Volvo Penta MC11C rev counter problem

[christopherb]

The rev counter has started to show 1200 rpm when I turn the key on and before starting the engine. When I start the engine, the reading increases in proportion to the throttle setting. The engine and rev counter are only a few years old and the rev counter unit itself seems in good order. I can't see any calibration screws on the back of the instrument. The connections to the unit and the alternator all seem to be clean and secure. Any suggestions on trouble-shooting this?

Edit: I've just seen other threads that say the rev counter isn't connected to the alternator but to a sensor on the emgine (flywheel?). I'll check that next time I'm on the boat but if that had failed surely the rev counter wouldn't work at all?

 

[Plum]

The rev counter has started to show 1200 rpm when I turn the key on and before starting the engine. When I start the engine, the reading increases in proportion to the throttle setting. The engine and rev counter are only a few years old and the rev counter unit itself seems in good order. I can't see any calibration screws on the back of the instrument. The connections to the unit and the alternator all seem to be clean and secure. Any suggestions on trouble-shooting this?

Edit: I've just seen other threads that say the rev counter isn't connected to the alternator but to a sensor on the emgine (flywheel?). I'll check that next time I'm on the boat but if that had failed surely the rev counter wouldn't work at all?

Different model but on my VP: 1) when I turn the key to stop the engine the rev counter stays at the revs that the engine was at before I stop the engine. There is no spring return. With the engine stopped when I turn the key on again the needle drives to zero, there is no physical "stop" that the needle rests against. 2) when I dismantled my rev counter to fit a new hours counter display, you have to push the needle onto the spindle at the zero position as that is the only way to set its correct zero display.

 

[christopherb]

Different model but on my VP: 1) when I turn the key to stop the engine the rev counter stays at the revs that the engine was at before I stop the engine. There is no spring return. With the engine stopped when I turn the key on again the needle drives to zero, there is no physical "stop" that the needle rests against. 2) when I dismantled my rev counter to fit a new hours counter display, you have to push the needle onto the spindle at the zero position as that is the only way to set its correct zero display.

It's not the same as mine. To clarify:

1) There is a physical stop at the 0 position.
2) When the power is off the needle drops to the 0 position and rests on the stop, but when I turn the power on it jumps up to indicate 1200 rpm.
3) There is no hours counter on the tacho, there is a separate hours counter installed at the back of the instrument panel, visible from inside the boat.

 

[christopherb]

I just asked ChatGPT about this and this was what it said:

Your Volvo Penta MD11C tachometer issue sounds like a grounding or voltage-related problem. Here are some possible causes and troubleshooting steps:

Possible Causes:​

1. Faulty or Missing Ground Connection
   
   • If the tachometer is not properly grounded, it can pick up stray voltage, causing incorrect readings.
   • A floating ground can result in the 1200 RPM reading when the engine is off.
2. Incorrect Power Supply Voltage
   
   • Check if the tachometer is receiving the correct voltage (usually 12V or 24V, depending on your system).
   • A voltage drop or irregular power supply could cause false readings.
3. Faulty Signal from the Alternator
   
   • The tachometer usually gets its signal from the alternator (W terminal).
   • A faulty alternator regulator or a failing diode can cause erratic readings.
4. Interference or Inductive Pickup Issues
   
   • If the tachometer is an inductive type (detecting pulses from the alternator), electrical noise or interference might cause incorrect readings.
5. Defective Tachometer
   
   • Internal issues with the gauge could cause it to display an incorrect base reading.

Troubleshooting Steps:​

1. Check the Ground
   
   • Ensure the tachometer has a solid ground connection. Try running a separate ground wire directly from the tachometer to the engine block or battery negative.
2. Measure Voltage at the Tachometer
   
   • With the power on but the engine off, measure the voltage at the tachometer terminals. If the supply voltage is unstable, the problem could be with wiring or the alternator.
3. Inspect the Alternator Signal (W Terminal)
   
   • If your tachometer runs off the alternator, check that the W terminal is properly connected and the alternator is functioning correctly.
4. Test with a Different Tachometer
   
   • If possible, connect another tachometer to see if it behaves the same way.
5. Check for Loose or Corroded Connections
   
   • Look for any loose or corroded wires between the alternator, tachometer, and battery.

If the tachometer reads 1200 RPM when the engine is off, a missing ground or incorrect voltage is the most likely issue. Start by checking the ground and voltage, then move on to the alternator signal. Let me know what you find!

When I next go to the boat, I'll try testing the ground connection and see if that helps. In view of the fact that it isn't driven off the alternator, my earlier investigations may have missed this.

 

[Bilgediver]

The rev counter has started to show 1200 rpm when I turn the key on and before starting the engine. When I start the engine, the reading increases in proportion to the throttle setting. The engine and rev counter are only a few years old and the rev counter unit itself seems in good order. I can't see any calibration screws on the back of the instrument. The connections to the unit and the alternator all seem to be clean and secure. Any suggestions on trouble-shooting this?

Edit: I've just seen other threads that say the rev counter isn't connected to the alternator but to a sensor on the emgine (flywheel?). I'll check that next time I'm on the boat but if that had failed surely the rev counter wouldn't work at all?

I am pretty sure this engine has an engine mounted transducer behind the rear cylinder head on the port side and has two terminals on the top. I doubt if there is a fault with the transducer or connections but something in the gauge itself.

 

[VicS]

The rev counter has started to show 1200 rpm when I turn the key on and before starting the engine. When I start the engine, the reading increases in proportion to the throttle setting. The engine and rev counter are only a few years old and the rev counter unit itself seems in good order. I can't see any calibration screws on the back of the instrument. The connections to the unit and the alternator all seem to be clean and secure. Any suggestions on trouble-shooting this?

Edit: I've just seen other threads that say the rev counter isn't connected to the alternator but to a sensor on the emgine (flywheel?). I'll check that next time I'm on the boat but if that had failed surely the rev counter wouldn't work at all?

The rev counter sensor on an MD11C is mounted on the hand starting gear cover. It is part # 21 in the parts diagram