E30 Adventures: On-Board Computer (OBC)

The on- board computer / clock / etc in my E30 325 was inoperative when I bought it. Occasionally you could see some sort of flicker in the LCD, but it looked like the liquid had been broken, and the backlight wasn't working.

Now, normally, this would not be a high-priority item. For some reason, I was intrigued. I was hoping that the dodgy installer who'd done the car stereo might have done something dumb and pulled off a wire, but that wasn't the case.

To get to the OBC on an E30 - trust me, do it this way:
- Take out radio
- Pop out light controls above radio (from behind)
- Remove 2 screws above the light switches
- Remove 2 switches on the bottom of the center console, on the lip under the heater
- Pop off heater knobs, pop off the plastic face where the sliders are (my radio removal tool came in handy here)
- Remove 4 screws holding heater to panel.

My OBC was held down with 4 screws, which are still a pain to remove, but easier this way than going in through the glove box like some recommend.

The OBC then has a removable light bar on the right-hand side; these are notorious for failing, and mine was no exception. I see BavAuto has them for like $35. Ouch.

I shined a light in mine and verified that it was dead, so all I could do at that point was put it all back together and go surf eBay. I managed to win a 13-button OBC listed as an E28 525 model for the bargain of $26 with shipping. (I think since it wasn't listed as an E30 it went unnoticed; E30 OBCs seem to go for $40-$50).

Got my new-to-me OBC and noted both lightbulbs in the light bar were blown; I'm guessing this can only happen during a jumpstart, since they're wired in series.

I decided to replace mine, not with a $35 BavAuto version, and not with incandescent bulbs, but with white LEDs. Radio Shack has a few white LEDs, and I ended up using the cheaper ones ($2 each (!) x2). They have lower total brightness but a 100-degree view axis.

In order to make this all work, I needed to find out two things:
1) What voltage is applied across the terminals? I see something marked "10V max". That would be swell if I was getting +10V, then 3x 3.6V LEDs would work great.
2) Which terminal is +? Not an issue for incandescents, but it is for LEDs.

So I made a test jig to bring the leads out of the light bar to my DMM. As it turns out, I should've just tacked wires to the circuit board; I ended up damaging the white carrier trying to get it out due to space limits.

For reference, the white lead in this picture is positive -- the one nearest the edge of the carrier. I noted that this is straight "R terminal" voltage, so 13V or so when in Run position. Let's see, two 3.6V LEDs in series is 7.2V. (13V - 7.2V) / 0.02A = 290 ohms of resistance needed to drop the voltage. Huzzah! A 300-ohm resistor right here in the junk box.

In order to wire the resistor in, I cut a trace on the circuit board, then used the resistor as the connection between the two LEDs.