Connecting a trailer to a modern VW

(without using the CANBUS)

When you try to connect your trailer to a modern VW the old fashion way (just connecting wires together), the car electronics will tell you something is wrong.
Also when I measured the signals on my Golf V and found they were quite different than what you would need for the trailer lights.
The best way to connect a trailer, is to buy a CANBUS connection set and hook that up according to the instructions.
An alternative is to take the signals from the tail lights and, with some electronics, regenerate the correct signals. If you know electronics, this would be a cheap alternative. (total cost of the electronics below is around 10 euro, while an original electronics box is over 100 euro. Is it worth the trouble? To be honest: maybe not. But to me it was just a challenge to get this working.

First let's look at the signals on the tail lights:

For the tail lights there are two bulbs on both sides. Both are driven by a 100Hz PWM signal with a 25% duty cycle.

When you switch on the lights, there is a soft start-up. I'm not sure if this is done with a current limitation or a dv/dt limitation.
At least this is a nice way to switch on the lights because it limits the current when the filament of the bulb is cold.

When the brake is applied one of the bulbs is switched over to 100% duty cycle. In the picture below the upper signal switches over to 100% duty cycle.

The turn signal light is is a straight forward on and off system and so is the fog light.

With this information I made a small electronics circuit to drive the trailer lights.
To detect the tail lights the circuit detects if the blue signal above is present. To detect the brake light the circuit detects if the duty cycle of the orange signal is above 50%.

The electronics use low resistance P type MOSFETs to drive the lights. They are easy to use and require a small board space. I used IRF types, but any type with a high enough breakdown voltage and low Rdson will work. The power is drawn from the battery only if one of the lights is switched on. There's no quiescent current. I fused every output to make sure that if one circuit fails, the other circuits will still work. The Vbat connection is the connection to the battery obviously. This fuse for this wire should be close to the battery.

The fog light is just an old fashioned connection. The wire is split and the car fog light is switch off when the trailer is connected. If the trailer does not have a fog light, a defective bulb message is displayed on the dashboard. (oh, well)
For the brakes C1 is charged slowly through R2 and discharged fast through D1. This will make an easy distinction between a 25% PWM (normal tail light) and a 100% PWM (brake light). The zener D4 protects the MOSFET from a too high Vgs in case of a load dump. (very high battery voltage for a short time) Diode D6 protects the circuit from inductive spikes when the lights are switched off. (long wires will have some inductance)

For the normal lights C2 and C3 are charged fast through D3 and D9 and discharged slowly through R11 and R27. This will detect the 25% duty cycle.

For the turn signal lights no filtering is needed.

For completeness I added the pin numbers and DIN number as used for a 7 pins European (Dutch) connection. You need to check if the connection in your country is the same.

Page Last updated: 07/19/2009