Trailer lights.

Just an update.
The boxes arrived, and one was what is called a bypass relay, the other for controlling the caravan battery charging when on the move.
I'm on a learning curve with this as I've never fitted or needed tow bar electrics.
The charge unit well and truly knackered. Looks like battery fumes got at it by the corrosion. But not needed for trailer only use anyway.
The bypass relay unit has 12v and ground, and 6 relays with outputs to the trailer socket with a hose pipe containing the 6 standard colours to sense each lighting etc circuit on the vehicle. A quick look at the PCB suggests it also senses the load from each trailer lighting circuit to give bulb failure warning.
It also has 2 grey and one white wire sort of added to the PCB - ie not within the same hosepipe. I'm thinking to disable the reversing sensors.
Powered it up from the bench power supply. Buzzer sounds loudly. Put some tape over it. ;-)
Applying 12v in turn to each of the hosepipe wires made some of relays for that circuit make, but not always the correct one, and some more than one.
Connecting all 6 sense wires together and to 12v would make me think all the relays would make, but not so.
It contains two ICs. One drives the relay coils and perhaps more. The other I assume does the logic and bleeper. Can't be bothered tracing out the circuit fully, as they've ground off the numbers from the ICs. ;-)
Would a standard 7 way unit cope with the reverse sensors too? They haven't worked for some time - and might be associated with this. Not having much success finding a supplier of such things that aren't part of a kit for a specific vehicle. And give a full description of what they do. The Amazon one recommended doesn't seem to mention reverse sensors.
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*Microsoft broke Volkswagen's record: They only made 21.4 million bugs.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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