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Trailer Wiring DIY
Trailer Wiring DIY
Install a trailer-light power converter in your 318ti.
Published by CirrusSR22
06-08-2008
Default Trailer Wiring DIY

Install a trailer-light power converter on your 318ti! Why? On cars that have check control (not the 318ti), this is required to prevent light out warnings. Using a converter also reduces the electrical load on the car's wiring and prevents possible electrical damage. Will it work without a converter? Yes, but this a more "complete" and proper method.

But why tow with a 318ti? Towing with cars is common in Europe, and I feel it's more reasonable to purchase a trailer for those RARE occasions I actually need a lot of cargo volume. The 318ti is rated to tow a 1366 lbs unbraked trailer with a max tounge weight of 165 pounds. These may be limited by your specific trailer hitch though.

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PARTS NEEDED
- Trailer lighting power converter. There are various manufacturers such as U-Hual (the one I used), Draw Tite/Modulite, Hoppy, etc.
- 12 AWG Butt Splices
- 16-14 AWG Butt Splices (optional)
- BMW firewall grommet if you have an automatic transmission. BMW part #21521156082
- Wiring kit. This appears to be a separate purchase with the Modulite unit. This consists of the 12 AWG wire which runs from your battery to the power converter which we will mount near the factory CD changer. It also includes the inline fuse holder which we will mount near the battery.
- Ring connector if you wiring kit does not include one. (not sure on proper size)
- Electrical tape.

TOOLS NEEDED:
- Wiring multi tool (Wire cutter, stripper, crimper)
- 10mm combo wrench. Battery Negative
- 13mm combo wrench. Battery Positive
- Philips Screwdriver. Dead pedal
- Multimeter


------------------------------------------------------------------------------------
Here are the Modultie instructions. http://www.hitchestogo.com/Instructions/N18176.pdf But we all want 318ti specific info!

#1. Determine a location for the converter. I chose the area where the factory CD Changer would mount in the rear left of the trunk. There are 4 unused mounting nuts if you do not have a changer. To access this area, you will need to remove the seat-back corner piece. This is accomplished by lifting up the rear seat (just yank, it snaps into place) and unscrewing the plastic seat-back retainer nut. The pieces will then lift out. After that is out, pull the trunk carpeting corner piece out.


--


#2. Ground the converter box to the chassis. There is a grounding post ready to go on the rear left wheel arch. See picture.



#3. Disconnect the negative battery terminal and safely inslulate the negative battery cable. I stick the loose cable in a thick rubber glove. 13 mm wrench

#4. If it’s not already attached, crimp the ring connector onto one end of the inline fuse holder and attach the ring onto the positive battery post. Remove the battery cable nut with a 10mm wrench. Here’s the temporary location of my inline fuse holder, plus a temporary (too large) ring connector. The white wire leading to the top of the image is the power supple wire which is discussed in the next step.

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  #1  
By CirrusSR22 on 06-08-2008, 04:45 AM
Default

#5. Now here’s a tricky part. We need to route the 12 AWG power supply cable from the battery to the power converter. It’s actually easier to route the cable from the footwell outward to the engine bay. I used the hole in the firewall that accepts the clutch master cylinder and hardline. If you have a manual (I have an auto), I’m guessing you have enough room to push the battery wire though that opening. If you have an automatic, purchase the BMW grommet for that opening. To install it, push out the paperboard perforated cutouts that are blocking the grommet hole and install the factory firewall grommet. Keep in mind there are two firewall openings, one for the clutch master cylinder/hard line and another higher up for the soft clutch fluid supply line. I chose the lower one, but the higher one should work too. See picture below.

Once you push the wire through the grommet, it will probably end up on the far left side of the brake booster. I routed the wire over the brake booster, through the hollow rubber hood seal and to the battery. Crimp the battery wire to the other end of the inline fuse holder.

At this point I removed the dead pedal and routed the battery wire along the already existing bundle of wires. You also need to remove the lower door edge trim panel. Force it backwards to release it from it’s clips. Once that’s away you can pull away the carpeting and route the wire behind it. At this point you will “rejoin” the factory wire bundle near the passenger’s footwell/side panel. Continue to route the battery wire along the factory wire bundle. It’s pretty self explanatory where to route the wire once you get it to the back seat. Finally crimp your battery supply wire to the converter battery wire Here’s some pics...

The wire leading out of the battery box and into the hollow hood seal.

Routing of white battery supply wire


Passing over booster and down towards firewall grommet.


Now inside the cabin looking forwards.


Didn’t get a shot of the dead pedal off, but this is a zoomed-out shot of the last one. The wire runs straight down behind the dead pedal.


Door trim panel removed.



