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  #1  
Old 01-29-2010, 12:04 PM
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Question 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.
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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

2 x 87 300SDL
1 x 87 300D
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.
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  #2  
Old 01-29-2010, 02:32 PM
Texholdem
 
Join Date: Mar 2007
Location: Dallas
Posts: 756
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
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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE
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  #3  
Old 01-29-2010, 04:18 PM
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Location: Cape Cod Massachusetts
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Quote:
Originally Posted by ah-kay View Post
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.
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!
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  #4  
Old 01-29-2010, 05:48 PM
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Why not just replace with an LED and a resistor?
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1990 300SE "Corinne"- 145k daily driver - street modified differential
1988 560SEL "Gunther"- 190K passes anything except a gas station
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  #5  
Old 01-29-2010, 06:02 PM
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Quote:
Originally Posted by tinypanzer View Post
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?
__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

2 x 87 300SDL
1 x 87 300D
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.
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  #6  
Old 01-29-2010, 06:11 PM
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Quote:
Originally Posted by tinypanzer View Post
Why not just replace with an LED and a resistor?
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.

Last edited by Billybob; 01-29-2010 at 06:17 PM.
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  #7  
Old 01-29-2010, 09:53 PM
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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.
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1990 300SE "Corinne"- 145k daily driver - street modified differential
1988 560SEL "Gunther"- 190K passes anything except a gas station
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  #8  
Old 01-29-2010, 10:16 PM
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Quote:
Originally Posted by tinypanzer View Post
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.
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  #9  
Old 01-29-2010, 10:45 PM
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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.
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1990 300SE "Corinne"- 145k daily driver - street modified differential
1988 560SEL "Gunther"- 190K passes anything except a gas station

Last edited by tinypanzer; 01-29-2010 at 11:05 PM.
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  #10  
Old 01-30-2010, 03:14 AM
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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.
__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

2 x 87 300SDL
1 x 87 300D
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.
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  #11  
Old 01-30-2010, 08:53 AM
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Clarify please, does the outside temp gauge backlight bulb need to be soldered, or does it plug in as other cluster bulbs do? Thanks.
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Owned since new and still going strong and smooth
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Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

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  #12  
Old 01-30-2010, 11:42 AM
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Quote:
Originally Posted by Cal Learner View Post
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.
__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

2 x 87 300SDL
1 x 87 300D
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.
Reply With Quote
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