1997 F-350 CCLWB 4X4....Originally single wheel w/PU but changed to Utility bed 7 years back. Wore bed out so decided to make flatbed dually...
Removed utility bed...disconnected rear light connector and wired up temporary lights to move. Rear lights worked correctly.

Built flat bed and converted to dual wheels with spring change....
again used temporary lights and they worked correctly..

Finished bed and started wiring up lights (All LEDS) and now have problem.

When the key is ON there is approx 9VDC on each of the rear turn/brake wires. When I activate the turn signals this goes away on the side being activated. It does not go away when the running lights are activated. This voltage powers 4 leds in the bulb, not all....all the turn/brake/runing/4-ways all work correctly.
If I activate the right turn signal the left still has the 4 leds lit, if I activate the left turn signal, the right has 4 leds lit...If I activate the running lights, the 4 leds stay lit on both sides...

Tried pulling each fuse on the panel to see if one of the circuits was feeding the problem...the 4 leds never went out....

Removed the trailer brake controller completely and the 4 leds still stay lit...

When the key is off the 4 leds are also off..

Checked all of the wiring and do not find any shorts...

This appears to be originating in a circuit that is not fused but is after the key....

I am very electrically literate but this does not seem to have any logical source...

HELP!!!!

 

I think the issue you're having is caused by the fact that you're using LED bulbs that don't have resistors built into them. Someone else will chime in here that has done this personally, but I think that's what you're going to find. I believe the resistors you need can be had separately and added to the lights you have.

The way these circuits are designed the brake light path is used as a signal path for some other things (cruise control especially). There are all kinds of problems associated to the braking circuit on these trucks. I bet if you were to drive it this way the torque converter would not lock up and the cruise would not work either.

I will let someone with first hand knowledge of the issue clear you up on the details of the solution, but I think that is the problem you're having.

 

Basically, the OBS trucks don't like LEDS and create all kinds of problems. Since you know everything works properly without them I'm thinking that is your issue. The PCM monitors the brake / blinker circuit so it knows if a bulb is burnt out. You can use LEDS if they are the kind that have a resistor is parallel but most just go back to standard factory bulbs and be done with it.

 

I had the same LED lights on the Utility Bed and did not have this problem. Been running the LED's for 4 years and this has never happened. I tow a tailer that also has LED lights a lot so I am constantly checking lights and with or without the trailer it has not happened before.

This dispelles the resistor issue and also the PCM idea in my mind.

Now when I hook up the trailer the same thing happens to the trailer LEDs.

tHis truck does not have Cruise Control...It is a basic truck with A/C, PS and PB. No ABS and manual trans.

I have been having an issue with the speedometer jumping around. Changed rear ends, sensor and speedometer. THe ABS light comes on which I read means there is a bad connection between the rearend and the unit....Have looked but can't find any problem. Even changed the sensor pigtai.

I guess I am not supposed to pass 500,000 miles......cause it was real close when the speedometer went nuts....Last time I changed the head and it worked...

 

Is there a chance that the third brake light is burned out? Whether the truck has the buttons on the dash for Cruise control or not, the PCM is still programmed with the cruise control features in it so its interior circuitry is still the same as a PCM for a truck with cruise control. It will still use that circuit as a return path.

Also, if the truck is lighting the ABS light, you do indeed have rear wheel ABS. If you look on the driver side frame rail right below the front of the cab I bet you will see the RABS (Rear ABS) valve with the brake lines entering and exiting it. The tail and brake light circuits can cause the dash ABS light to come on as well. I don't think ABS was an option on a SRW F-350 it was standard equipment I believe. Dually or F-Super Duty would have been another story, but since yours started life as a SRW F-350 I bet you have Rear ABS.

 

When you use LED's in certain places, it doesn't let the relays work right and screws everything up.
There is not enough load from the bulbs, so what happens is, well what your seeing.

Yeah, they will work fine on the trailer, because the trailer is a whole separate circuit.

I've seen tutorials on modifying the relays in the vehicles just to make turn signals work right. (not for our trucks)

In certain places you can use LED's, but for the most part you have to buy certain lights for other places from reputable sources.

There used to be a MFG here in Michigan that made great bulbs.



Now let me explain why they put resistors in the LED towers.

I=E/R

The relays work on Impedance, or draw on the circuit.

When you change your Resistance in the circuit, you change your Impedance.
Thus the metal in the relays does not heat up and break.
Its really neat to play with these sensors. You can use a lighter and break the circuit.

When you put the LED's in the truck, your not allowing the Relay to work any longer.
The lead for the trailer works after this lead, so when you had normal bulbs in your truck, they worked fine because the main relay was working fine.
So when you put LED's in, its not tripping the relay and thus sending the same signal to the trailer.

 

I have a 2005 F-150 just bought some LED tailgate lights that hook up to my 4 pin trailer harness. Well I plugged them in and they work if I don't cut my lights on. When I cut my lights on and turn on my blinker the left one does not blink and the right one blinks slow. If I cut the lights off the blinkers work fine. I have seen several threads with people having the same problem when they hook up to trailers but there was no answer. I have the factory 4 plug harness I was wondering should I check for a ground on the harness or is it a relay?

 

If you hook a trailer up to your 4 pin harness do the lights work like they should? Sounds like cheap LED's to me.

 

I had a trailer plugged in a couple of years ago and had some issues but have not tried recently.

 

I would try plugging it into a trailer and see if it works like it should. That or test the connection with a test light or meter and see if you're getting the voltages there that you should be getting. If it is the stock wiring the connections shouldn't be the issue, although if something got loose somewhere and you got some water in the connections, it doesn't take long for moisture and DC voltage to make lots of green corrosion out of copper.

 

racenitro,
Did you get your speedo fixed? I am having the same issue, started Friday on the way home.
Did a search and found the following post "PSOM Replacement" by duggyb. He was having similar issues and changed the PSOM to fix it.
Now I am wondering if that is my problem. Anybody have input on this????