Faulty Tach and overheat alarm sounding

I have an '06 Johnson 90HP saltwater addition. PROBLEM! boat is used regularly and 3 wks ago took boat out and it ran perfect as it always has but only for about 10 minutes. I noticed the Tach seemed to be reading to low for the MPH. The tach does not have sys check lights, only the alarm beep when key turned to on. The overheat alarm sounds and the boat shudders so I put in neutral and turn off immediately. I restart the engine with no alarm & notice tach is stuck around 2000 rpms and does not move until we are on plane again. Alarm sounds again, engine shudders so I kill it a 2nd time. put boat on trailer and delivered to my mechanic. He replaced, impeller and thermostats even though I had checked them out and proved to be good. Also added new water pressure gauge and installed Temp Gauge.
Took boat out yesterday and boat ran great for about 10 mins before heat alarm and shudders began. this time the Tach seemed to be operatingly properly until overheat alarm came on. this time the tach was stuck around the 3200 rpm reading with the engine turned off. If I went over about 7 mph(according to GPS) the alarm would come on. While the engine is running I kept checking the TEMP GAUGE and you can see that the t'stats are opening and closing from the rise and fall in temp. Max hi-temp around 175 deg and low temp about 145 to 150. Put it on trailer and brought it home. I did read somewhere that a faulty tac could cause these issues.
Need help!!!! - Ron

Re: Faulty Tach and overheat alarm sounding

Good to have a manual on the motor. Dont have the 06 manual but the tach can be disconnected to test, probably at the Rectifier. Yes a bad tach can cause these problems. It could also be the rectifier.

Re: Faulty Tach and overheat alarm sounding

James R said:

Good to have a manual on the motor. Dont have the 06 manual but the tach can be disconnected to test, probably at the Rectifier. Yes a bad tach can cause these problems. It could also be the rectifier.

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Thanks James R, I will check it out and post what i find.

Re: Faulty Tach and overheat alarm sounding

Look for a short in your system if you don't find any shorts the next things to do is start testing components. I would check both temp sensors to make sure they are operating correctly. Your engine is equipped with Slow Mode which cause the shuddering engine as it prevents it from self destructing if it thinks the motor is overheating. The temp sensors have tan wires running from them. Check Out page 59 in the CDI troubleshooting guide it is in my sig

Re: Faulty Tach and overheat alarm sounding

Results-changed out t'stats with new ones and installed new tach. I set the dial on back of Tach to NO.6 as instructed by boat mechanic and wouldn't you know it, Tach did not work. checked old tach and it was set on NO.5 so I set the new Tach to NO.5 and now tach works like a champ. As far as the new t'stats go i mistated in my original thread that they opened and closed between 145 and 170. Actual temps are approximately 130 to 150. Engine runs like new with no slow mode or alarms going off.
Thanks for everyone's advice and expertise
ron