Trailer Relay

[Chris Holmes]

Back in the dark ages I had a Rover 214 with a tow bar. But it didn?t have an indicator to show when you had a trailer attached.

It was possible to buy a replacement indicator relay which had an extra output to light a bulb on the dash when it detected that there was an extra bulb in parallel with the indicators.

How would this have worked? Would all of the indicator current have been sent through the coil that activated the switch for the extra contact, and only when the extra bulb was connected would the current be sufficient to throw the switch?

Or something else?

[williamwright]

I don't know but mine has the same thing except it sounds a buzzer.

Bill

[Harry Bloomfield, Esq.]

Chris Holmes explained :

Yes, through a relay with a low resistance coil winding.

They used the same idea for detecting when a car lamp failed long ago. It's all done by electronics now.

[Dave Plowman (News]

You need an indicator light to tell you you have a trailer attached?

[Harry Bloomfield, Esq.]

My fancy system indicates nothing at all if everything is working properly - I'm not sure it is entirely legal to the letter of the law.

Rather it monitors all of the trailer lighting constantly and puts an icon up on the screen, indicating which bulb has failed. Once on, it stays on screen, until the engine is restarted - so no chance of it missing an intermitant problem.

[Dave Plowman (News]

The bulb failure unit on the old Rover uses reed relays. And a vast amount of extra cabling.

[Harry Bloomfield, Esq.]

Dave Plowman (News) was thinking very hard :

No extra wiring in mine - all done digimagically.

[Steve Walker]

There is a legal requirement to indicate whether or not the trailer indicators are working.

It used to be done with an extra dash-light, later and more simply with a buzzer in the boot (where all the wiring was) and these days, the car's electronics usually cope with it - either by a similar check or by a short pulse every few seconds to prove that all the trailer lights are working (and incidentally, by showing that the trailer is there, modifying the operation of the braking and stability programs.

[Brian Gaff (Sofa]

So what is this supposed to do? If its just so direction and hazard indicators on the trailer work, normally these seem to be part of the standard connection and run in parallel with the cars ones just like all the other lights do. However if its an indication only that a trailer is hooked up that would be easy to achieve with a link in the plug when its attached. Some car stuff seems to be unreasonably complicated for what its supposed to do.. Brian

[Brian Gaff (Sofa]

That could be done by a chip called a window comparator, witch is capable of monitoring the current or resistance depending on how its set up, of each light, but do people really go to this complexity these days? Brian

[Harry Bloomfield, Esq.]

Brian Gaff (Sofa) explained on 19/04/2021 :

Obviously they do, or I would not have one. It is the OEM BMW optional CAN-BUS part. My car already has/had a full road lighting monitoring system It cost me around £130 extra for the extra towing module, compared to the basic towing adaptor sold by the accessory shops. A single extra wire relays data back to the lighting control module in the dash. Worth it to know my lighting is fully working on my trailer.

[Harry Bloomfield, Esq.]

Brian Gaff (Sofa) was thinking very hard :

No, they are supposed to monitor the current, to provide feedback that the trailer indicator is actually drawing some current/ working. No buzzer/WL flashing = cable fault or lamp failed.

[bert]

In article <s5h283$961$ [e-mail], Chris Holmes [e-mail] writes

My Defender still has that and it is much better than the faintly audible though not if passengers were talking system which replaced it.

[bert]

In article [e-mail]>, "Dave Plowman (News)" [e-mail]> writes

Last time I looked, which was some time ago you were required to have an indication that your trailer indicators were working. Originally that was visible and still is on my Defender but then switched to audible, to save the manufacturers putting in a cable and dash light which in most cars was never used. Don't know what it is now but on my 2015 T5 Transporter it is still audible.

[Andy Burns]

My car has a a single extra "trailer indicator" lamp on the dash, and if there is a fault, it displays a message to the effect that trailer lights aren't working ... not sure if the circuitry for that already exists, or was fitted as part of the towbar kit.

[bert]

Defender (Old type) don't do messages.