check back in and show what I did. At first I had suspected the illumination control module, but then I got one from a junkyard, swapped it in and no change to the problem. After studying the diagrams in the "Combination Meter" part of the "Wiring System" section of the USDM FSM, I became convinced that the "Dimmer Circuit" shown as driving the backlight bulbs for the odometer and clock LCD displays was part of the circuitry on the main circuit board of the instrument cluster. No power was getting to the "twist in" contacts for those bulbs, so I just decided to bypass what was supposed to drive those lights entirely.

Here's my modification of the twist-in collar to remove the incandescent bulb and the metal parts. I then drilled through the back of the collar so the wires driving the white LED could come out the back. The LED pressed into place with a snug fit. https://photos.app.goo.gl/afHo4xauPTr5XX1U9 

Because the LED had a distinct projection pattern that I wanted to scatter to spread the light out more, I sanded the front of each LED. The LEDs I bought: https://www.amazon.com/gp/product/B01NAHYUBD - used the white ones.

Then I tapped into the dimmer-ring-controlled voltage that drives the four main illumination bulbs. The LEDs I used were advertised as 12 volt LEDs, and came with a 1k ohm resistor wired in series with each one. They were too bright for what I wanted, so I put a 2.2k ohm resistor in the wire that drives both LEDs. Effectively, that means that each LED is driven with an overall 5.4k ohm resistor, so the current will be very small. The light is still plenty bright though, and when the dimmer ring on the turn signal lever is adjusted, the brightness of the odo and clock displays matches the other illumination levels pretty well.

Here's a picture of my hack wiring on the back of the instrument cluster: https://photos.app.goo.gl/3LgDfKUF3wrx9GU36

The tape I used to hold everything in place: https://www.amazon.com/Scotch-Glass-Cloth-Electrical-scotch/dp/B005YSL2SM

Edited November 3, 2019 by brycarp
Add what kind of tape I used.