Help: Wiring a harness for a trailer

[Evoforce]

@Lucianbeebe did you get the trailer wiring adapter where you can use incandescent or LED? I found that you have to have your headlights on for the kit to operate properly. If you don't have your lights on, you get odd operation of the trailer lights.

 

[Lucianbeebe]

@Lucianbeebe did you get the trailer wiring adapter where you can use incandescent or LED? I found that you have to have your headlights on for the kit to operate properly. If you don't have your lights on, you get odd operation of the trailer lights.

No I just tapped into the rear light wires and ran them straight into the wiring harness for the trailer. The trailer is LED so I was hoping to not have to do much more.

 

[Evoforce]

No I just tapped into the rear light wires and ran them straight into the wiring harness for the trailer. The trailer is LED so I was hoping to not have to do much more.

I don't believe anyone has found that to work properly but I may have missed it. I tapped into the hot for the wiring kit off the lift gate.

 

[FMinMI]

MODEL S TRAILER WIRING PROBLEM SOLVED



After further consideration I surmised that the trailer tail light circuit needed to combine the signals from both the left and the right tail light signals (see chart in my previous post) in a logical "AND" circuit (where the left and the right tail light circuits both needed to be powered in order for the trailer tail lights to be powered).

I implemented this using a simple low power 12 VDC relay where the left tail light circuit powered the coil of the relay and the switched the signal from the right tail light circuit. This simple circuit eliminates the problems with the use of trailer lights without needing to turn on the running lights or parking lights. The coil on the relay that I used has a resistance of 1200 ohms and therefore draws only around 10 milliamps when the tail light circuit is on and the circuit isolator draws only a few milliamps to switch on the tail lights (which are powered by the isolator directly from the car's 12 volt system).

The same fix may be used either with a circuit isolator (such as the Tekonsha Product Code: 119191-22, which is rated for up to 4.2 amps for turns/stops circuits and 7.5 amps for the tail light circuit and may be used with incandescent trailer bulbs) or for direct connection to a trailer with low power LED bulbs. (I am using the Tekonsha device to protect the Model S wiring from any problems or potential short circuits in the trailer wiring and to enable the use of some small incandescent lights on my hitch mount bike carrier.)

I know this post is old but was hoping you (or someone else who did this wiring) could help, as I have a very odd problem that sounds similar to what you have seen.

Although my trailer is all LED, I decided to buy an isolation trailer adapter from Hopkins just in case I forgot and tried to connect a trailer with incandescent bulbs to it. On the Tesla, I found the right turn, the left turn and the parking light wires and connected them to the respective "ins" of the adapter (right, left, parking). For power, I tapped into the power lift +12V to run the adapter (plus I found/confirmed a good ground as well).

When I put on the right turn signal, it works fine. When I press the brakes, both lights come on and work fine. When I turn on the parking lights, both trailer lights come on at the expected lower intensity as do the side lights on the trailer. And with the lights on (parking or headlights), all the lights (left, right, parking, brake) work fine too.

But here is the really weird part - with the lights off, if I turn on the left turn signal, it blinks as expected BUT the right parking light (lower intensity) also blinks. If I turn on the parking or headlights, it then works fine. I tried my harness with 2 different trailers that have LEDs - in both cases it does the exact same thing. I am at a loss as to why, with this isolation kit, it wants to flash the right parking light as well as the left turn signal light (which of course is also the brake light on that side). I did 'tap' into the parking light wire on the right side of the Tesla if that makes any difference.

Any idea as to what is wrong?

Thanks.

 

[Evoforce]

I know this post is old but was hoping you (or someone else who did this wiring) could help, as I have a very odd problem that sounds similar to what you have seen.

Although my trailer is all LED, I decided to buy an isolation trailer adapter from Hopkins just in case I forgot and tried to connect a trailer with incandescent bulbs to it. On the Tesla, I found the right turn, the left turn and the parking light wires and connected them to the respective "ins" of the adapter (right, left, parking). For power, I tapped into the power lift +12V to run the adapter (plus I found/confirmed a good ground as well).

