I'm trying to get a touchscreen for my truckputer to trigger the reverse camera feed on my 2009 F-150. I originally hooked it up to a wire that was identified as the reverse sensor wire and has successfully been used on other installs with aftermarket headunits. After I hooked everything up, I quickly discovered that my monitor would trigger on my blinkers and brake in addition to reverse. I confirmed with my meter that the brake light was a low positive voltage and the blinkers pulsed near 12v. Apparently, aftermarket headunits are less susceptible to this, but my liliput touchscreen detects the voltage and switches outputs. After testing dozens of wires, I finally just removed my taillight found the wires feeding directly into the reverse bulb. I tested one and it had the same signature as before with pulsing on the blinkers. Then I realized that the other wire must be canceling that out somehow since the light doesn't flash with the blinkers, and if I measured voltage with respect to the second wire and not to chassis ground, it read a steady 12v on reverse and nothing with the blinkers or brake.

Now my question is, how do I get this to work with my monitor that only has input for a +12v wire? It is using my vehicles ground and not this wire as its reference.

 

Guess I could use a relay. May try that.

 

Ok, just to be clear:

• backup light was measured to chassis ground and found to be receiving less than 12v when turn signals on
• backup light was reading 0v when referenced to the 2nd backup light wire, and 12v when in reverse
• the trigger line you want to use for your monitor activation matches the above behavior

just making sure I understand you. Is this correct?

 

i had this problem as well with my camera install and after looking at the wiring digram i found it easier to just tap into the reverse on the trailer tow plug

 

No. The backup light wire does measure +12v when measured to chassis ground. However, it also reads greater than 0v (pulsed) when the blinkers are on. This is the same behavior I found in the light blue wire on the passenger side that other forum members have used, which I didn't notice until I had it hooked up to my monitor. After discovering this behavior with the light blue reverse signal wire, I checked the reverse wires at the rear view mirror harness and the trailer control harness. They had the same exact behavior. Oddly, I also checked the trailer hitch harness reverse wire (center pin) and it also pulsed with the blinkers on, so I'm not sure how jhunt47 got it working.

The only way I've found to get a +12v signal if and only if the truck is in reverse is to use the reverse bulb positive with the second wire feeding into the reverse wire as the reference.

 

How is this different from what I said?...


Anyways,


What is the pulsing voltage? If it's low, you might be able to put an RC circuit in there to dampen the pulsing. See here for more info:

http://en.wikipedia.org/wiki/RC_circuit

If you can give me an approximate pulse voltage and an approximate pulse time (how long the pulse lasts), I can do a quick calculation and let you know the parts you'll need.

However...if you don't have the ability to measure the pulse timings...

Maybe a better idea, if you're only reading a voltage of 5v or so when the line is pulsing, you might consider putting several diodes in series instead. This will lower the voltage by the same amount regardless of voltage, pulsing, etc. (assuming the voltage is high enough to overcome the diodes).

Usually the turn-on voltage of a regular diode is between 0.7v and 1.3v. So string enough of these together to eliminate (or severely decrease) the pulsing voltage, while maintaining a working trigger voltage (i.e. so your "real" 12v signal still triggers the unit).

 

It wasn't. I misread your first bullet. I'll have to double check my the readings for the blinker pulses tonight. I want to say it was around 10-11V, but I'm not sure I checked what full range was when reading the bar graph.

 

Ok, sounds good to me. Do you have access to an oscilloscope by chance? Or know someone who does? That would be the best tool to diagnose the signal, and check for pulse duration.

If it ever stops raining here in NC I'll check my truck and see what I get. I imagine the wiring for this is identical between the 2009/2010.

 

Is this the kind of pulsing you guys are talking about?

4-ways


Turn signal

 

Yes I think so. Please correct me if I'm wrong, but you're talking about the brake lights getting brighter when they are blinking and/or brakes applied?

 

Well, the running lights and license plates lights pulse with each blink of the turn signal. For example, if the left turn signal is on as in the video above, the right tail light pulses. From this thread it sounds like the reverse lights do the same thing. Just figured this might all be the same issue.

 

Hmm, this is all very interesting. I'll have to take my truck to the lab tonight and scope out what's going on here.

 

FYI, I did get it working this weekend using a relay. Used the light blue wire (reverse trigger) running along the passenger side (same one I had originally hooked up to the monitor directly) along with chasis ground to energize the relay's coil and provided a constant 12v wire to the switched input.

 

Awesome! Any pics of it up and running?