D4 Electric Trailer Brake Wiring

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Originally Posted by Romer

hi sniegy
thanks for this post it has answered a lot of my questions. I have removed the LHS tail light and tape from the harness for a few centimetres but there is no split in the cable. I have removed the inside panel and located the harness from the trailer plug. If I cut the blue cable and connect the controller output to the blue cable will this work. Then splice into the red for the input for the controller.
any advice appreciated
Romer.

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He hasn’t logged on since 2020.

The splice is there, either outside or inside the vehicle body.

Romer have a look at the pictures in post 684. On the later D4s the splice moved from originally outside, below the tail light to inside the rear LHS compartment where the tow hitch receiver is stored. It is usually wrapped in a white sticker that you need to remove.
Also there is a great summary PDF of this thread in post 450

Thanks to all the contributors to this thread for the wealth of great information.

When my D4 in connected to my camper trailer and the tail/side lights are on then the trailer brakes are activated. The trailer lights are LEDs. The tail/side lights and brake lights share the same physical LEDs. Note that the trailer brakes work correctly when the tail/side lights are off. My brake controller is a Redarc Tow Pro Classic.

I ran an experiment with trailer connected, tail/side lights on and the brake lights/pin 6/red wire disconnected in the trailer plug. Adjusting the brake controllermeasured the voltage on knob produced no voltage on the trailer brakes wire. When brake pedal was pressed the brake controller worked correctly. So my conclusion was that the problem was caused by the LED trailer lights.

To investigate further, I disconnected the trailer plug from my D4 and applied 12V to the tail lights/pin 7/brown wire and measured the voltage on brake lights/pin 6/red wire. It was 8.8V. I also applied 12V to brake lights/pin 6/red wire and measured the voltage on tail lights/pin 7/brown wire. It was 7.2V. So my conclusion is that when trailer is connected and the tail/side lights are on then the brake lights/pin 6/red wire activates the brake controller.

Do I need a diode in the brake lights/pin 6/red wire to fix the problem?

Thanks in advance for any thoughts or ideas.

Regards, John

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Originally Posted by Enduro

Thanks to all the contributors to this thread for the wealth of great information.

When my D4 in connected to my camper trailer and the tail/side lights are on then the trailer brakes are activated. The trailer lights are LEDs. The tail/side lights and brake lights share the same physical LEDs. Note that the trailer brakes work correctly when the tail/side lights are off. My brake controller is a Redarc Tow Pro Classic.

I ran an experiment with trailer connected, tail/side lights on and the brake lights/pin 6/red wire disconnected in the trailer plug. Adjusting the brake controllermeasured the voltage on knob produced no voltage on the trailer brakes wire. When brake pedal was pressed the brake controller worked correctly. So my conclusion was that the problem was caused by the LED trailer lights.

To investigate further, I disconnected the trailer plug from my D4 and applied 12V to the tail lights/pin 7/brown wire and measured the voltage on brake lights/pin 6/red wire. It was 8.8V. I also applied 12V to brake lights/pin 6/red wire and measured the voltage on tail lights/pin 7/brown wire. It was 7.2V. So my conclusion is that when trailer is connected and the tail/side lights are on then the brake lights/pin 6/red wire activates the brake controller.

Do I need a diode in the brake lights/pin 6/red wire to fix the problem?

Thanks in advance for any thoughts or ideas.

Regards, John

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Diode protects the CJB.

You need to split out the cable joiner.

The problem is caused by improper wiring, not the lights.

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Originally Posted by Tombie

Diode protects the CJB.

You need to split out the cable joiner.

The problem is caused by improper wiring, not the lights.

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Thanks Tombie. I did more investigation of the trailer wiring. I tested the trailer lights is isolation that is with the lights are not connected to the trailer wiring. One of the LED trailer lights is faulty and generates 8.8V on the brake lights wire when the tail lights are on. Problem solved!

Hi all. I have just fitted a tow pro to my D4 after what I thought to be thorough research of this thread locating splice at the rear tow hitch storage area. Cut brown wire joining to blue ect.
I have no led colours on the button so hooked up a globe to test the controls at the trailer plug and got a bright light with brakes on but no variable brightness using the dial. I then cut the brown/blue wire feeding number 5 pin and I expected no power at the trailer plug but it’s still live So my thoughts are my tow pro is dodgy at the button but confused about the brown wire feeding number 5 pin. Any thoughts on this would be fantastic I’d like to avoid pulling the rear bar off if possible.
cheers

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Originally Posted by Bigjeff

Hi all. I have just fitted a tow pro to my D4 after what I thought to be thorough research of this thread locating splice at the rear tow hitch storage area. Cut brown wire joining to blue ect.
I have no led colours on the button so hooked up a globe to test the controls at the trailer plug and got a bright light with brakes on but no variable brightness using the dial. I then cut the brown/blue wire feeding number 5 pin and I expected no power at the trailer plug but it’s still live So my thoughts are my tow pro is dodgy at the button but confused about the brown wire feeding number 5 pin. Any thoughts on this would be fantastic I’d like to avoid pulling the rear bar off if possible.
cheers

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Do you have a photo of when you joined the brown? I had to locate where it splits which was further inside the loom factory tape.

