On 18/04/2021 14:34, Dave Plowman (News) wrote:
In article ,
Chris Holmes wrote:


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?

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

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.