Hello! I have an issue that I am chasing down that was first discovered when I noticed my trailer brakes getting activated when the headlights are turned on. I have troubleshot it to the point of proving that the brake signal coming into my brake controller truly does see 5 volts when the headlights are activated. 0 when they are off. Is this just a bad ECM? According to the wiring diagram the lowside of the brake switch goes straight as an input to the ECM and if I am getting 5 volts to ground out of it when I turn on the headlights it seems as though that is the only possible answer? Thoughts? It is pin 2 (BOO) I am looking at. Willing to try anything before trying to find an ECM for this thing (1994 F350 Revcon RV).

 

I would say you have a poor ground for your headlights.

(edit) verify your grounds on the park light circuit, which would be on with your headlights. Turn your park lights on only and see if you have 5 volt on the brake controller circuit. If you do, then the issue is on your park light circuit, and the ground or bad connection on a bulb socket would be a good starting point to look at.

 

Here's a good thread on grounds. Check out posts 6 and 7. https://www.ford-trucks.com/forums/1...nd-points.html
Might not be identical to your F350 chassis.

 

Sounds like a short to me
None of that wiring is connected IIRR with a conventional brake controller on a 1994
That diagram does not show the headlamp switch or the stoplamp switch wiring
You need an ohmeter and to do some continuity checks

 

Thanks for the suggestions all. I want to reiterate that when I turn my headlights/running lights on that I get around 6 volts OUT of what should be an input to the ECU, pin 2 (BOO). I cannot see how a bad ground could cause the ECU to send voltage out of an input?

 

You have to realize what you are dealing with. You are dealing with transistors and logic gate circuits. They are not a true switch which would fully create an electrical open/isolation point. Those transistors are just as good of a conductor as they are are an insulator. In aviation I have had some logic gate circuits be considered "electrically" off, but there is still 4-5 volts on that circuit.

I'm not saying that is what is going on, but it could be a possibility.
Here is an easy way to verify. Drive with the headlights off and reach cruise speed so that it goes into overdrive and lock up on your converter. Watch you tach or listen to the engine, and turn your headlights on. There should be roughly a 50 rpm increase when you turn your headlights on. If there is that means the torque converter went out of lockup. If not, then what you are seeing on pin 2, the roughly 6 volts, is a 'red herring'. It is not effecting your issue.

The main reason I told you to check grounds is that your brake controller IS a transistor logic gate circuit controller. It is tied into your park/brake light circuit. And if you have a bad ground/socket then that bad connection point will not be able to carry the full amperage load, and will find an alternate path that has less resistance to complete the circuit. Brake controllers have been known to be that alternate circuit.

 

Thank you for the reply. I am only getting the voltage out of the Brake Pedal input when I turn the headlights on. No voltage present on that pin with the headlights off. If it is some sort of bleed through/false positive, it is enough to still trigger my brake controller that is tied to that signal as well. I am toying around with the idea of putting in a blocking diode on the input so that voltage can still go in, but none can come out.

 

I missed your 2nd statement of the bad ground/sock on brake lights. Will also look into that. Thanks.

 

Also make sure one or more of the park/turn bulbs is not shorting between the filaments. Seen that a few times.