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  #1  
Old 11-22-2011, 05:43 PM
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W124 License Plate - One Lamp Works - Hack wiring?

1992 300E - I have one license plate lamp that works. The other socket has a valid ground but no +12V.

I have the trunk lining off and upon inspecting the the wiring I see it quickly goes off into the shrouded black yonder. I don't see any visible damage to the wiring loom at obvious spots.

Rather than track down where the wire is broken (I have no idea how to do this if it is inside the body work somewhere), I am wondering about just stealing power from the other lamp. This will obviously light the lamp, but will not turn off the warning light on the dash.

I really am not concerned about being warned if just one of my license plate lamps is working, and wondering if I could hack the wiring so that the warning light will go off if either lamp is working.


Looking at the wiring diagram (above), can I just short the two wires going into the Lamp Failure Monitoring Unit? Or will this give an incorrect resistance reading or something and cause the failure warning to still come on?


If this is a bad idea, alternate suggestions or hints at tracking down the break instead are appreciated.

If it seems like a good idea, I'd appreciate a pointer from someone who knows where the Lamp Failure Monitoring Unit is located.
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  #2  
Old 11-23-2011, 08:16 AM
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Of course the wiring is designed to allow for specific current consumption, but since license tag bulbs are only 5W, you shouldn't have any issues!
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  #3  
Old 11-24-2011, 12:18 PM
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To trace a broken wire or dodgy connection you would use the continuity function of a multimeter. Connect one lead to each end of the suspect cable with the circuit switched off and see if the meter's buzzer sounds. No buzzer means no continuity and a break in the wire as long as there are no switches or resistors and diodes in the circuit. It is easier to try this test in short sections between known joints, for obvious reasons.
Sedans are known to suffer harness failure near the boot lid hinge.
The lamp out warning module is behind the instrument cluster if I remember correctly.
Do both lamps use the same fuse?
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  #4  
Old 11-24-2011, 01:02 PM
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N7 is behind the fuses in the relay compartment on my '93 TD.

Pretty sure bridging the lamps will set off the warning...it did when I had to bodge my fogs for the test

If you have both ends of the wire you can at least check if the wiring is broken somewhere between, whether it's worth tracing through the loom or running a new supply is up to you eh?

cheers!
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Old 11-24-2011, 08:04 PM
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N7 has YOUR Number

He's correct:
"Pretty sure bridging the lamps will set off the warning...it did when I had to bodge my fogs for the test."
IT WILL !

Best Advice:
"Sedans are known to suffer harness failure near the boot lid hinge."

A resistor of the appropriate "Drag"
(Such as would be used to "Fool" N7 when using L.E.D. s )
is about your only "Hack" option.
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  #6  
Old 11-25-2011, 11:54 AM
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If there's a known weak spot (i.e. trunk hinge) and one wire may already be broken, then how long before another goes?

You could try a hacked fix but I'd recommend at least opening up the shroud on the harness near that area. A DMM with the pointy probe tips let you penetrate the insulation without stripping the wire.

I'd use the wiring chart you've got and see what wires you expect in the harness (eg. brake, tail, rear fog, b/u, license, ground) and then check continuity before and after the hinge.
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  #7  
Old 11-25-2011, 12:21 PM
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Both lamps use the same fuse, but each has its own wire connecting it to the lamp failure monitor.

I can't think of an easy way to hack the wiring to make the warning lamp work except for restoring it back to stock.

-J
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  #8  
Old 11-25-2011, 01:32 PM
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Appreciate all the replies! They inspired me to try to fix it right.

I first detached the ribbed cable shroud where it meets the smooth shroud that disappears into the body (at yellow arrow in photo below), this was just taped on.

I was able to expose about an inch of wiring. I pulled on the appropriate wire to see what happened... and it popped out.

I then used the broken wire length as a guide and carefully cut away a section of the ribbed shroud with a razor blade and a wire cutter.



As others suspected, I had other problems waiting to happen -- possible shorts of hot to ground wires.

I spliced the broken wire with a crimp-on heat-shrink butt connector, put some electrical tape of the broken insulation on the others (the wires seemed to still be ok for the moment).

Wrapped it all with some rubber splicing tape, pushed the shroud over the rubber, then another layer of splicing tape on top.



Sitting inside the trunk and closing the trunk lid quickly reveals why this is a problem spot.

I doubt my fix will last another 19 years as the bent wires are probably brittle and the butt splice has also made things more stiff, but I guess I'll know where to look in the future!

A more permanent fix might be to splice all the wires, adding a loop in such a way so the loop gets smaller/bigger when opening/closing the trunk rather than forcing wires to bend.

I hope the photos help someone else in the future, thanks again for all the replies.
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  #9  
Old 11-26-2011, 03:40 PM
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Yeah!

Wunnerful Inverted Photography.

That cracked insulation doesn't make me feel any better about my Older Chassis.
(Of course,Stuttgart never intended for us to be working on these Chassis at this late date.)
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  #10  
Old 11-27-2011, 08:35 AM
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I had lost one of the license plate lights on my 94 E320 cab.
As a quick fix, I jumpered the feed from the one working to the dead one.
Downside was that this triggered the lamp monitor.

I later discovered that the headlight switch was failing, and not sending power to the second light.

Jim
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  #11  
Old 11-27-2011, 09:05 AM
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I'm glad that it was an easy fix for you.
I discovered the harness issue at that location on our 300 when at nine years old, smoke started billowing out of the right hand side of the dashboard and steering column cowl on a country road.
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