I have a 1997 F350 dually Power Stroke 7.3L 4X4 cab and chassi with a mason dump. Bought it new in 1998. I'm original owner. Recently I converted to LED tail lights and since then, the tail lights have been on dim when the key is on. Also I have noticed the ABS light has been on.

I checked the circuit with a digital volt meter and found 8 volts on the brake lights when the key is on and no brake pedal applied.

What could be causing this?

 

You LED lights are causing that. There are electronic circuits on the same circuit as the brake lights. There is the ABS system(to know when you push on the brake pedal) and there is the cruise control system(to know to dis-engage when you hit the brake pedal). You also may have a trailer brake controller on the brake line also to control the trailer brakes.

With the LED lights on the end of the circuit, they are not presenting enough load to the brake wire. In other words, with the old fashioned lights, they draw a lot of current to burn and are a decent load on the circuit. The little bit of stray current from the other stuff on the line was always there, but it just went through the large bulbs and then went to ground, and it was not noticeable because it was not enough to make the old fashioned lights burn.

You LED lights take very little current to burn. They are getting the same stray current that was always there, but they are burning because of it. I bet if you unplug the lights all together and read the wire with your meter, you get 12v. The LED's are presenting enough to load to bring it down to 8v.

To fix all this you can buy "load resistors" and wire them in parallel with the LED lights. These increase the load on the circuit make everything work correctly again. I am surprised your turnsignal flasher still works, it's usually messed up too by the LED lights, but you must have a heavy duty or electronic flasher on it.

 

You were reading my mind. Only I figured out another problem that was added. I don't have cruze control. But anyway, the third brake light on the top of the cab was burnt out. I replaced the bulb and everything is working fine. It fixed the ABS light, the red glow in the brake lights when the key was on and also stopped the transmission from unlocking overdrive everytime I took my foot off the gas pedal. So in essence, your idea of a resistor is correct. I may add one anyway just for insurance and to make the blinkers blink faster.

 

Yes, you put a regular lightbulb in the third brake light, and that must have been enough load to bring it under control. So long as you leave that a conventional bulb, you will be ok, but if it ever burns out, you are in trouble again. I believe the brake signal goes to the tranny to disengage the torque convertor.

If you want a little bit more insurance, do a web search for "led load resistor" and see what comes up.

 

I have the same problem. My high mount stop light was burnt out. It did not bother me because the head ache rack blocks it. After reading this, I put a new bulb in the HMSL. LED light are out. But this did not solve the problem. There should not be any voltage on the 511 circiut until the brake light switch is activated. I traced it to the PCM. Not sure were to go next. Check this link.

https://www.ford-trucks.com/forums/1...t-problem.html

 

You did positively trace it to the powertrain control module? As I said in a previous post in this thread, I am sure it's going to be a ABS, trailer control, or some other device on the brake wire circuit.

There is not a problem here. You have modified the truck by putting LED lights in the circuit. You either get rid of them and put the regular bulbs back where they belong, or you increase the load in the circuit, either by using load resistors or regular bulbs somewhere in the circuit.

 

I did not modify the cab or chassis, it came without a body or lights. As far as I know Ford did not tell the body manufacturer not to install LED lights. Low voltage on this circiut will and is causing other problems.

 

Check the link I posted on this thread. I go thru the elimination steps.

 

I can guarantee Ford did not design this truck's lighting circuit for LED lights. People with even later model trucks(and cars also) have trouble when they install LED lighting. Most at least have to buy a electronic flasher to make the turnsignals work. This is a common problem with most vehicles.

 

So you think Ford thinks it is OK to have 6 to 8 volts bleeding from the PCM to the brake light circuit? I am just trying to find out why. Is it an engine sensor or some other input to th PCM that is getting to the 511 circuit via the # 92 pin on the PCM connector? Should I just replace the PCM?

 

Yes, the voltage bleeding is ok. There is a certain amount of leakage coming from the electronics. The original bulbs take a lot of current to work, and this small leakage does no harm. The large stock bulb will pull the circuit down to probably less than a volt, and there is no harm done. Your LED lights do not present enough load to the circuit, so this leakage becomes a problem.

You can monitor this circuit with your meter, and you can add more or less LED's, and watch the voltage go down the more LED's you add. You can also watch it go down even more depending on the size of regular light bulb you install.

 

I understand the incandecent bulbs create more resistance and will not burn bright enough to see. I am happy my LED lights no longer burn dimly when the ignition is turned on. But how long will the incandecent bulb in the HMSL last acting as a resistor for the brake light circuit? The bulb was burnt out and the socket shows signs of heat damage.

 

Install a load resistor as you were told to and the problem won't come back.

You're making this a lot harder than you need to.

 

I get the easy fix. I did that by replacing the HMSL bulb. I understand the LED lamps can cause problems with the T/S flashers or may cause a “burned out bulb” warning light to come on. I don’t understand why there is power (no matter how little) on a circuit that should not have power with the ignition switch in the run potion or that power bleeding is ok. Sorry for being a knuckle head. Just want it to be right. Franklin2 thanks for the input, as I said I am happy the lights are off now.

<TABLE style="BORDER-TOP-WIDTH: 0px" class=tborder border=0 cellSpacing=1 cellPadding=6 width="100%" align=center><TBODY><TR title="Post 10798942" vAlign=top><TD class=alt2 width=125 align=center>Franklin2</TD><TD class=alt1>Yes, you put a regular lightbulb in the third brake light, and that must have been enough load to bring it under control. So long as you leave that a conventional bulb, you will be ok, but if it ever burns out, you are in trouble again.</TD></TR></TBODY></TABLE>

 

I am so glad I found this thread I have had my dash tore out three times and have spend a metric ton of time and money chasing this problem. my HMSL bulb is out and i have led teillight on my flatbed. I am going to go out and replace the HMSL bulb and hope it fixes my issue. if not where can I get the load resistors at?