ground to plus 12v?

The question:  Is there such a thing as a ground wire that can switch to a plus 12v?

I'm not sure I need to go into all the details of my car and my system just to ask the question but I'll give a little history anyway just to understand where the quesiton comes from.

I've got a 2007 Ford Five Hundred. I've installed a Pioneer AVH-P4200DVD head unit using a < color=#417394>PAC C2R-FRD1 factory wiring harness adapter. The HU works great. I'm very pleased and highly recommend it.

The question is related to the Audiovox rear view camera that I have installed. The HU has a wire that senses for a reverse signal so that it knows when to show the video from the rear view camera. That wire is connected to a wire on the PAC adapter that shows it is for the reverse signal. The problem is that the PAC adapter is being sent the reverse signal via the CANBUS data wires and that is causing the HU to think the vehicle is constantly in reverse. I've contacted PAC about the issue last week and I have not heard from them. The power to the camera is tapped into the wire that powers the reverse lamp so the camera comes on and functions beautifully when the reverse lights come on.

Being a forum guy, I immediately posted the situation on a car audio forum and also on a Ford Five Hundred forum to see if anyone had personal experience with this. I received a reply on the car audio forum from someone with a completely different vehicle saying that he found the wire at the back of the vehicle that was a ground to plus 12v and hooked his HU up to that with success. I've contacted him via that thread for clarification but have not received a reply yet.

This got me looking on the web for what a ground to plus 12v wire would be. I've searched a few different forums with no luck. I've never heard of such a thing but that doesn't mean much knowing that I don't have much of an electical background.

Anyhow, that's why I'm here. Any help would be greatly appreciated. It is frustrating when something is hooked up and is ready to go but there's just one thing that is preventing it from working correctly.

Thanks in advance.

Jeremy

Thanks for the quick reply Kevin. The Pioneer installation manual only states, "Of the two lead wires connected to the back lamp, connect the one in which the voltage changes when the gear shift is in the REVERSE (R) position. This connection enables the unit to sense whether the car is moving forwards or backwards." The fact that it says it was just looking for a change in voltage made me think that it didn't really matter. The PAC installation sheet doesn't say anything about what their module outputs to that wire - either positive or negative or otherwise. I wish PAC would return my email question.

Jeremy

What does your volt meter say?  The "typical" output is 12vdc when in reverse and open (0vdc) when in any other gear.  Is the input programmable on your Pioneer?

I haven't put a volt meter on the wire yet. I did finally get to talk to a guy at PAC. He was very cool. He said that the green wire outputs 12v. He also recommended checking it with a volt meter just to make sure the unit is working correctly. If it is, then I'm not sure where to go yet.

Yes, the HU has a setting for the input: Battery or Ground. The problem is that I have the same problem no matter which setting it is on. Which seems to lead to a problem with the HU.

The camera itself is powered by the positive lead that runs the left rear backup light and it turns on and off correctly when you put the car in reverse. So it makes sense what you say about the "typical" output. It appears that the wire that runs back there works just fine. Now I just need to check to see if the green wire from the PAC unit is actually doing the same thing. If it is then something may be wrong with the HU.

Looks like we crossed our posts. I'll give that a shot and see if that corrects the problem.

Thanks!

So I'm guessing you didn't properly check the signals with a volt meter before running the wire? 

If you find out what the voltage is doing at the head unit you can most likely "fix" the issue with a relay.

Actually, I checked the signal from the PAC green wire to see if it was outputting 12v like it was supposed to. It wasn't. So I just ran the wire back to the back assuming that since the camera powers on with that wire then it will work. I didn't. So, you are half correct Kevin. I did check signal but not on the new wire.

I have actually spoken with a Pioneer support person and the guy was actually really nice. He explained how the unit works first (which was interesting) but then he said to make sure that PURPLE / white wire was getting either positive or negative 12v and then set the HU based on that. So, now I'll give that a try and see what it's doing. He said if it's getting some kind of signal and it still doesn't work then I'd have to send the HU in for service.

Well, I'm not a pro at this stuff so I'm not completely ashamed to admit that it was a user error. One of my connections wasn't crimped well enough. I re-crimped and I finally got a 12v reading. Sure enough, the HU works the way it's supposed to.

I'm not one to give up immediately and blame one part or another so taking the time to look into it and get some help here was very much a learning experience. Thanks for the help!