I have reviewed all of the posts for the overheat alarm but have not seen my exact question asked. I have a 89 Merc 100 hp. The other day the overheat alarm went off but i have a temperature gauge and the temp was normal. I shut down the engine while checking out the situation. The alarm would sound when only the key was on. After about 15 mins I was able to start the enigine again and continue motoring. after about 5 mins the alarm sounded again. All through this time the temperature gauge was showing normal operating temperatures. Any advice.