2002 Mercury 50HP Bigfoot 4-Stroke Tach Reading 7000rpms WOT - Alarm

[FlipFlops365]

For some reason my boat started reading around 7000 rpms at wide open throttle after driving for 5 minutes. It was fine all day until we started to head home after the boat was sitting for a few hours at the sandbar. After a short time WOT the alarm sounds and it slows down the motor. Normally I am right at 6000rpms WOT. When I would turn the boat off and start again and make sure my tach stayed around 5000rpms the alarm wouldn't sound. However, I had 8 people on my 20ft pontoon so we could only hit about 8MPH.

This happened a few weeks ago and I turned off the boat, started it up again and it was fine the rest of the day. Any ideas? Can something trick the tachometer into thinking it's running higher RPMs than it really is?

Any help is greatly appreciated. Thank you for your time.

Tom

 

[Texasmark]

Re: 2002 Mercury 50HP Bigfoot 4-Stroke Tach Reading 7000rpms WOT - Alarm

Kind of stepping out on this one. On 2 strokes the tach gets fed by impulses from the flywheel via a switch box in the engine. These impulses go into the tach and are counted. The tach has a selector switch which determines how many pulses equal how much deflection.

I'd say you have an intermittent problem with your tach, but with the other things you mentioned, I don't think so.

What is the nature of the alarm....solid or beeping? Solid is usually over temp and beeping would be low oil for 2 stroke engines,,,,,don't know if oil is monitored on 4 strokes.

Do you have an over rev. limiter on that engine? If so, and your engine slows down something is feeding it erroneous information causing the engine to slow down, so it could be wiring making intermittent connections, a bad switch box, or possibly even a stator up under the flywheel.

Nobody responded to this so I just tried to help. Am in unfamiliar ground here. If nothing else it will bump it back to the top where it will get more visibility.

Mark

 

[FlipFlops365]

Re: 2002 Mercury 50HP Bigfoot 4-Stroke Tach Reading 7000rpms WOT - Alarm

I've had the boat out a few more times and noticed that it alarms even if the tach isn't reading wrong numbers. It's a constant alarm and according to the Mercury manual could be (oil pressure, coolant sensor, overspeed, high voltage, overheat).

I never had these issues until I installed a Blue Sea Add-a-Battery which has a dual circuit and allows you to run two batteries (one for the house electronics and the other for the motor). It charges both batteries but keeps the motor on it's own battery so if you leave the stereo on all day at the sandbar and kill the battery, you can still start your motor. I'm going to double check all of my battery connections and report back.

Kind of stepping out on this one. On 2 strokes the tach gets fed by impulses from the flywheel via a switch box in the engine. These impulses go into the tach and are counted. The tach has a selector switch which determines how many pulses equal how much deflection.

I'd say you have an intermittent problem with your tach, but with the other things you mentioned, I don't think so.

What is the nature of the alarm....solid or beeping? Solid is usually over temp and beeping would be low oil for 2 stroke engines,,,,,don't know if oil is monitored on 4 strokes.

Do you have an over rev. limiter on that engine? If so, and your engine slows down something is feeding it erroneous information causing the engine to slow down, so it could be wiring making intermittent connections, a bad switch box, or possibly even a stator up under the flywheel.

Nobody responded to this so I just tried to help. Am in unfamiliar ground here. If nothing else it will bump it back to the top where it will get more visibility.

Mark