Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

Help: Wiring a harness for a trailer

nrcooled

P#8946 VIN 03225
May 22, 2012
575
33
NoVA
I picked up the EcoHitch for the Model S and I need to get the harness wired in for the trailer lights. This is a light duty open motorcyle trailer that only needs rear lights and turn signals.

What I can't get a good answer to is wiring the car up for the trailer:
1. For a basic flat four wiring setup do I actually need a power source other than pulling power off of the existing lines from the tail lights?
2. If I do need a power source, I plan to fuse it, but where do I tap to get power on the Model S? Typically I would tap off of the battery of the car at the posts. I am not looking forward to digging the 12V out and tapping that since it is already a troublesome part on the MS.

My assumptions:

- Standard wiring harnesses pull power from the rear tail lamps to power the trailer lights
- Advanced wiring harnesses use a power block which is powered from a 12V power source to power the lights bypassing the use of the rear lamp power.

Any help is appreciated
Please correct me if I am wrong.
 

mknox

Well-Known Member
Aug 7, 2012
10,103
1,870
Toronto, ON
I'm not even sure there are discrete circuits for running lights and brake/turn signal lights going to the tail lights. When my car was brand new, I had wonky turn signals that would flash at a wildly varying rate. It was fixed by pushing new firmware to the tail light itself. This makes me wonder if the tail lamps just have power and some sort of communications feed.
 

nrcooled

P#8946 VIN 03225
May 22, 2012
575
33
NoVA
I'm not even sure there are discrete circuits for running lights and brake/turn signal lights going to the tail lights. When my car was brand new, I had wonky turn signals that would flash at a wildly varying rate. It was fixed by pushing new firmware to the tail light itself. This makes me wonder if the tail lamps just have power and some sort of communications feed.

Darnit! This isn't a good start...

Maybe I should start digging around the fuse box to see if there is a discrete circuit for the turn signals.
 

mknox

Well-Known Member
Aug 7, 2012
10,103
1,870
Toronto, ON
Darnit! This isn't a good start...

Maybe I should start digging around the fuse box to see if there is a discrete circuit for the turn signals.

Don't hold me to it! I haven't had the car apart or anything, but with so much of the car under software/firmware control, it wouldn't surprise me if the tail lights were little microprocessors too.
 

landover

Member
Jan 17, 2014
89
0
Pleasant Prairie, WI
I'm not sure who but I know there is a company working on a taillight setup for the model S. I know this doesn't help you now. But when I do mine in a few months I will just run a 12 gage wire to the front of the cars 12 volt battery. If you have a volt meter, try using that to make sure there is actually 12 volts or (13.8) what ever it might be.

I am interested to see how this goes. So ether direction you go please keep posting.

Thanks, Tim
 

rdrcrmatt

Member
Jun 27, 2013
608
33
Milwaukee
sounds the tail lights are can-bus driven..

A buddy of mine works for a bus company and he can reprogram the can bus devices to think they are anything else on the can bus. He said he made the turn signals think hey were stop lights.. so they lit up when the other tech hit the brakes.
 

nrcooled

P#8946 VIN 03225
May 22, 2012
575
33
NoVA
For information's sake I am cross-posting from the Tesla Forums for anyone else that wants to tackle this job. Here's the reply that I got from a member who has installed a hitch and harness.

Pungoteague_Dave said:
There is plenty of power at the tail lights to run two extra sets of regular LED trailer lights. I posted about this elsewhere, but did not outline the wiring connections. It is very easy. The tail lights are accessible through press fit panels in the car's rear inside corners. I removed the plastic panel across the rear bottom of the tailgate opening. It snaps out, no tools. I then ran a standard trailer wiring harness, available at any car parts store, to both sides. I picked up the tail light and left brake and turn signal at the left light connector and the right turn signal at the right connector. This is easily done by removing the electrical connector from the tail light and then use a small volt meter (Radio Shack) to test which wire controls which lamp. Have someone sit in the drivers seat and operate the signals, brakes, tail lights, etc., and label the wires. Then tap in with posi-taps (no wire cutting required) and test with a trailer hooked up. I hid the wires behind the panels and left the tail with the plug hanging out into the rear footwell about a foot. When using the trailer we simply hang the harness out the hatch and plug in. There's plenty of space provided by the rear hatch seal for this, and no need for a permanent exterior outlet. Worked fine on a 2,400 mile trip towing a trailer.
 

wycolo

Active Member
May 16, 2012
3,095
429
WA & WY
Flat 4 does NOT provide constant +12v from battery. Neither does flat 5, it just adds a BACKUP lamp feed. You do not need constant 12v on a dinky trailer normally.

As the TM poster says replace all bulbs in your trailer with LEDs and MS should not complain about the extra 'load'.

