Head unit wiring puzzle.. any electrical wizards here?

[yche476]

So I've spent the past few days trying to put a new JVC kw-r710 onto my Dad's 2008 Mercedes Benz B200 Turbo.

It had a aftermarket Japanese pioneer head unit but was faulty.

I've connected up all the harness wires but I had no luck turning on the headunit until today. The head unit only turns on after I connect both red (acc) and yellow (constant) wire from the JVC harness to the yellow(constant) wire from the Benz. This implies that the red acc wire from the benz is not giving out power.

I've used a voltmeter to measure the voltage coming out from acc wire from the benz to be about 11V (I don't really know why this is a bit below 12 or if it even matter, but this reading is only when the JVC headunit is not connected to it. When I connect the acc wire from the benz to the JVC, I measure the voltage at the junction, it reads a 1 Volt potential.

My question is why does the acc wire read 11 volt when not connected but is dropped to 1 volt when connect. I did some electrical engineering sometime ago but I am scratching my head over what can cause the voltage coming out of the Benz to drop to 1V when connected to the head unit.

Any suggestions would be greatly appreciated.

Kind regards,

Brendan

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[Worlddre 5,000+ posts]

that is likely a CAN wire that you are trying to pull acc from. Im not familiar with that model of benz as we do not have them here in the states but typically you can grab acc from the cig light in a mercedes should be a light green wire.

 

[keep_hope_alive Premium Member]

yep, some modern cars use CANbus wiring to the head unit. sounds like you're not getting a true ACC wire and dre has the right idea.