Does anyone notice the weird voltage on reverse light?

[Tony_1992]

Hey guys, I'm recently adding an aftermarket touch screen stereo radio, with back up cam. Everything works fine except that the screen doesn't automatically switch to reverse view when I switch to reverse mode, which is weird. So I checked up the voltage signal of reverse light, and I found something.

When the car is in not in reverse mode, the positive(+) and negative(-) pins of reverse light are both 12V to the ground(chassis), so that there's no voltage across the light bulb so it doesn't work. And when I turn the car into reverse mode, the negative pin of reverse light becomes 0V, thus lights up the light bulb.
But here's the thing, the stereo radio only recognize 12V as a effective signal for altering to reverse view, which means it'll never work as expected due to the electric design of Lincoln LS.

I really cannot understand why they design the electric system like that. Shouldn't it be better to use 12 as a triggering signal, rather than a inverted scheme(negative as triggering signal).

By the way, does anybody have any idea where to find a really high-voltage-triggering 12V, when the car is switched into reverse mode?
Thank you!

 

[Gary Vogeler]

Tony,
The reverse light is likely wired this way due to the battery being in the trunk. I would expect the fuse for this circuit to be in the rear fuse panel close to the battery. The reverse light is between the fuse panel and activation switch that is most likely at the gear shift.
A layout like this minimizes the amount of wire necessary therefore reducing weight, cost and complexity of the harnesses. If it were wired with the ground directly off the light to the chassis, power and a return wire to the light would need to be run from the back of the car to the switch and then back to the light.
I never purchased the shop manuals so I can't confirm this, maybe someone who has the shop wiring manual can confirm.
If my guess is not correct, I still expect this to be driven by economic or weight minimization design goals.
To get around your problem, you will need an inverter circuit or find another trigger of correct polarity as you have asked about.
I'd check Cruchfield or another online retailer for an inverter.

 

[1LoudLS]

the battery in the truck has absolutely nothing to do with the design of the system to turn on and off things by switching the ground wire on and off vs the more common method of switching the power wire on and off.

most good radio's have the option to accept either a ground signal or a 12v signal, since your doesn't, the solution is simple... use a relay triggered off of the reverse light, have that relay provide 12v when triggered. run terminals 85 & 86 to the positive and negative wires at the bulb socket, run terminal 30 to the 12v source (which can be the same as the 12v light bulb feed) and then run terminal 87 up to the radio's reverse trigger input. terminal 87a will not be used


also there is no "high voltage" trigger in a 12v system, just the low 12 volts that run the system (aside from the ignition system anyways) there are however low and high CURRENT wires in this car...

 

[Tony_1992]

the battery in the truck has absolutely nothing to do with the design of the system to turn on and off things by switching the ground wire on and off vs the more common method of switching the power wire on and off.

most good radio's have the option to accept either a ground signal or a 12v signal, since your doesn't, the solution is simple... use a relay triggered off of the reverse light, have that relay provide 12v when triggered. run terminals 85 & 86 to the positive and negative wires at the bulb socket.


also there is no "high voltage" trigger in a 12v system, just the low 12 volts that run the system (aside from the ignition system anyways) there are however low and high CURRENT wires in this car...

Using relay is a good idea. I even think about using transistor and resistor to build a NOT gate.

 

[Gary Vogeler]

Relay is a good idea.

Have to disagree with LoudLS. I've seen high side and low side switching utilized in my years in automotive and see it even more in aviation where weight is so critical. Can't believe cost and weigh were not drivers in the decision to wire the circuit this way. Ford took the rear headrest out to reduce weight.

 

[joegr]

The "negative" switching is because it is simpler (fewer transistors needed when switching 12V power from 5V or 3V logic circuits), and it is more robust (a short to ground won't hurt it). This is not weird, or even very uncommon. I switch ground in all of my designs when it is practical.

There is a "positive" trigger for the reverse signal. It goes to your autodim rearview mirror. (The purpose is to undim the mirror when in reverse for safety.)

I used a relay at the light so that I could use it to switch power to the camera too.

 

[Tony_1992]

the battery in the truck has absolutely nothing to do with the design of the system to turn on and off things by switching the ground wire on and off vs the more common method of switching the power wire on and off.

most good radio's have the option to accept either a ground signal or a 12v signal, since your doesn't, the solution is simple... use a relay triggered off of the reverse light, have that relay provide 12v when triggered. run terminals 85 & 86 to the positive and negative wires at the bulb socket, run terminal 30 to the 12v source (which can be the same as the 12v light bulb feed) and then run terminal 87 up to the radio's reverse trigger input. terminal 87a will not be used


also there is no "high voltage" trigger in a 12v system, just the low 12 volts that run the system (aside from the ignition system anyways) there are however low and high CURRENT wires in this car...

Hi, you mentioned terminals 85, 86, 30 and 87. But I don't know what they refer to. Could you tell me where to find the circuit diagram that correspond to your instruction? Thank you.

 

[joegr]

Hi, you mentioned terminals 85, 86, 30 and 87. But I don't know what they refer to. Could you tell me where to find the circuit diagram that correspond to your instruction? Thank you.

Those are the standard terminal numbers for the 12V/30A relays. Look closely at the relay and you'll see them.

 

[Tony_1992]

Those are the standard terminal numbers for the 12V/30A relays. Look closely at the relay and you'll see them.

View attachment 828487342

OK I got it. I thought these are number of part of the car.
For convenience I bought this 4 channel relay module. Though only one channel will be used.
https://www.amazon.com/GERI-4-Channel-Module-Optocoupler-Arduino/dp/B00VND5ZPI/ref=sr_1_6?ie=UTF8&qid=1491237033&sr=8-6&keywords=4+channel+relaysories

 

[joegr]

See attached for more wiring details.

 

[Tony_1992]

See attached for more wiring details.

Thank you so much for helping

 

[abes_linc]

check this out, I had the same problem and I think I just wired the positive to the bulb wire with a switch now when ever I hit the switch the cam comes on I can check tomorrow to see exactly how I did it..