When you have removed the entire MS rear bumper etc then you can tap into 1. the running light line. 2. the left brake light. 3. the right brake light. 4. a good ground. 5. [optional] the backup light line.

Are you saying EcoHitch does not offer details on doing this? On the grounds that it is only to be used as a bike rack?
--
 

nrcooled

P#8946 VIN 03225
May 22, 2012
575
33
NoVA
Flat 4 does NOT provide constant +12v from battery. Neither does flat 5, it just adds a BACKUP lamp feed. You do not need constant 12v on a dinky trailer normally.

As the TM poster says replace all bulbs in your trailer with LEDs and MS should not complain about the extra 'load'.

When you have removed the entire MS rear bumper etc then you can tap into 1. the running light line. 2. the left brake light. 3. the right brake light. 4. a good ground. 5. [optional] the backup light line.

Are you saying EcoHitch does not offer details on doing this? On the grounds that it is only to be used as a bike rack?
--

You are correct about EcoHitch not providing information. They are not interested in providing any support for powering a trailer. So much so that they wouldn't even sell me a harness!

I did confirm that they are comfortable with the ability of the hitch for towing but they are not comfortable with the wiring.
 

wycolo

Active Member
May 16, 2012
3,095
429
WA & WY
Finally finished the trailer wiring project. All work is performed from *inside* the ModelS. If your hitch is already completed and the rear bumper put back in place, no problem, the wiring is all done from the inside only.

As per the quote just below by Pungoteague Dave, the simplest plan is to replace all the filament bulbs in the trailer with LEDs. That way they will not overload the MS circuitry. I used LED replacement bulbs mfg by jamstrait.com and sold thru O'Reilly/Checker Auto Parts. The cable to trailer is a 48 inch with flat 4 socket. The colors you find on your MS tail light wires may vary from mine (just under 2000 VIN). The wires I chose give full brilliance on PARK, TURN, 4way, and BRAKE, so no PCM waveforms on these! The Right Turn wire is snaked thru a convenient cross-pocket in the aluminum body. I used a 10 ga stranded wire to snake with, the big white one seen here:

Snake.cable.jpg
snake.access.jpg
cable.for.trailer.jpg
aaaMS.trailer.wiring1.jpg


Yellow = Left Turn
Brown = Running Lights
White = Trailer Frame Ground
Green = Right Turn

The trailer cable fits easily under the hatch door and stores in the trunk.
--
 
Last edited:

nrcooled

P#8946 VIN 03225
May 22, 2012
575
33
NoVA
Finally finished the trailer wiring project. All work is performed from *inside* the ModelS. If your hitch is already completed and the rear bumper put back in place, no problem, the wiring is all done from the inside only.

As per the quote just below by Pungoteague Dave, the simplest plan is to replace all the filament bulbs in the trailer with LEDs. That way they will not overload the MS circuitry. I used LED replacement bulbs mfg by jamstrait.com and sold thru O'Reilly/Checker Auto Parts. The cable to trailer is a 48 inch with flat 4 socket. The colors you find on your MS tail light wires may vary from mine (just under 2000 VIN). The wires I chose give full brilliance on PARK, TURN, 4way, and BRAKE, so no PCM waveforms on these! The Right Turn wire is snaked thru a convenient cross-pocket in the aluminum body. I used a 10 ga stranded wire to snake with, the big white one seen here:

View attachment 47569View attachment 47570View attachment 47571View attachment 47572

Yellow = Left Turn
Brown = Running Lights
White = Trailer Frame Ground
Green = Right Turn

The trailer cable fits easily under the hatch door and stores in the trunk.
--

You are a saint amongst men! I was going to tackle this job this weekend and you just prevented me from having to use the voltmeter.
 

wycolo

Active Member
May 16, 2012
3,095
429
WA & WY
Voltmeter might indicate 12vdc but if the source is PCM'ed only a bulb would show the power/brightness level accurately. See red test light in photo. But with so many wires going into each rear light I'm guessing TM is not using PCM at all. Hope your color codes are the same as mine. Other mfr's new cars use PCM over a single wire for: running/brake/4way.

Suggestion, you might want to CUT the rug to expose the rear light wires rather than have to pop out the carpeting from each trunk pocket like I did. Just cut a Tee or X to expose the harness. Once you very carefully begin the cut, slip in a big spoon to keep the utility knife from cutting any wires. Rug cuts very easily (I did it in the Frunk). Save you massive amounts of effort needed to pop out the carpeting. Seriously! If I have to go back in there I'm using the razor!
--
 

wycolo

Active Member
May 16, 2012
3,095
429
WA & WY
CORRECTIONs to MS car wiring for lights on a trailer:

[Trailer GREEN] - Right TURN - connect to: TAN/Black tracer [going into passenger rear light]

[Trailer YELLOW] - Left TURN - connect to: PURPLE [on MS left rear light]

[Trailer BROWN] - TAIL lights - connect to: PURPLE/Grey tracer [on MS left rear light]

The harness wires I originally chose seemed to work OK and I did test drives at night with good results. But I noticed that the trailer side tails remained ON after the car was turned OFF. So not proper; these were using the 'bulb out' sensor leads of the MS system (thats what those extra wires are for!).

