Talk Morgan Forums Thread Like Summary - 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.

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 as elsewhere, the KISS principle rules