View Full Version : Load rating on rear fog light wiring?

08-15-2010, 11:46 AM
Looking to signal a relay off the rear fog light wiring to control a rear external flasher unit. Tried to send signal to relay and only got a short signal from the fog light wiring and then nothing. Seems like the computor is telling the system there is a short somewhere? Is the system that smart?
Thanks Tom:thinking:

Aqua Puttana
08-15-2010, 02:38 PM
Is the system that smart?
Thanks Tom:thinking:

From my experience with the T1N wiring the system may very well be that smart.

I'm not clear on what you're doing. Are you only triggering the relay or is the entire load of the flash unit being added to the existing circuit?

The reason I ask.... you may get away with just the thieving relay current. The entire flash unit is most likely enough load that it would be seen and would cause the computer to react. FWIW. vic

08-15-2010, 03:48 PM
Most later model MB vehicles measure the current draw/voltage across each lighting circuit. I found this out when changing a tag light bulb in the E320. I used a LED which draws a percentage of the current than the standard 194 incandescent. Not only did the LED throw a bulb out warning on the dash, but it was flashing like it was saying wrong bulb moron. They make replacements with built-in resistance so aftermarket lights will work properly with no error message. Measure the load from the existing lamp and install a LED in the rear fog position and use the all, (adding resistance if necessary), of the available excess current to trigger the flash unit.

08-15-2010, 03:57 PM
I'm only trying to trigger the relay, should be limited current? I did try to combine the left rear fog light and the lower right un-used light together on the brake circuit, didn't like that either? Just thought more brake lights while they are thier not being used.

Thank you for the reply.

08-15-2010, 04:11 PM
The lighting controller doesn't like the combining of circuits. Figure out how much voltage you need to trigger the relay. I'm not sure how much the rear fog lamp draws, (haven't taken mine out), but a 1156 uses about 27 Watts. Even a high power LED will only use 3-5 watts depending on configuration, you should have plenty to work with.

Aqua Puttana
08-15-2010, 04:37 PM
A different tack. Maybe try wiring a powered trailer module into the circuit instead of a relay? What is normally the running light circuit of the trailer module could be used to feed power to the flasher unit from the dedicated power wire feeding the module. As long as the trigger signal (fog light) is present the module will put out power to the running lights (flasher in your case). For a couple years now I have used a powered trailer module for my trailer lights. The computer has not noticed the trigger currents. vic