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MG MGB Technical - 1977 horn wiring

As title I have a 77 bgt. I purchased a new indicator horn actuator mechanism ( stalk type ) it has two wires, purple/ black and plain black on the connector block ( male) which corresponds to the female block.

The horns themselves are the single wire type and work when 12v is applied.

Close to the mounting point of the RH horn is a single black/purple and a single purple. I get continuity when these are connected and the horn stalk pushed but cannot for the life of me work out where the main feed is or should be to actuate the horn.

Is the horn stalk connections (PB & B ) for the earlier 2 wire horns ? At the connector under the dash the black wire disappears to god knows where, certainly not in the loom going forward.

Spent ages connections up relay too with 12v but no joy and am at my wits end.

The advance auto wire diagrams make it look all too simple ( late MGB and 1977 onwards.

Any help at all would be appreciated

Regards
P Liles

It sounds like your wiring is indeed for the two wire horns. Purple is a permanent fused 12v supply. Purple and black goes to the horn switch and black is ground. The purple and the purple/black would be connected to a two-terminal horn. Pressing the horn button completes the circuit to ground. There would also be a short two-wire loom to connect the two horns together.

It begs the questions, what was your old switch like and have the horns ever worked?

If you’re using a relay, you can use the purple and the purple/black to operate the relay and use it to switch 12v to the horns.
Dave O'Neill 2

Horns were 2-wire until the start of the 1979 model year in May 78. The Advance wiring diagrams copy Haynes which miss out some late changes, the horns being one of them, later Leyland Workshop Manuals do have the correct drawings

As such you have the correct stalk switch for a 77 as that puts an earth (shared with many other dash components) out to the horns and they are are backed by a 12v supply from the purple circuit in the fusebox.

However if your horns only have one spade terminal on each they are the later one-wire horns that require a switched 12v supply on the purple-black and are backed by an earth that they pick up from their physical mounting.

To use the one-wire horns with the earlier switch you will need a relay to reverse the logic. That will need a purple (12v) supply to terminal 85 and the purple-black from the stalk switch to 86 so the earth from the horn switch operates the relay. There will also need to be a purple supply to terminal 30 on the relay and terminal 87 will go to the purple/black to the horns.

If you mount the relay down by the horns it avoids cutting wires by the steering column but exposes the relay to the weather. Mounting it behind the radiator diaphragm panel will alleviate that but still need a bit of jiggery-pokery down there to feed the original purple and purple/black wires back to the relay 85 and 86, then a new wire from 87 to the single spade on both horns. By cutting wires by the right-hand horn you can avoid running the new wire across the front of the car from the right-hand horn to the left hand, your choice.
paulh4

…or fit a pair of two-terminal horns.
Dave O'Neill 2

I'm too tight to even contemplate that ...
paulh4

Thanks guys particularly Paulh4, yours worked when all other schematics did not. TFFT. Still a million other little jobs, mostly trying to source the lost heater control dial/ knob nut. Anyone know the thread size ?
P Liles

Andy Jennings used? If you say you only want the nut he's probably got a totally knackered control he could take the nut off.

Seems to be 27H1249, available from the usual suspects.
paulh4

This thread was discussed between 28/05/2025 and 02/06/2025

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