"The alarm horn is driven by t
"The alarm horn is driven by the gauge or in my case, the tach.
Are you using the small 2" gauge with the vertical row of four lights, and a connector on the back that has two rows of four pins? Not the tach?
There is nothing on the engine that can generate the short burst to the horn at power up except the circuitry in the gauge or tach.
In fact, with a jumper wire, you can jump directly from 12v to the purple wire and cause the system check to run thru it's checks and bleep the horn. If that doesn't work, it has to be a bad ground, or a bad gauge or horn itself.
Do the lights cycle at power up or is it just the horn that doesn't work?"