When I put on the right turn signal, it works fine. When I press the brakes, both lights come on and work fine. When I turn on the parking lights, both trailer lights come on at the expected lower intensity as do the side lights on the trailer. And with the lights on (parking or headlights), all the lights (left, right, parking, brake) work fine too.

But here is the really weird part - with the lights off, if I turn on the left turn signal, it blinks as expected BUT the right parking light (lower intensity) also blinks. If I turn on the parking or headlights, it then works fine. I tried my harness with 2 different trailers that have LEDs - in both cases it does the exact same thing. I am at a loss as to why, with this isolation kit, it wants to flash the right parking light as well as the left turn signal light (which of course is also the brake light on that side). I did 'tap' into the parking light wire on the right side of the Tesla if that makes any difference.

Any idea as to what is wrong?

Thanks.

You must have the running lights on.

 

[FMinMI]

You must have the running lights on.

But why? And why doesn't the left parking light flash when I put on the right turn signal?

Thanks.

 

[RichardC]

Frank, you may want to review the posts on the previous page, which provide further background and details. The source of the problem is the fact that the Model S illuminates the tail light as a turn signal, as well as the brighter brake / turn signal light, which means that the tail lights on both sides blink when the turn signal is used when the running lights are off.

As detailed above, I used a low power 12 vdc relay to create a logical AND circuit with the tail light from one side switching the relay while the tail light circuit from the other side provides the power which is routed through to the tail light input on your isolator. In this way the tail light is not illuminated when only one of the Tesla tail lights is lit (i.e., when it is being used as a turn signal), but is lit when both of the tail lights are powered (i.e., when they are being used as tail lights, and not as turn signals).

You will need to run a wire from the second tail light on the other side of the to run to the relay (I mounted the isolator and relay on the passenger side and powered them from the panoramic sunroof circuit, through a separate fuse, and ran a wire with the tilight signal from the driver side through the rear frame to the relay).

It has been working perfectly now for a couple of years.

I hope that helps.

Richard

 

[FMinMI]

Frank, you may want to review the posts on the previous page, which provide further background and details. The source of the problem is the fact that the Model S illuminates the tail light as a turn signal, as well as the brighter brake / turn signal light, which means that the tail lights on both sides blink when the turn signal is used when the running lights are off.

As detailed above, I used a low power 12 vdc relay to create a logical AND circuit with the tail light from one side switching the relay while the tail light circuit from the other side provides the power which is routed through to the tail light input on your isolator. In this way the tail light is not illuminated when only one of the Tesla tail lights is lit (i.e., when it is being used as a turn signal), but is lit when both of the tail lights are powered (i.e., when they are being used as tail lights, and not as turn signals).

It has been working perfectly now for a couple of years.

I hope that helps.

Richard

Hi Richard,

I did read all the posts as well as yours, but I didn't understand what you were doing. Any chance you have a schematic (or can draw one) of what you implemented? That would help me understand what I need to do (instead of always making sure parking lights/headlights are on). No rush...

Thanks.

 

[RichardC]

Hi Frank,

I don't have a schematic but have quickly prepared a rough sketch as follows:

 

[Evoforce]

Otherwise, you must just turn on your headlights after using the trailer wiring kit. You may not have a sunroof either as described above. i used the hot off the liftgate for the trailer wiring kit.

 

[FMinMI]

Otherwise, you must just turn on your headlights after using the trailer wiring kit. You may not have a sunroof either as described above. i used the hot off the liftgate for the trailer wiring kit.

