Temperature gauge backlight light bulb
 

Fixed my odometer on the 190D. Would like to fix the backlight on the outside temperature gauge while the cluster is out. It does not come on at night and the bulb is found to have burnt.

What is the part number of the small light bulb that is soldered onto the PCB board? Is it 12v? Can I replace it with one of the small light bulb, say, the one in the climate control unit? The size is roughly the same, just need to straighten up the legs.

Do you want to solder it to the PVB? the legs of a bulb such as for the climate control seem very short to me.
Part # from ******** az:
0008250094

Instrument Light; Frosted Bulb for Instrument/Dashboard Lights

$0.69

The bulb is smaler than the CC bulb I think, and probably less wattage also. In the past I've been able to find the smallest 12 V bulbs at Radio Shack
12V 60mA
Microlamps (2-Pack) Model: 272-1092 | Catalog #: 272-1092 $1.79 for two

This bulb is also a suitable replacement for the $19 bulb used in Volvo seat heater switch illumination!

Why not just replace with an LED and a resistor?

I thought about that if I cannot find a sutiable light bulb. What is the current drawn? Shall I limit the currrent to be below 40ma?

The problem is that LED's are effectively not adjustable with regard to their light output because the MB interior lighting circuitry is voltage variable not current variable. The circuitry for the interior lighting varies the voltage levels to achieve dimming effect with incandesant bulbs. LED's have a threshold voltage and either turn on if it is exceeded and turn off it is not. An LED's brightness is affected by the flow of current to it.

Another potential issue would be the positioning of the LED, most LED's are not omni directional in the orientation of their light output. Usually there is essentially a "primary cone of light output" that could require a specific placement of the LED to maximize its ability to illuminate the entire display area.

Yeah, I forgot it was a dimmed circuit. You actually can dim LEDs quite a bit, but at some point they just cut off. LED dimmers actually work by pulsing the voltage to the LED instead of reducing it.

So, your points are quite valid. The dimming will work to a point, then it will just shut off.

Although, with a resistor in series with the LED, you get your current drop when you dim the voltage. Trying to get the right value resistor might be a pain. Perhaps bench test with a potentiometer to find the sweet spot?

Y'know..... The easiest thing to do might just be go to Pick n Pull and grab one. I've got several of them and I don't think I was charged for any of them.

The problem with LED is its operational range of the voltage between V min and V max is quite limited, the current is a function of voltage divided by resistance, with the resistance constant (inline resistor) and the voltage variable (the swing between high and low interior illumination voltage) only over a small range, the resultant variation in current which determines the output of the LED is almost unnoticeable.

In that case, a second resistor with one leg connected to ground could be used to create a load after the first resistor which would in turn provide a more substantial voltage/current drop.

Now what would be totally AWESOME is if you used a two color LED and made the display change colors when you get below freezing.

I found a spare temperature gauge from a w126 chasis with a broken LCD but good light bulb. I swapped the light bulb and now I have backlight.:) I think you need to have a small and low wattage light bulb otherwise it may bust the electronics. It is all surface mounted electronic stuffs, I am impressed.

Clarify please, does the outside temp gauge backlight bulb need to be soldered, or does it plug in as other cluster bulbs do? Thanks.

You need to open the cover and solder the new one in. On the 190D, the bulb is buried under 2 flat ribbon cables and is a bit of tough to get to. It is not a plug in.