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


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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.


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#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.