Hi Frank,

I don't have a schematic but have quickly prepared a rough sketch as follows:
View attachment 482969

Hi Frank,

I don't have a schematic but have quickly prepared a rough sketch as follows:
View attachment 482969

Thanks Richard. So then you are saying that if I were to 'remove' the parking light lead from the input of the isolation adapter, then both turn signals should work properly, correct? (just as a test)

 

[FMinMI]

Otherwise, you must just turn on your headlights after using the trailer wiring kit. You may not have a sunroof either as described above. i used the hot off the liftgate for the trailer wiring kit.

I do have a sunroof...

 

[RichardC]

Thanks Richard. So then you are saying that if I were to 'remove' the parking light lead from the input of the isolation adapter, then both turn signals should work properly, correct? (just as a test)

Frank,

Yes, that will prove that the tail light connection is the source of the problem with the turn signals (and that the above circuit will fix the problem).

 

[RichardC]

Otherwise, you must just turn on your headlights after using the trailer wiring kit. You may not have a sunroof either as described above. i used the hot off the liftgate for the trailer wiring kit.

Evoforce,

You are correct that turning on the lights all the time would prevent the problem from being visible, but was masking the symptoms, rather than fixing the problem did not strike me as a valid solution (especially because the Model S likes to manage your headlights lights for you).

 

[JustADude]

I'm trying to wire up my April 2015.
The driver side tail light has red, pink, black, white, and purple with a stripe I believe.

Multiple posts say to connect the wires to the two purple wires but that's incorrect for me.

Could anyone help me out?

 

[FMinMI]

I'm trying to wire up my April 2015.
The driver side tail light has red, pink, black, white, and purple with a stripe I believe.

Multiple posts say to connect the wires to the two purple wires but that's incorrect for me.

Could anyone help me out?

I would not trust the wire colors others mention. I found 3 different posts and none were correct. Just use a VOB or 12V tester and find them that way. And look 4 posts above - even when you find the correct wires, you will still have an issue unless you manually turn on the parking lights unless you do the circuit shown. And do buy a trailer wiring isolation 'brick' - don't try and go directly from the current 26 (?) gauge wires to the trailer lights, even if LED. You are asking for trouble. You can get the isolation unit for about $30 on eBay or Amazon.

 

[JustADude]

I would not trust the wire colors others mention. I found 3 different posts and none were correct. Just use a VOB or 12V tester and find them that way. And look 4 posts above - even when you find the correct wires, you will still have an issue unless you manually turn on the parking lights unless you do the circuit shown. And do buy a trailer wiring isolation 'brick' - don't try and go directly from the current 26 (?) gauge wires to the trailer lights, even if LED. You are asking for trouble. You can get the isolation unit for about $30 on eBay or Amazon.

I think I figured it out. I got a harness that can do single bulb taillights and it also requires it's own 12v

 

[FMinMI]

I think I figured it out. I got a harness that can do single bulb taillights and it also requires it's own 12v

That sounds interesting but I don't understand how it works. Can you post the link to it? Curious minds want to know... Thanks.

 

[Aussiesb]

There is a circuit board in the back of the lights. So they don't work like "normal" lights as we know it. I have a Tekonsha ZCI control module and it simply does not work.

 

[LuvsToSail]

In case anyone is still wrestling with this, I have just wired up for trailer lights on a 2020 Model 3 using the relay system and avoiding adapters that may or may not work. After reading YouTube posts I use 3 relays powered by the left, right, brake and tail light wires of the M3 (tapped using suitcase connectors). The trailer lights are powered by a small car-starter battery that I have had lying around since I had an ICE car, so the trailer is electrically completely isolated from the car. And no need to run a power wire back to the trunk. Each of the wires from the car's rear lights goes through a diode to keep them separated from each other in the case where two signals have to merge to power the common brake and turn lights. The trailer lights use LEDs, so the drain on the portable battery is not an issue, especially as one uses a trailer only a tiny fraction of the time.
All parts available from Canadian Tire and Radio Shack (as it used to be called in Canada). $50 for the relays and sockets, and virtually no limit to the current draw available to the trailer lights. The relays draw 100mA each, but you can get far lower-power relays if you want to go looking.