Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Renewing the twin aero horns

Due to a gradual perception of poor performance of the old hooters I decided to renew them with a new set of Fiamm horns. These are pretty standard items and I am delighted with the result. Feeling the old wiring was not up to the task of supplying sufficient current I put in a new supply lead direct from the rear battery via a 20 amp fuse along the underside of the near side chassis and directed it through a Bosch Relay. A simple enough job unless you happen to accidentally touch ground with the live wire and blow a couple of fuses in the process !
I mounted the horns on the alloy flat surface of the bulkhead using a couple of small wooden platforms glued on with araldite to save drilling into it. I chose this location because the bonnet side louvres open facing forwards so the sound is able to project forwards through them and at the same time they give protection from wet and grime.
I attempted to use the ground to switch on the circuit but it failed to work for me ( it might have been due to trying to achieve a circuit through a blown fuse ). However the upper diagram was used with the live interrupted by the switch and that works just fine. The relay pin 87A is not used. Also note the on/off switch used is a press-for-on button switch.

Apologies for the criticism Graham but there seems to be a degree of re-inventing the wheel here.

Wonder why you felt the existing horn wiring was not up to the task? It worked fine for me when I dispensed with the Roadsters twin two tone horns and fitted Siebel twin air horn (which likely draw more current than your Fiamm's) although I added a ground switched relay and upped the existing horn circuit fuse to 20 amps. Note the factory wiring provides power to horn circuit only when ignition on or when first accessory position is selected. Use of this would have saved your fireworks, blown fuses etc.

It appears you have dumped the horn push in favour of a momentary switch with live connections . Was this because for whatever reason you couldn't wire the relay coil to trigger from the ground provided by the horn push as this is how it should have been done.

Not sure why you have posted relay diagrams showing changeover circuits employed, noting that top one is wrong, strictly speaking, in showing reversed relay coil pin connections .

Happy to provide you with instructions on wiring your horns correctly if you decide to simplify, just send me a PM. It may save your car going up in smoke!

Richard, thanks for the warning - I realised there had to be a reason for the ground switch method but in my frustration simply fought for hours attempting the right way without success. (Hence the pants-down emoji)... Please do provide further instructions for us to learn the best way. I was trusting the fuse to protect everything against a catastrophe whilst avoiding the steering wheel push which I find needs a firmly centered thump to make it properly work. I now have a choice of which horn to press.

Essentially its this Graham.



Shown here is a simple on/off relay but a 5 pin changeover type as usually fitted by MMC works equally well - just ignore the 87a contact. Important you use a diode suppressed one as factory fitted though to avoid pulses that may damage ECU.

The device are the horns in this case and the ground trigger source the horn push. BTW might be worth taking your steering wheel off to see reason for difficult operation. I found no need for extra wiring simply using the two horn feeds, both light green/white in wiring loom, to connect to pin 86 (coil live) and pin 30 (main power to relay). These feeds are already fused although you may need to uprate fuse. The purple/black wire is ground switched feed from horn push so it goes to pin 85. Ensure second one in loom (present even if only one OE horn fitted) is isolated.

Finally in car electrics the KISS principle applies

Thanks for this clarification Richard. I have never been brave enough to take the steering wheel off and shall investigate further. You mention a diode in the relay and I am wondering if perhaps this could have prevented current going the wrong way using a ground earth on-button during my first attempt.