Impossible LED problem!

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Apr 4, 2011
I have a 2012 Subaru Forester toad.  Just hooked up BLUE OX LED towing lights in the existing tail light housing.  The wiring runs thru the car interior and is connected to a 4 prong socket at the front of the car.  I made up my own 4 wire connector harness to go from the motor home to the car.  To tested the lights, I hooked up a 12 volt battery to the motor home end of the harness and the towing lights worked (parking and turn/brake).  When I hook up the  motor home to the car, the turn/brake lights work but not the parking lights.  The parking lights will not work when hooked up to the motor home, just a separate 12 volt battery.  Yes, I have power (parking lights/brown wire) at the car end of the wire harness when hooked up to the motor home!  When connecting the harness to the car or motor home (with the motor home parking lights on) the car parking lights will momentary flash on, but not stay on.  I also checked continuity of the harness with a ohm meter.  Every thing should work.  I even stuck pins in the wires to check the voltage.  This is impossible, it will work off a separate 12 volt battery but, not the motor home.  Has anybody else had an issue like this?   
Did you consider that LED lights have a pos and neg side? jm2cw. At the end towing light end of the circuit reverse the wires and see if this solves your issue.
Yes, I am aware of LED's polarity issues.  Each LED socket has a white ground wire, a brown wire w/ resistor to reduce voltage (parking light) and a black wire (turn/stop light).  The lights work just fine when I connect my lawnmower battery to the motor home end of the harness.  I will double check the LED wiring at the taillights tomorrow.
What you need is 3 more LED's (or a test light)

I have a small device that has two Flat 4 connectors (One is the trailer end, one the RV end) 3 LEDs are mounted in one of the plugs.. Tail, Right and Left.

When the tail lights on my towed failed to light up back on 2006 I put this between the Motor home and the towed.. Turns out the TAIL light on the tester did not light up either.. So I noticed the brake system I had took care of the car's battery and turned on the parking lights,  Got it fixed some 2,000 miles later.

(Getting it fixed at 3am on Jan 2 was NOT going to happen don't you know)

I still have the tester,, Flat-4 testers are easy to find, but the in-line ones are harder..  I might have a spare non-in-l;ine one lying about.

Or you can use a test light.. The turn signa (As you knwo) is right next to the Ground, which is the "Odd" pin on the plug/socket.
Just a guess that the car is parked behind the MH for testing. Try running a ground from the car frame to the MH frame as if the tow bar was hooked up. Because the lights will blink on it sounds like you are loosing the ground maybe?
It does sound like you're missing a ground.  Maybe the shell on the motorhome connector isn't grounded - relying on the tow bar for the ground is bad practice and will likely lead to flickering lights as the tow bar moves around.

The reason the LEDs are responding differently than incandescent bulbs is because LEDs are polarity sensitive and unlike a regular bulb won't light up if the polarity is reversed.  When you remove the ground the current returns through the opposite hot line feeding that bulb - i.e. if you energize the stop/turn signal line the current returns on the tail light hot line, and vice-versa.  This messes up the polarity through at least part of the circuit.
I had a similar problem while wiring my new MH to my old toad. My 4-wire plug is designed so that the housing setscrew can be inserted into two different threaded locations on the internal wiring plug and I hadn't noticed that. After a couple of hours of doing exactly what you are going through I realized that I was inserting the internal plug upside down (180 degrees out of sync) into the outer housing.  With the plug upside down, I had the same combo of lights that you appear to have -- brake lights, turn signals but no running lights. Actually, I believe it puts the running lights in series so you may get a current reading without noticing that, during the day, the running lights were very dimly lighted.
This is frustrating.  I called Blue Ox and they recommended turning the bulbs 180 degrees around.  That did not work.  The turn/stop work just fine.  The parking light voltage is only reading 0.08 volts.  When I remove the connector harness from the motor home and hook up wires from my spare lawn mower battery to the MH end of the harness, the parking lights work!  When the harness is connected to the MH I stuck a pin into the brown and white wires (parking lights) and the voltage reading is 11 volts.  I have also connected a seperate ground wire from the MH to the car.  NO difference.  The LED lamps only have 2 contacts.
The parking light voltage is only reading 0.08 volts
The LED lamps only have 2 contacts.

And... Your description suggests (to me) that the polarity is reversed somewhere. Personally, I often have a problem getting used to the digital voltmeters as they always give the reading in volts but you have to pay good attention to the '-' sign. I like the older mechanical meters better that made the polarity obvious by trying to read backwards!! But that's me!!
FWI the LED lamps have three contacts. The two on the base and the metal shell (gnd) of the lamp.
See picture / schematic. Suggest measuring with an ohmmeter from lamp socket to Motorhome connector. It really acts like you are missing a ground path.
Did you run a discrete ground from front connector to each lamp? One line from front to back with jumpers between lamp sockets? Or did you ground sockets to chassis locally at sockets and front connector. You may need to use ohmmeter to verify the ground connection from the Motorhome connector to chassis gnd also.