With the corrected connections AND using DRLs during daytime and headlights at night the trailer lights now work correctly. Set to "AUTO" in Settings!!
--
 

Jayb737

Member
Jul 18, 2013
11
2
Houston
CORRECTIONs to MS car wiring for lights on a trailer:

[Trailer GREEN] - Right TURN - connect to: TAN/Black tracer [going into passenger rear light]

[Trailer YELLOW] - Left TURN - connect to: PURPLE [on MS left rear light]

[Trailer BROWN] - TAIL lights - connect to: PURPLE/Grey tracer [on MS left rear light]

The harness wires I originally chose seemed to work OK and I did test drives at night with good results. But I noticed that the trailer side tails remained ON after the car was turned OFF. So not proper; these were using the 'bulb out' sensor leads of the MS system (thats what those extra wires are for!).

With the corrected connections AND using DRLs during daytime and headlights at night the trailer lights now work correctly. Set to "AUTO" in Settings!!
--


i just did this hook up. It works pretty good. These are small wires so I think all trailer lights need to be LEDs. I replaced my trailer light bulbs with led light bulbs. Just like the regular bulbs but they are LEDs. Available at the auto parts store.
 

mudfud

Member
Jul 17, 2014
77
16
SF bay area, United States
I just got the ecohitch installed and the trailer wiring done. Wiring was very time consuming.

The wires in the new Tesla (since new sensors, late September) are a different color than those posted above.

I had the trailer guys install a isolation box so the car doesn't see any extra current draw from the trailer lights. Turned out that one of the left rear wires had both turn and brake signals on it. We tapped into the side markers to isolate right and left, brake signal came from rear LED bar above the window. 12v supply came from auxiliary port. Lights worked with a test trailer w incandescent bulbs.

The ecohitch looks solid. I ordered the "undercover" to keep dirt out of the opening in the cover panel.

If you're in the SF area, I recommend Midnight Trailer in Redwood City.
 
  • Helpful
Reactions: pl804

ybryany

New Member
May 13, 2016
1
0
Santa Monica, CA
Thanks for all of the info! I used a generic wiring harness kit on my RAV4 which had relays and requires a 12v power source (used a cigarette outlet adapter and plugged it in when needed). I'm sure you could use the same generic harness on the MS so you don't have to replace the bulbs in your trailer with LEDs. But I'd prefer to do the LED swap out anyways, the only thing is I only tow occasionally and I rent my trailers and I don't want to have to swap out their bulbs when I rent. I'm not even sure if they'd let me rent a trailer with the MS since it's technically not rated for towing. I don't see why it wouldn't be a problem though, if you show them the hitch specs and you have a working wiring harness, they usually are happy. One thing is the hitch has to have a place to put 1-2 safety chains on.

Unrelated - I'm waiting for someone to tow a generator that they use to charge their MS. That'd be even sillier if they figured out how to charge WHILE driving...
 

lousecco

Member
Nov 11, 2015
6
2
San Jacinto, Ca
Finally finished the trailer wiring project. All work is performed from *inside* the ModelS. If your hitch is already completed and the rear bumper put back in place, no problem, the wiring is all done from the inside only.

As per the quote just below by Pungoteague Dave, the simplest plan is to replace all the filament bulbs in the trailer with LEDs. That way they will not overload the MS circuitry. I used LED replacement bulbs mfg by jamstrait.com and sold thru O'Reilly/Checker Auto Parts. The cable to trailer is a 48 inch with flat 4 socket. The colors you find on your MS tail light wires may vary from mine (just under 2000 VIN). The wires I chose give full brilliance on PARK, TURN, 4way, and BRAKE, so no PCM waveforms on these! The Right Turn wire is snaked thru a convenient cross-pocket in the aluminum body. I used a 10 ga stranded wire to snake with, the big white one seen here:

View attachment 47569View attachment 47570View attachment 47571View attachment 47572

Yellow = Left Turn
Brown = Running Lights
White = Trailer Frame Ground
Green = Right Turn

The trailer cable fits easily under the hatch door and stores in the trunk.
--
I just tried this on my 2013 MS and it bench tested great. Ready to get my 5x8 mesh trailer now. This was exactly what I was waiting for and the eco hitch is also great. Thanks for all your help BIGLY!!
 

Products we're discussing on TMC...

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top