car radio battery wire 0v when harness is disconnected from the car radio, but after harness connected to the radio , it has 12v, why??!!

[lh333_34]

Hi everyone
I am new in this forum and this is my first thread.

Car model: Toyota echo 2004 sedan.

I used a multimeter to test the car radio battery wire ( blue/yellow) when it was disconnected from the car radio, its reading was 0v. ( as shown in photo 1 ) before and after ignition, isn't it supposed to have 12v even being disconnected from the original radio?

but after I connected the original radio to the harness, its reading was 12v before the ignition (as shown in photo 2) , and 12v after the ignition ( as shown in photo 3) .

I would like to find out why it is like this?

When I tried to connect an after market radio to the harness, the battery wire still read zero before and after ignition, because of this, the after market radio could not be powered up. ( I tested it on direct 12v power supply, the after market radio was working) .

I am looking forward to getting some advice from experienced members on this forum.

Thanks in advance.


photo 1 ( harness disconnected from the original car radio )

photo 2 ( before the ignition)

photo 3 ( after the ignition )

 

[lh333_34]

My question can be simplified as below diagram shows

 

[1aespinoza]

question can be simplified as below diagram shows

It is probably an anti-theft feature. On some cars, when you purchase a replacenent OEM radio, it has to be programmed to your ECU.

 

[Bigballs420]

Could be as simple as one of these

https://www.grainger.com/category/electrical/industrial-controls-automation-and-machine-safety/relays-and-accessories/current-sensor-relays

 

[lh333_34]

Thank you all for your input....It happened that I was following wrong instructions from a webpage saying that the black wire is the ground...and in reality it isn't !

The ground for my car actually is the white/black one! The black is for power antenna.

Once the true ground is identified, puzzle is solved...

 

[audiobaun]

Interesting find