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Originally Posted by imaz

Do you have a photo of when you joined the brown? I had to locate where it splits which was further inside the loom factory tape.

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Attachment 178101Hi imaz. I just dropped the spare and followed brown to the 12n plug. Looked original tested again and it started working at times still with no leds on the dial Further persistence and it now seems to do as it’s meant to just without the dial lighting up now have what sounds like a relay clicking at the park brake Buggered if I know

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Originally Posted by Bigjeff

Attachment 178101Hi imaz. I just dropped the spare and followed brown to the 12n plug. Looked original tested again and it started working at times still with no leds on the dial Further persistence and it now seems to do as it’s meant to just without the dial lighting up now have what sounds like a relay clicking at the park brake Buggered if I know

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The dial will light up once connected to a trailer with electric brake, and the first time will take some time to calibrate. You can not do any simulation with a test lamp and expect variable glow. Press the dial, it should light up blue since it’s not connected to anything.

Since you are going to tow, you may know but I’ll remind you - add some resisters or make up a resister plug for the indicator circuits. Assuming the trailer in tow uses LED tail lamps, else tows like crap.

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Originally Posted by sniegy

Morning all,
Thought i would post up some photo's i have for the new D4 Electric trailer brakes.
The wiring has changed in the Discovery 4 which is a good thing as it is easier to wire.
Now lets begin & hopefully it will be clear for all to follow.

Trailer Plug info(7 Pin Plug):- Australian Regs:-
Pin1 Yellow Left Indicator
Pin2 Black Reverse/Auxilary
Pin3 White Earth
Pin4 Green Right Indicator
Pin5 Blue Electric trailer brakes/Auxilary
Pin6 Red Brakes
Pin7 Brown Tail lights


As per usual 4 wires:-
Black +ve direct to battery via Circuit Breaker.
White -ve direct to battery/battery post.
Red Connection to brake light wire in trailer loom.
Blue Connection to Pin No. 5 on Black trailer plug socket.
You will need to run Black & White wire to the front of the vehicle & the Red & Blue wire to the rear of the vehicle under the L/H/S tail light.
All 4 wires will run from the position you decide to place your Electric trailer brake unit. (normally at the L/H top edge of the panel under the steering wheel).
When removing this panel just grab the top section & pull in an arc downward, if you brake, lose the clips the part number is FYC500040.

In this photo below i have had to find the splice joint in the wiring loom for the park light circuit (all European vehicles run a left & a right park light circuit in there vehicle wiring loom)
It is basically one wire feeding 2 Pin outs. (Pin 7 is our park light circuit & Pin 5 is our supply for our Electric trailer brake varying voltge).
Remove the L/H/R tail light assembly by undoing the 2 phillips head screws & then prising the light gentry from the body with a flat blade & cloth inserted between them so as not to scratch the paint work.
Cut away the black tape on the loom that goes down to the bottom connector, you will come across the splice joint (brown cable with glue coming out on connection-you will notice it starts as a black wire only & after this splice joint it will have a brown & black coming out). The black wire goes to Pin no. 7 & is your park light wire, the brown wire is the other park light wire that we do not need & connect the blue wire to for your trailer brakes. Cut wire as close to splice as possible, tape splice joint up & connect your blue wire to the brown wire that now goes to the plug in the wiring loom.

https://www.aulro.com/afvb/images/im.../2016/11/3.jpg

What we have done is basically supply the cut brown wire the varying voltage for your trailer brakes via the blue wire we have run.
The red wire just connects to the red wire in the same loom that goes to the bottom plug, this is the brake light wire pick up.
In this photo you can just make out the 2 connections & the cut of the brown wire.
https://www.aulro.com/afvb/images/im.../2016/11/4.jpg

https://www.aulro.com/afvb/images/im.../2016/11/5.jpg

The photo above just shows the exit point on the firewall (rubber grommet) that i passed the cable through. This is on the drivers side brake booster compartment.
Below shows how i cable tied up nice & neatly the Black & White wire to connect directly to the battery via a Circuit breaker & Earth post next to the Main battery box.
https://www.aulro.com/afvb/images/im.../2016/11/6.jpg

On the new vehicles there is no 2nd fuse box in the rear nor the need for the Diode.

Cheers all.

p.s. sorry for the poor quality images, from my phone. Will hopefully get better pics when i can.

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so helpful, but on my d4 2015 I can not figure how to lower the panel under the steering wheel, on my 2010 it just pulls down?