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The LED bulb base is just like a "peanut" bulb with only 2 wire contacts.  The body is plastic.  The MH has a 7 connector plug in the rear and I am using a 7 to flat 4 adapter in the MH socket.  The connector wire harness to the car is a standard 4 wire trailer lead.  The ground wire is used.  I ran a continuity test from the front harness socket of the car to the LED socket contacts.  The ground terminal has 0.8 ohms and the parking light terminal has infinity (no contact).  I stuck a pin into the brown parking light wire before the LED socket and resistor and got 0.8 ohms.  The brown bulb lead has a resistor to reduce voltage (brightness).  I don't understand how you can only have 2 contacts with a ground/park/brake light.  Its like the park and brake elements are connected together.
I don't think it is a ground problem on the toad because you have tested the toad and cable wiring from the MH end of the cable using your motorcycle battery.

Try this: Plug the cable you made into the MH only and turn on the MH headlights and running lights. Find the running light voltage at the other end of your new cable (toad) noting the polarity and pin location. Using your motorcycle battery, apply its voltage and correct polarity to the correct pins on the toad connector. Your toad running lights probably won't light. The fix would then be to check the wiring of your new cable plug to the MH plug.
I connected the lawn mower battery to the proper pins in the toad socket and the parking lights work.  The wiring harness polarity and pin voltage is good at the toad end when connected to the MH.  The parking lights will not work when connected to the MH.  Impossible!! 
I just took the guts (pin socket) out of the toad socket and connected the harness to the MH.  Got 11 volts, proper polarity and pins.  All connections are souldered and heat shrinked.  NO lights.  Still waiting for Blue Ox tech support.
I know how you feel -- I went through this.

You may have a partial short in the MH plug which won't show up until the circuit is loaded, meaning current is actually being fed to the bulbs. A digital meter won't "load" the circuit. I would open the MH plug and inspect it for strands of wire bridging connectors especially the hot (-12V) to the running light wire. If it looks good I would use the ohm meter (lights off) between each set of contacts on the MH plug.
Thanks for all the suggestions.  The problem is in the LED socket.  The socket has 3 leads, ground, parking and brake/turn.  The LED lamp has only 2 contacts (positive and negative).  The parking light lead has a small resistor to reduce the voltage and produce a dim light.  The brake/turn signal lead goes straight thru producing the brighter brake light.  With only 2 bulb contacts, the brake and parking light leads are connected in the LED socket.  You now have your parking lights connected to your brake lights.  When you turn on the MH parking lights the voltage goes thru the resistor and gets connected to the brake light lead and travels back to the MH brake lights leaving you with 0.1 volts at the LED lamp.  This is why everything worked when I tested the system using my old mower battery, the brake light leads were not connected to anything.  I assume you have to use diodes to prevent feedback to the MH.  Be very careful when installing LED towing lights with only 2 contacts.
I am looking to purchase a car dolly and asked a similar brake light question to "American Car Dolly" and here was his reply:

All trailers (American Car Dolly), have a single bulb with two filaments which are your tailight and then the blinker/brake lights. What you need if you have separate blinkers and brake lights you need a tail light converter which can be bought at Autozone, Orileys, Pepboys etc., The best kind to get is the one with 6 wires going to the RV and reduces to 4. The sixth wire is a wire that goes to 12 volts and brings the voltage back to the true voltage it should be instead of cutting it down like the cheap converters that go from 5 wires on the RV to 4 to the trailer.

Have you considered a tail light converter??
Just a thought and don't know if this even relates to your problem???

Good Luck

Very interesting.  I'd have thought that the path of least resistance would have been through the LED, not back through the MH brake lights.  I wired my existing incandescent towed taillights using lots of diodes.  Sounds like you've learned that add-on (2 contact) LED's also need diodes.  Thanks for posting the explanation and solution, I've learned something new today as well.
Without a diode in line with the brake/turn signal line, the motorhome brake lights are connected in parallel with the LEDs.  The dropping resistor that makes the LED tail lights glow less brightly when the tail lights are on by themselves restricts the current going to the LED to make it burn more dimly.  It won't pass anywhere enough current to run the motorhome brake lights in addition to the LEDs.

This is a design flaw in those particular LED.  The manufacturer should have anticipated this condition and included the blocking diode on the brake/turn signal line.  Or you can do it yourself - just get a couple of diodes at Radio Shack (one for each side) and put it inside the taillight enclosure between the brake line and the LED light.
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