Volvo tachometer

[bowring]

Looking for help to diagnose my faulty tachometer problem

Volvo D2-55

Tachometer runs at full RPM
sometimes reads correctly but only for a short time, then full
Sometimes doesn’t work at at
The hours are showing

Thinking it could be the sensor? - what does that look like and where to get a new one

Any advice would be great - thanks

[Pandor]

Are your batteries fully charged? When the batteries are fully charged, the internal regulator disconnects from battery, this also stops the pulses needed to the tach. After the battery voltage drops, the regulator reconnects to the battery and pulses to the tach are restored. It is an impractical feature, but that is how it works.

[bowring]

Originally Posted by Pandor

Are your batteries fully charged? When the batteries are fully charged, the internal regulator disconnects from battery, this also stops the pulses needed to the tach. After the battery voltage drops, the regulator reconnects to the battery and pulses to the tach are restored. It is an impractical feature, but that is how it works.

Yes they are charged but it still does it when they aren’t, most of the time, straight way and if not then within 1 minute of starting

Also if it disconnect then wouldn’t the reading be 0

[Pandor]

The tach wire usually connects to alternator post labeled ‘W’. With the engine running, you should see 5-8 volts AC when you test it with your multimeter.

It could just be a loose connection at the alternator or at the instrument. But I’d look at the alternator first.

[Wotname]

Originally Posted by bowring

Looking for help to diagnose my faulty tachometer problem

Volvo D2-55

Tachometer runs at full RPM
sometimes reads correctly but only for a short time, then full
Sometimes doesn’t work at at
The hours are showing

Thinking it could be the sensor? - what does that look like and where to get a new one

Any advice would be great - thanks

I think it is unlikely to be the sensor.

I can't easily see how any sensor or wiring fault could vary between full scale, no scale and correct rpm reading. The symptoms suggest (to me) an indicator fault.

[Pandor]

Tachometers do go bad, but you’ve got to start the troubleshooting somewhere. To properly test a tachometer wire will require a Digital Multi Meter capable of measuring AC voltage. Note that a typical tachometer signal is not true AC voltage but pulsed, DC voltage. A Digital Multi Meter is not fast enough to read the pulsed DC voltage and this is why you need to use the AC voltage setting. Set the meter to AC voltage. Connect the negative Meter lead to a good ground on boat and the positive lead to the tachometer wire. Start the engine and wait for it to idle down to normal idle speed. At this point the meter should be displaying a fairly constant AC voltage. If this is the case, rev the engine up and down and observe the reading on the meter. The AC voltage should rise when the engine RPM’s rise and fall when the engine RPM’s fall. If so, then yea, it is likely the instrument that is the problem.

[DougR]

The tachometer drive on the D2-55 is completely unrelated to the alternator.
The tach is driven by a inductive sender mounted in the flywheel housing “counting” pulses generated by machined slots in the flywheel. The photos show the sender and it’s location in the housing.

If your engine model is D2-55A,B, or C, then the signal from the sender was sent directly to the tach where it was converted to needle readout. If you have a later model engine, then the signal was sent to the MDI box, converted into CanBus language (J1939) and placed on the can bus where it travels to the instrumentation.

You can check the output of the sender as mentioned up thread ( pulsed DC), and also check connections depending on the vintage of the engine. Given the symptoms of the instrument, I’m inclined to agree with those who speculate that it’s an instrument issue.