Re: 1991 johnson 150 warning alarm
With the key ON, engine OFF, unplug the wires from the VRO pump, oil tank, and temp switches, then see if you still have the alarm. If yes, find the short in your wiring harness or cable to the remote control, tan wire to ground. If no, plug in the wires one at a time starting with the temp switches to see what is causing the alarm. I had an oil tank give me a constant alarm one time, so I just left it unplugged as the tank was in a visable location.
When the alarm horn is working correctly, it gives a short chirp when the key is turned on to let you know it's working. The Radio Shack and other alarms/buzzers won't do that. If you use a non-OMC/BRP alarm, leave the black wire disconnected from the horn. Purple is +12v, tan becomes ground when a sensor activates. The black is ground for the test chirp in the OMC/BRP horns.
Just reread your post, you say the motor is new. Run a premix of 50:1 fuel and oil with the VRO connected when you first start it. It may take a while for the VRO oil sensor to start working, and the factory recommended using premix until the VRO is verified working by the oil level dropping in the tank. It may take a bit of running to verify as the engine uses about 1 gal. of oil to 50 gals. of gas. The extra oil won't hurt anything except maybe the mosquito population.