**Continued next page**
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  #2  
By CirrusSR22 on 06-08-2008, 04:48 AM
Default

I had two scrap pieces of 12 AWG so they were splice together under the rear seat. You can see how the battery supply wire can now rejoin the factory bundle. I’m guessing if you were to remove the front seat you could follow the factory bundle right from the firewall.



Finally the battery supply wire (now orange!)can follow the factory bundle to where we will mount the converter.





#6. Reconnect the negative battery cable.

#7. Time for some actual work! We need to splice the converter’s wiring into the car’s wiring. It uses these signals for logic, and powers the actual trailer lights through the battery wire we just ran to the unit. I initially tried to solder my wires and use heat shrink tubing but my soldering gun was too weak. The converter kits usually come with “taps”, but they seem quite cheesy. I chose to cut my car wires and use crimp style butt splices to complete the connections. I made the connections at the bundle that slopes downward from the wheel well.

Please don’t take my word on these wire colors!!!! Take out your handy Bentley manual and use your multimeter to make certain your are splicing into the proper wires.


(CONVERTER WIRE : BMW Wire Color)
BRAKE WIRE : BLUE w/ RED STRIPE
LEFT TURN : BLUE w/ GREEN STRIPE
RIGHT TURN: BLUE w/ BROWN STRIPE
TAIL LIGHT: GREY w/ YELLOW STRIPE. This wire leads to the right tail light. You should also be able to use the GREY w/ VIOLET wire which leads to the left tail light

Here’s where we will splice into the factory bundle.


Here’s the bundle after the splices are complete. Re-wrap your bundle with electrical tape when you are done.



#8. After all wiring is complete, install the supplied fuse in the inline fuse holder and test your lights. My U-Haul converter came with handy LED’s built into the connector end to see what’s going on. If not, plug the converter into you trailer and check their operation. Hopefully all is well!!!

#9. Now comes time to mount the converter in the trunk. I made a metal holder that bolts into the lower two nuts of the CD changer location. If you actually have a CD changer, you’ll have to get creative. If worst comes to worst, use the double sided tape on the back side of the converter and stick it somewhere. Here's the mount I made.



And the converter stuck to it.



#10. Finally you’ll need to find a way to route the four-flat wire outside of the car. I haven’t really explored this yet, and I assume you could just close the hatch on the wire and it would be fine on a temporary basis.

Enjoy! Remember..... Buy trailers, not trucks!!
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  #3  
By thesk8nmidget on 06-08-2008, 05:04 AM
Default

nice write up! i would run the wires a little differently but other than that nice!

fyi

if you pull the seat you can see there are guides and kinda like tracks where all the wires go and you can run your wires with the stock harness.
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  #4  
By CirrusSR22 on 06-08-2008, 05:12 AM
Default

Yeah that's what I figured. I didn't feel like taking the seat out.

Here's my temporary solution to routing the four-flat wire. There's an opening already in the side carpet, then I screwed a hook through the carpet and backed it with a little piece of wood. For the time being I'll buy a cheap four-flat extension and route it out the trunk opening so the main four-flat wire won't be damaged.

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  #5  
By R111S on 05-03-2009, 01:01 AM
Default

Thanks for the nice write-up and pics. I used the Wire Color Codes you listed...same as my '97.

I took a somewhat different route to the lights and wiring for my Trailer. I bought a pair of Red LED Blazer Trailer Lights (Model: C783-Left and C793-Right) and a pair of Amber LED Marker Lights (Model: C531A) from my local TSC Store. I also used two 4-Terminal Plug/Wire sets...one for the left and one fo the right.

I wired the Red Lights in parallel with the L/R Tail/Stop Lamps and the Amber Lights in parallel with the L/R Turn Lamps. I did not use an adapter...the LEDs draw very low current so there's not a significant load added to the Tail Light Circuits. The Blinkers work just fine...neither slower nor faster than normal. Tail/Stop Lamps look good too.

The LED Lights are more expensive than normal Incandescents units but they will more than likely out last the car, turn on quicker than Incandescents and therefore ~16 Feet sooner at ~60 MPH, and draw less power for a few drops of fuel savings.
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  #6  
By John Firestone on 05-03-2009, 12:42 PM
Default

BMW sized the existing, rear wiring to accommodate a trailer (and is what they use), but I think anything you can do to unload the overworked brake light switch is a good thing!

The official method to get power for the trailer would be to add fuse contacts and a 15A fuse to the assigned position F2 in the main distribution box, with internal, power bolt X0 supplying the fuse. But then you have to deal with the rats nest inside the fuse box, and reaching it.

I use sheets of hook and loop fasteners (e.g. Velcro) to attach electronics without drilling holes. I try to orient the sheets so that they attempt to sheer past each other in a frontal crash, but can't.

Nice write up and pictures!
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  #7  
By mnguyen84 on 05-10-2018, 04:34 AM
Default

on the passenger side there's a red/white wire that always has current. is that normal? what does it do?
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