trailer wiring - be careful here!

[Mike Arman]

Watching the thread develop on wiring to trailer lights on our cars here . . .

There are several problems which must be addressed.

First, the inevitable flames about daring to put a trailer hitch on our
cars in the first place. These can safely be ignored, since it is just noise.

Second, the separate turn signal/brake light setup compared to the combined
system used on most American cars and (evidently) all trailers here.

Third, Autocheck.

Fourth, Bosch and Audi, who have evidently taken the "just-in-time" idea
and converted it to "just-in-adequate" with regard to excess current
carrying capacity for anyone who would dare to put a trailer hitch on their
car in the first place.



For problem number two, separate turn signal/brake light (on the car)
versus combined (on the trailer), the so-called "converter box" is used to
make this work. I have in front of me a "Lite-Mate model 504 Taillight
Converter" which solves THIS problem, but not the last two (which I'll come
to next). It cost me about $15 at Discount Auto Parts, and these devices
are widely available.

The converter box has a four prong trailer wire socket on one end, and five
wires on the other, being ground, brake, park/tail, right turn and left
turn respectively, and they are supposed to be connected to those places on
the car.

This will work if your car has big hefty wires (which Audi does not!) and
does NOT have an autocheck (which Audi does). This is why the "$15 solution
to the problem" is still in front of me, and not on the car.




Problem number three is the Autocheck. Autocheck senses and compares the
currents between the bulbs, and if they don't match, it complains.

On type 44s, the rear autocheck box is just forward of the driver's side
trunk hinge, under the fabric/cardboard cover on the driver's side of the
trunk. It compares the current drawn by the brake lights (teminals 58R1
{Gy-R}, 54R {Bk-R}, 58L1 {Gy-Bk}and 54L {R-Bk}), and if they differ, it
squawks. Note that the tail lights are not routed through the autocheck box
(terminals 58R and 58L, Gy-W and Gy-Gn respectively).

The power into the autocheck box from the brake light switch is terminal 54
(R-Bk).

Now ignoring the lack of capacity of the wiring of the car, we could
connect the converter box as follows:

White to ground

Red (brake lights) on the converter box to terminal 54 autocheck box (this
is *before* the autocheck, so the added power drawn by the trailer lights
won't be seen by the autocheck, and it will stay happy).

Brown (tail/park) to 58R or 58L on the autocheck, same comments as above,
remembering that these bulbs are not sensed by autocheck anyway.

Green (right turn) to Bk/G, right turn signal on car, not sensed by autocheck,

Yellow (left turn) to Bk/W, left turn signal, same comment.





HOWEVER . . . We still have problem number four. I don't think the wiring
harness really has sufficient extra capacity to carry the extra trailer
lights, especially if the trailer also has side markers like most boat
trailers.

The wire sizes are as follows: Brake light, 1.0mm, all others are .5mm.
Existing loads are 21 watts (brake lights) which at 12 volts is just under
2 amps per bulb, and 5 watts each (most other bulbs), which is just under
half an amp each, plus the license plate lamps.

The trailer load is 2 amps for each brake light/turn signal and half an amp
for each other light. (figures approximate).

When we add those loads to the existing loads, I'm pretty sure the wiring
harness will promptly fail the smoke test. I also note these wires are
fused at 5 amps each (15 amps for the brake lights), so we're getting
pretty close to the capacity of the fuse when we add the trailer load to
the existing load.



The answer, therefore, is going to involve some relays, and a separate hot
lead, run from the battery (and properly fused) to power the trailer
lights. The resistance of each relay coil can be about 300 ohms, which at
12 volts will draw 40 milliamps or one half a watt (which hopefully will
remain within the capacity of the wiring).

We'll need a relay to work the brake lights, a relay to work the parking
lights, and a pair of relays to work the turn signals. Which immediately
brings us to another gotcha: Wiring the turn signal and brake light relays
in parallel will make the turn signal filament burn steadily when the
brakes are on, and wiring them separately will make the trailer's turn
signal flash in reverse to the car's signal. (car on, trailer off, trailer
on, car off.)

Two tentative solutions to this new problem are, one, do the relays and use
them to feed the $15 converter box, or two, use lights on the trailer which
have separate brake and turn signal bulbs!


I'm not yet sure exactly how I'll work this out, but when I do (next few
weeks, time permitting), I'll post a schematic which will solve this
problem once and for all.


Best Regards,

Mike Arman