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Old 03-20-2002, 11:45 AM
Posts: n/a
Question CLK55 reverse light wire tap reads 9V?

I'm trying to tap onto the reverse light to hook up a backup sensor. My multi tester reads only 9V on the wire I found. (Yes, I checked hot bus voltage and it IS 12V). I read in another forum that a computer determines voltage feed to certain systems. I DID change the bulbs from the std 1156 24W to 55 W halogens. Can't see a thing in the back of this car with the top up. Can anybody shed some light? (no pun)

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Old 03-20-2002, 02:33 PM
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Join Date: Feb 2002
Location: St. Louis, MO
Posts: 340
I have a 1997 E320. Recently the dealer replaced some front and rear light wiring harnesses because there was a problem with bulbs burning out prematurely. Supposedly, the harnesses have some in-line resistance to drop the voltage applied to the bulbs. You may have the same arrangement.
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Old 03-20-2002, 10:26 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Well, if you are pulling 55w bulbs on the same circuit, then I can see how it would have 9 volts. As amperage increases, voltage decreases. Simple electricity 101. You are pulling a higher load on with the 55w bulbs causing a voltage drop. I would either use a relay to fire the bulbs, or change them back. You are putting stress on something....
Donnie Drummonds
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Old 03-20-2002, 10:34 PM
Posts: n/a
Well, I flunked electricity 101. No summer makeup offered at the time either.

They did warn against stressing the wiring but I figured I only use reverse a few seconds at a time.
I guess this compounds my problem, specially if I add another load (the sensors) to that circuit.
Do you mind walking me through the relay suggestion?
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Old 03-21-2002, 11:28 AM
Q Q is offline
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Join Date: Aug 2000
Posts: 839
To use a relay, you will need to find the pinout diagram on the net. You will need a standard 30amp relay (found in most auto parts places), some female crimp-on connectors, and some good 14 gauge multi-strand wire.

You will need to cut both of the wires that goes to the reverse light bulbs. Better yet, if there is one wire that then splits to both bulbs, cut the single wire. If you must do both, you will need to insulate one of the ones that goes back to the wiring harness to prevent a short, as you will only need one wire.

Now, install the existing hot wire to the hot lead of the relay. Install a wire to the ground lead and then connect the other end of that wire to the car's sheetmetal as a ground. Make sure you have a good clean metal connection. If you don't, you will have trouble with it.

Now, you will need a 12V wire from the battery to the relay's NO (normally open) connector. A fuse as close to the battery as possible is mandatory to prevent a fire if there is a short!!!! For two 55 watt bulbs, a 10 amp fuse is cutting it pretty close, but I would start there and move to a 15 if it fails.

The opposite side of the NO connection should go to the lights.

Now, when you turn on the reverse lights, the relay's normally open internal switch closes and connects the reverse lights to the battery with a fused connection that allows the correct amount of current flow without over heating your wiringing harness.

Can anyone else add to this?
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Old 03-21-2002, 11:46 AM
Posts: n/a

I think you pretty much covered it.
It sounds pretty straightforward to me. I'm running out to get the parts today.
Luckily, there is a small fusebox right next to the battery, which is on the right side
in the trunk. Pretty close to where I need to be.

Thanks again.
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