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MG MGB Technical - Hella horn installation

I have a 1973 MGBGT which I am finally ready to upgrade the horns. I bought a set of Hellas. the original passenger side has 4 terminals which I assume provides the jumper to the other horn. Each new horn has just two terminals. with 22 stamped on one terminal and 06 on the other. I already have a relay for horns in my updated fuse box. Which wires go were. All I get is some clicking or at best a weak sound out of one horn as I alternate wires.
Ben Stone

Do the factory horns sound? Have you confirmed that the Hella horns work when connected to 12v?

The more current the horns take the more sensitive they are to any extraneous resistance in the circuit. Are you sure it is the relay that is providing _switched_ power to the horns and not the button? On a 73 that is an earth not 12v. Both my 73 (different to yours) and 75 (should be the same) have had high-resistance earth problems through the columns and I've had to provide relays, these are operated by the button earth and put out a body earth to the horns.

On a 73 you should have two purple wires on one side of the right-hand (looking forward) horn supplying permanent fused 12v and two purple/black wires on the other side of the horn supplying the switched earth.

With the wires connected to the factory horns connect a voltmeter between the purple terminal and earth and you should see 12v. Sound the horn - and assuming they do sound - there should only be a small reduction in voltage. If you see a reduction of several volts then there is a bad connection back through the fuse to the 12v supply.

If that voltage is good put the meter on the purple/black terminal and again you should see 12v. Sound the horn, and this time ideally the voltage should drop to zero. If there is more than a few tenths of a volt shown then there is a bad connection back through the earth circuit.
paulh4

Thanks Paul, Yes I am thinking I have a low voltage /bad connection issues, but haven't had time to do any meter checks. Only one of the old horns worked. I did have some jumper connectors to get all wires connected.
Ben Stone

Adding a horn relay makes a world of difference.
Glenn Mallory

This thread was discussed between 17/12/2022 and 31/01/2023

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