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Old 12-17-2015, 03:43 AM
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Posts: 138
Tail light resistors ??

Hi all,

Can somebody please explain the presence and purpose of what I think are resistors in the tail lights of w113 and some w111 cars? Please see this photo. The two things circled in the link. If anyone also knows the specs of the little blighters I'd also love to know, thanks.
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Old 12-17-2015, 11:36 AM
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Could those be relays instead of resistors? Is there a part number on them? With a part number you can check the EPC.

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Old 12-17-2015, 12:02 PM
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Thanks for the reply. I have this info from an electrical whizz I know:

Those two diodes were indeed ORing diodes (as I suspected earlier) for the purpose of having two otherwise separate circuits lighting one filament...however, as shown in the wiring diagram extract, one diode is not connected (NC) or used. (This is likely due to different configurations of the vehicle, for different years and or sales markets/countries)
This means that because your vehicle has the other diode still connected, it is dropping some voltage to the lamp, resulting in that lamp being less than at its fullest brightness...this unnecessary diode should simply be jumped (or a connection) should be made across it.

I recommend the following to confirm wiring as shown in diagram is indeed same as your car: Determine which is the diode still in-circuit and connected a test jumper across it (or jump across both)...connect to vehicle elec sys normally and test all lighting functions, they should work as expected, and you might even see a slight improvement in brightness on the lamp involved because it is now getting full voltage! There is no risk of damage here...the worst that could happen is that lighting does not work as expected, or absolute worse, you could blow a fuse if something was very wrong (unlikely!). In that case report back with results.

Having confirmed proper operation, make the jumper permanent, and obviously, if you are reproducing this fixture a second time, in this case, you don't even need the diode...or to worry about it!

I hope that helps someone.
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Tail light resistors  ??-280sl_wiring_diagram_extract_rear_lights.jpg  
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Old 12-17-2015, 07:10 PM
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Join Date: Jan 2012
Posts: 7,378
Does the car in question have functioning Euro parking lights? This is where you turn the headlight switch counter clockwise and either the left or right parking light lamps illuminate.
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