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Strange Backup Camera Wiring Harness Test Result


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charlieworton 
Member - Posts: 20
Member spacespace
Joined: June 09, 2013
Location: Alberta, Canada
Posted: May 07, 2018 at 7:15 PM / IP Logged Link to Post Post Reply Quote charlieworton
Hi - I just bought a Chinese backup camera to connect to my Chinese AV head unit. The cable supplied has a proprietary plug that enters the camera. Coming out of the plug is a red wire, a black wire, and a video cable. Presumably the red and black wires - about 5 feet long - are intended to provide power to the camera, and the video cable - perhaps 15 feet long - is to supply the video image to the car stereo display.
The other end of the video cable contains a yellow video RCA plug, and a red wire, about 1 foot in length.
Somewhere I read that the red wire is a second power wire, and can be connected to the red ACC wire on the back of the car stereo. To verify this, I checked the resistance from the red wire by the RCA video plug, to the end of the 5 foot long red wire. I expected that the resistance would either be very low - effectively zero - or infinite. Either they were connected, or they weren't.
What I got, was a repeatable 3.6 ohms.
OK. So now we need to check the ground. Figuring that the 5 foot long black wire might be common to the RCA video cable external sheath - the outer ring of the fitting - I measured the resistance from the 5 foot long black wire, to the RCA plug external tabs. Again, I was expecting zero resistance, or infinite; what I got, was 5.6 ohms.
wth?
What I'd like to do is run the video cable to the head unit, tie the red wire to the ACC red wire on the head unit, and make certain I had a good ground on the head unit. That would be really quick and easy. But would it work?
Thanks for your clarity, and wisdom - Charlie
charlieworton 
Member - Posts: 20
Member spacespace
Joined: June 09, 2013
Location: Alberta, Canada
Posted: May 07, 2018 at 7:18 PM / IP Logged Link to Post Post Reply Quote charlieworton
Seeking to be respectful and compliant with website rules - the vehicle is a 2006 Mercedes E350 (W211 chassis) with a standard Mercedes wireless key fob. Peace - Charlie
i am an idiot 
Platinum - Posts: 13,334
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: May 07, 2018 at 8:26 PM / IP Logged Link to Post Post Reply Quote i am an idiot
Connect the one by the camera to the reverse light. The 1 foot one at the rca is what you connect to the reverse trigger wire of your deck. Those 2 wires are connected together. The wires are tiny. There is resistance in the wire.
charlieworton 
Member - Posts: 20
Member spacespace
Joined: June 09, 2013
Location: Alberta, Canada
Posted: May 07, 2018 at 11:05 PM / IP Logged Link to Post Post Reply Quote charlieworton
OK - so you're suggesting the red and black wires are connected to the backup lights in the trunk, so the camera will turn on and generate a video signal when the backup lights go on. I'm good with that.
But what does the red wire do... well, I guess it would become energized when the backup lights go on. Are you suggesting that when that line goes from zero to 12 volts, it is supposed to signal the deck that the display should be showing the signal from the backup camera?
I guess this would be a way for the deck to be able to switch automatically to "display backup camera" mode.
Am I understanding this correctly, or am I way off in the weeds?
Thanks - Charlie
eguru 
Copper - Posts: 340
Copper spacespace
Joined: February 04, 2018
Location: Quebec, Canada
Posted: May 08, 2018 at 8:16 PM / IP Logged Link to Post Post Reply Quote eguru
Your logic is correct.

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