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brian99's avatar
brian99
Explorer
Feb 01, 2015

Issue with trailer lights

I'm stumped and hoping that someone can offer me a suggestion for my issue with my trailer lights.

Tow vehicle: 2006 Mazda MPV

Trailer: 2009 Jayco Jay popup

Wiring Harness: CURT Manufacturing 56010 T-Connector (http://www.amazon.com/gp/product/B001GMX9DG/ref=oh_aui_detailpage_o01_s01?ie=UTF8&psc=1)

"Reese Towpower 74184 Plastic 4-Way Flat to 7-Way Round Vehicle End Adapter" (http://www.amazon.com/gp/product/B000CPI6S6/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1)

Wiring harness was installed with no issues - and connected the wiring harness white wire to a good ground on the tow vehicle. I just unplugged the taillight assemblies on left and right side and plugged in the wiring harness connectors in line so there isn't a whole lot that could go wrong here.

The Reese 4 flat to 7 round adapter also has a white ground connector, so connected that to a good ground no the tow vehicle. The other 3 extra wires on the adapter are not connected (these are for brakes, backup light, and 12v power, which are optional)

When everything is connected the tail/running lights work. But the blinkers and brake lights do not work.

HOWEVER, the blinkers DO work in this scenario: If the ground connector on the Reese 4 flat to 7 round is disconnected, and the blinkers are turned on, they will blink very dimly on the trailer. While the blinkers are blinking dimly if you connect the Reese adapter ground wire to a ground on the car the blinkers on the trailer start blinking normally. If you then switch the other blinker on it won't work UNLESS you first disconnect the Reese adapter ground, turn the blinker off, turn the other blinker on (blinks dimely), THEN connect the Reese adapter ground and it will blink normally.

So basically the only way the blinkers will work is if the Reese adapter ground is disconnected, the blinkers are turned on, they then blink very dimly, and if you then connect the Reese adapter ground to the car the blinkers start working normally. But if the blinkers are turned off and back on again they won't work again until the ground is first disconnected and then the blinker is turned on, then the ground connected.

I suspected a ground issue on the trailer and I used an ohm meter and verified that on the trailer side the ground connector on the trailers plug is connected to the chassis of the trailer and has a good connection. And I also did the same on the vehicle side to verify that the ground pin on the 7 way adapter is connected to the vechile ground. I have also used a multimeter to verify that the pins on the car side have voltage across the negative and left/right blinkers.

The blinkers on the car are always working and blinking normally throughout all of this.

Any ideas? Thanks in advance for any suggestions!

20 Replies

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  • Artum_Snowbird's avatar
    Artum_Snowbird
    Explorer
    Feb 01, 2015
    I think I would get a bulb with two wires on it, and put one end into the trucks ground feed, then go around the trucks various wires to see if they have power on them when they are supposed to.

    If that works, then take short pieces of wire and instead of plugging in the two connectors, run a ground wire from the truck plug to the same plug on the trailer, and then try each light filament again.

    With your battery test you proved that the ground is good and the wiring is good on the trailer, but you did not prove that the ground connection between the truck and trailer is good.
  • brian99's avatar
    brian99
    Explorer
    Feb 01, 2015
    Ok, connected the 4 way to 7 way adapter to the trailer side, and lights work when I manually put 12 volt in to each of the 3 functions on the 4 way plug.
  • enblethen's avatar
    enblethen
    Nomad
    Feb 01, 2015
    Now connect the adapter to the trailer and test through the adapter.
    Test using the four wire ground, then test with the large loose ground wire.
  • brian99's avatar
    brian99
    Explorer
    Feb 01, 2015
    Good idea - I took the battery on the trailer and only hooked up the ground wire for it so the trailer chassis was grounded. Then ran a temporary wire from the possitive terminal on the battery and stuck the other end of that wire in to each of the "Left brake/blinker", "Right brake/blinker", and "Tail lights" on the trailers 7 way plug and they each worked perfectly and were bright.

    So I think that 100% clears the trailer as OK, and shows the the trailer grounding to the lights is good as well. I also used the ohm meter to test all 7 wires through the Reese 4 flat to 7 round adapter and they all mapped through as they should, so I think the adapter is OK.

    Is there anything else on the vehicle side I should try or check before replacing the CURT wiring harness?
  • enblethen's avatar
    enblethen
    Nomad
    Feb 01, 2015
    No, I would not condemn the harness yet!
    Check the harness to the trailer. From the connector one the trailer go through each function by applying 12 volts to each of the functions.
    This will show that the wiring in trailer is good.
    Dumb question! Have you thought about cutting the four pin flat off and installing a seven pin blade female car end connector?
  • brian99's avatar
    brian99
    Explorer
    Feb 01, 2015
    Thanks for all the suggestions! I bought a test lamp and tested at the flat 4 wire and all lights worked properly on the test lamp.

    I did some more testing with the ohm meter, and found that with all the lights turned off, on the flat 4 connector coming out of the CURT wiring harness that both the white and the brown(tail lights) wires have zero resistence with the car ground. Would this mean that the CURT wiring harness is probably bad? I still don't understand why it is working with the test lamp, and why the blinkers work on the trailer if I hook up the Reese adapter ground after the blinker is on.
  • Artum_Snowbird's avatar
    Artum_Snowbird
    Explorer
    Feb 01, 2015
    Here is what you should do. Take the covers off your trailer lights and look at the bulb. Inside the bulb there will be a thick filament, and a thin filament.

    When you are trying to brake or turn, the power comes to the thick filament, and hopefully glows brightly and goes through it and returns on the ground wire from that bulb.

    When the bulb glows dimly, and you take the cover off you will likely find that the thick filament is glowing very faintly, and now the thin filament is glowing dimly. The reason for this is because the ground from that bulb is not connected to the trailer and back to battery ground. The power comes in to the thick bulb filament, can't get out on the ground, so it tries to exit from the other filament, the thin one. When this happens, the power usually goes to another thin filament on the other side, and it glows dimly too.

    The three filaments are now in series, and in a series circuit the most resistive and the thinnest bulb filament drops the most voltage, and will therefore glow brighter than the thick one.

    To prove this is indeed the case if you can see both filaments glowing like I said, connect a good ground from a connected jumper cable to the case of the bulb and see if the bulb starts working properly.

    Then fix the ground connection.
  • enblethen's avatar
    enblethen
    Nomad
    Feb 01, 2015
    Suggest you test the output at the four wire with a test lamp.
    Photo on Curt site indicates that you do not have amber turn signals, is this correct? If it is, then you do not have a 3-2 adapter. These are not Asian, this was a big thing a while back to switch to amber turn signals, but it faded away.
  • Matt_Colie's avatar
    Matt_Colie
    Explorer II
    Feb 01, 2015
    Brian,

    I suspect that the reason you are stumped is that the device is bad.

    The Asian Adapter that Don mentioned is supposed to be in the Curt device - according to their specifications.
    Why? The simple answer is that some vehicles have what are call 3 filament taillights (your Madza) and others (your trailer) have two filament taillights. If a vehicle has only one bulb that is a dual filament and it does stop/turn/tail, that is a two. If it has two bulbs (usually in an amber turn signal, that is a three.

    Inside that little box is a collection of electronics to make everybody work together. It seems that they are not. If you point your browser at www.curtmfg.com and look for support on Monday, you should be able to get someone to tell you it is defective and where to ship it.

    Good Luck

    Matt
  • donn0128's avatar
    donn0128
    Explorer II
    Feb 01, 2015
    Did you install the Asian to American adapter? Without them most Asian vehicles wont correctly power trailer lights.

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