Temp Display - Back light blue led upgrade
 

DIY for replacing the boring factory incandescent bulb for back lighting the temp display. This is from a W201.

Pic1 - Display removed, incandescent bulb removed. Opening for led bulbs shown.

Pic2 - Tried 3 types of blue leds, the smallest ones shown the best results. The small led size allowed the them to be placed further away from the display...therefore no bright spots.

Pic3 - Replaced onboard current limit resistor with 220 ohms. 3 leds in series dropped 9.5V, wanted 18mA. (13.5V - 9.5V = 4V, 4V / 18mA = 222 ohms).

Pic4 - Blue leds installed, you can see the leds sticking out in the opening. The outer leds are spread outwards for even light distribution.

Pic5 - Blue leds turned on.

Pic6 - Blue leds turned on.

Pic7 - Blue leds turned on, display back on, shown pretty even light distribution, no bright spots.

Pic8, 9, 10 - A few pics with blue leds turned on, temp display on.

Nice work. Awfully warm in your shop...!

Rgds,
Chris W.
'95 E300D 380K

Anodes? Cathodes? What's the correct polarity?

Hehe, it was like a sauna.....no, just kidding. :D

MB uses a thermistor (resistance varies significantly with temperature) as a the temp sensor. I used a 1.2K ohms fixed resistor as the temp sensor for testing...this value = 47 degreeC.

So, if I was to use a 12K ohms, I could have gotten the display to indicate a freezing temp.

Anode = longer lead, positive voltage
Cathode = shorter lead, negative voltage

To connect 3 leds in series....connect 1st led's cathode to 2nd led's anode, connect 2nd led's cathode to 3rd led's anode. Than connect 1st led's anode to positive (12V) and 3rd led's cathode to negative ground.

Don't forget the current limiting resistor should be connected in series too, but in this case...just swap the onboard resistor to 220ohms.

Great idea, excellent write up with pics.

Nice work.

That appears to be an older style temp gauge. The newer style one is much more compact and modifying it like you have the old one might prove difficult for the more directional LEDs.

Some pics here:
http://www.peachparts.com/shopforum/tech-help/313607-outdoor-temp-display-repair.html

Yes, from a 85' W201. Looks like for later models they really integrated all the circuitry into a single die, wire bonded & encapsulated the die.

I would assume the incandescent bulb is mounted on the other side of the circuit board & the surface mount current limiting resistor is external and not integrated into the die.

The leds I used are small 3x2mm rectangular leds, therefore 3 leds side by side is only 6mm wide...so no problem fitting through the opening. Than I would remove the lcd display on the other side (to expose the leds) & use small pliers to spread the outer leds apart.