Brake lights getting cross fed power?

[kc0stp]

http://www.ford-trucks.com/forums/1304495-tail-brake-lights-getting-crossfed-power.html

Instead of retyping it all theres the current thread, any electrical gurus have any ideas?

 

[typ4]

Mine does the same thing untill I hook up the trailer. It may be the cruise control, I havent worked it out yet , no time and It is just daytime running lights.
There is no pcm on a 91

 

[laserjock]

To summarize, seems like you have eliminated the multifunction switch itself by replacement. You put LED lights in and the problem started after you put the right side in?? Was it there prior or you didn't check?

You have voltage in places you shouldn't with key on and key off.

You have a lot of corroded connectors.

The LEDs take way less juice to fire off so you may have had the problem all along and didn't notice. Might be worth disconnecting them and seeing what you have in other places like the front turn signals.

I would check your trailer plug and wiring carefully (old and newer)

I would check your ground straps too. A bad ground can lead to a floating voltage. Trailers are notorious for back feeding through tail lights and stuff due to bad grounds.

If you pull the turn signal bulbs up front, does it still do it? That might help isolate the problem. I don't think it could back feed through the LEDs. Depends on how they are put together.

Hope that helps.

 

[racin460]

I did the same thing led and the last ring of lights on the bulb light up. led take les current to light I have not got that far but I want to see if there is any power there with a volt meter then get a resistor to pull out the residual power and try it. if you figure any thing out before me please post so I know.

 

[laserjock]

Spent a few more minutes thinking about how I would trouble shoot this.

Are you having any battery issues?

Might be worth looking for a draw which is an indication of a short somewhere. Key off I would expect maybe 50-100 mA draw basically for the radio since there is no PCM. Probably less. If there is more start looking for it.

I just went through this on a subaru which turned out to be a bad alternator....again.

I'd start there. Unhook it from the system then see what you have. If you haven't 3G 'd it I'd pull the wires off the regulator too. Still the same hook it back up and head to the fuse block.

Pull the fuses out one at a time and look for the draw to go away. Put it back if no change (radio circuit should have small draw).

No love yet. Start pulling the power feeds off the starter solenoid. I'd start with the GP feed. Lots of possibilities in that system to cause problems. Then any auxiliaries you might have and finally main power. If that doesn't get it, about all that's left is the starter.

Hopefully by this point you have at least found the offending circuit.

Good luck. Hope that rambling helps. Might at least start a good conversation.

 

[typ4]

No multifunction on a 91.
Its not a draw or anything like that, ive done some checking , fuses and all made no difference. it still points to the cruise module/ actuator. Trailer isnt backfeeding, when trailer is hooked up lights work fine. Needs a load on the wire to make them go out.
Its just a design flaw since LED'S werent popular when these were built.

 

[C.J.]

I ran into the same issue when I put LEDs on the back of my box van. Only happened if the engine was running. My mechanic said it was from stray voltage in the system. I just figured it was poor mans day time running lights I never did have any issues and was happy as heck with my new lights that I didn't care.

 

[kc0stp]

Already pulled individual fuses with no change, tried brake switch and turn signal switch neither made a difference. Back when I was running a single battery itd go dead on me but with both batteries in Ive left it sitting for months and had it fire right up (replaced both batteries so guessing it was just a bad battery). It was probably doing this back on the incandescent but 4.5 volts wont get them bright enough to notice especially with the pos lights this flatbed had. Honestly if it was the tail light circuit I would just ignore it at least for a while but being the brake circuit want to get it fixed.

All that said I do think you guys are onto something with the required load bit, if I unplug the main wiring harness where it connects to the bed/trailer wiring the voltage goes up to 9 volts (I will admit I just gave that a quick look so no guarantees) Im tempted to get the other 2 lights wired in and see what it does...

 

[79jasper]

Funny you have this problem.
On my 94 psd, I bought those waterproof led tail lights, the exact same thing happened. (Just the tail light function, two LEDs per side)
I guess I didn't notice it when I hooked them up, but noticed at my gf's house later that night.
My trailer plug was for sure cross feeding. So I just cut it out at the time.
Between that and hooking up the front and rear side marker lights (4 total, incandescent) and hooking up the headache rack lights (one on each side, incandescent) it quit doing it.

So no help really, just my experience. I'm changing the rest to led eventually, so it may come back, but then again, 8 lights may be enough to shut it down.


Sent from my iPhone using Tapatalk

 

[laserjock]

No multifunction on a 91.

Sorry that's what the parts guy called the one on my 88. Probably bad lingo. Turn signal/horn switch under the steering wheel.

If your goal is simply to make the lights go out, you can probably just add a resistor to put a constant load on the system or put a diode in line with the lights to keep if from back feeding from ground (not sure how your lights are constructed internally so that may not make any difference). I'm not sure that's actually solving the problem though. Be it design flaw or actual problem, you still have voltage somewhere it's not supposed to be when it's not supposed to be there. Cruise module might be a good bet but I would have thought pulling the fuses would have shown that. It should have lost power when you pulled the fuse for the cruise module right?

Anyway, good luck. Hunting down things like this can drive a man crazy.

 

[typ4]

Yeah these new parts guys have no idea what came on older cars and trucks.

The diode thing doesnt work, tried it. I havent unplugged the cruise yet to see if the lights go out, maybe its the flasher for the turn signals letting thru some transient voltage. When I unhook the camper ill do more tests.

 

[franklin2]

Yeah these new parts guys have no idea what came on older cars and trucks.

The diode thing doesnt work, tried it. I havent unplugged the cruise yet to see if the lights go out, maybe its the flasher for the turn signals letting thru some transient voltage. When I unhook the camper ill do more tests.

I believe you will find it is the cruise control doing this. It takes a input right from the brake circuit to disable the cruise. The lightgreen wire from the brake circuit goes right into the cruise amplifier. Another possibility would be the ABS circuit board.

To the guys with the problem; Does your cruise control still work? It may not with these lights installed. I have heard other stories of people's cruise not working, and they happen to remember they had just replaced a burned out taillight bulb, and for various reasons it made the cruise control not function until the got another bulb and installed it.

 

[79jasper]

I don't have cruise. Lol


Sent from my iPhone using Tapatalk

 

[jaluhn83]

Cruise control. IIRC, the system senses if the brakes are applied by sending a small current through the brake lights - if the lights are off, the small resistance of the bulbs causes this pin on the cruise control module to be pulled down the ground, and when the brakes are applied the brake light switch puts 12 volts onto the circuit with a much greater current so the small current from the cruise control has no effect and the input on the cruise is 12 volts which is sensed as 'brakes on' by the system.

This is also why failed brakes lights bulbs sometimes will cause issues with the cruise control not working.

My guess would be that the LED bulbs have a high enough resistance and light with a low enough current that they're coming on now with that small normal current from the cruise system whereas before you didn't see anything because the regular bulbs wouldn't light up at all.

Is it enough of an issue where you need to fix it? You could probably fix it by putting a resistor across the brake light wires, but I'm not sure what specs to give you on one - need to do some research.

Or just disconnect the cruise control.....

 

[kc0stp]

So where is the connector for the cruise control at anyways? (no I don't plan on leaving it unplugged, just to test it out)