How to connect the towbar electrics? (bulb failure system)

t_w · Post by **t_w** » Sun Jul 22, 2007 11:09 am

Well, holiday is coming and I'm going to travel with my 480 and a little trailer towards Italy...

So, this weekend I bought a towbar... (I can do this, cause I'm Dutch

)

The towbar is original Volvo...

But:

How do I connect the electrics, so that the bulb failure system still works?

I've done a search, but could'nt find an answer...

I've read something about some sort of a 'relay box' which is needed?

Who will help?

480_rocket · Post by **480_rocket** » Sun Jul 22, 2007 11:29 am

I don't think you could run it through the bulb failure unit. What you could possibly do is get another bulb failure unit, and wire them together, should be possible!

t_w · Post by **t_w** » Sun Jul 22, 2007 11:43 am

Ok, but I could imaging that Volvo thought of some sort of sollution for this problem... Maybe some sort of 'box' which hast to be connected in between?

Robou · Post by **Robou** » Sun Jul 22, 2007 8:39 pm

The bulb failure detector reacts on a difference in current between the left and right side of the car, except for the high-level brake light, for which a resistor is build in. So whenever one adds a load, it should be done symmetrically.
The indicator lights are not run through the detector.

You should be able to connect tail- and brakelights parallel to the existing ones, providing the loads are equal on both sides. And, of course, don't forget to adapt the fuses.

aibast · Post by **aibast** » Sun Jul 22, 2007 8:42 pm

didn't volvo have special something for towbars electrycs?

t_w · Post by **t_w** » Sun Jul 22, 2007 8:56 pm

Robou wrote:The bulb failure detector reacts on a difference in current between the left and right side of the car, except for the high-level brake light, for which a resistor is build in. So whenever one adds a load, it should be done symmetrically.
The indicator lights are not run through the detector.

You should be able to connect tail- and brakelights parallel to the existing ones, providing the loads are equal on both sides. And, of course, don't forget to adapt the fuses.

Well, thats the 'problem'.

The connector for the towbar has 7 pins,
-mass
-light left
-light right
-indicator left
-indicator right
-brakelights
-constant 12v supply

That means, that the 'normal lights' won't be a problem, but the brake light does, since I only got 'one pin' in the connector which feeds the two brakelights of the trailer...

@ aibast,
I'll give the local Volvo dealer a call in the morning, perhaps they can tell me more... I'm curious what the price of such a device is

Robou · Post by **Robou** » Sun Jul 22, 2007 10:39 pm

Connect a diode in each wire from the car to the one wire of your trailer connector. These diodes are cheap and will do the trick. The specs are low voltage, 5 Amp, this will ample do for two 20W bulbs.

If you don't know what a diode is and/or where to get them, pm me.

t_w · Post by **t_w** » Mon Jul 23, 2007 6:56 am

Ok, that's something I didn't think about! Thanks for the answer!

I know what a diode is, but I think I don't understand how this should work.

Is it the trick that when I use a diode, the 'bulb failure system' doesn't 'see' the extra load of a second bulb? Cause the diode prevents any 'feedback' (don't know how to discribe it) to the bulb failure system?

Robou · Post by **Robou** » Mon Jul 23, 2007 8:53 am

Both diodes conduct, as long as they're identical, half of the necessary current for the two trailer bulbs, so the detector should keep on doing it's work, which I presume was your objective.

If not, why not make connections before the detector?

t_w · Post by **t_w** » Mon Jul 23, 2007 9:59 am

Robou wrote: If not, why not make connections before the detector?

How could I be so stupid not to think of that

Thanks! I think that's by far the best solution!

Ok, now I'm curious if you have a bright solution for this one:

In the Netherlands, a trailer has to have a fog light.
But, when the fog light of the trailer is on, the rear fog light of the car is not allowed to be on...
Of course I could do this with a manual switch, but why go the easy way if the hard way is nicer

So, what I want is the following: When I connect the plug of the trailer, the power supply for the vehicle's rear fog light should be interrupted...

ps. when I'm done mounting the towbar, I'll put a 'how to' on the forum, the info of this thread will be in there also...

Grtz, Thijs

Robou · Post by **Robou** » Mon Jul 23, 2007 8:50 pm

I wonder if this subject has any common interest and deserves a place in a "how to", but anyway:

One is only allowed to use a fog light when the rear lights are on, so if you have a spare connector in your trailer plug feed the live of your trailer's rear lights back to the coil of a relais, of which the n.c. is connected to your car's fog light and the n.o. to your trailer's fog light.
This is the only d.i.y. solution I can think of.

Any other possibility should use a voltage drop in the wiring, which triggers some simple electronics which in turn triggers the relais.