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post #1 of 12 Old 05-05-2019 Thread Starter
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Missing RPMs

Try to solve this one....
Shiva, unfortunately was not used for 1 year due to my back surgery. We sailed her to winter storage on 1 Nov 2017 and everything mechanical/engine wise was working perfectly.

Boat was hauled and then launched on 1 Nov 2018 and we motored to in water winter storage on a freshly painted bottom. 34 year old Volvo MD17D started right up much to my surprise. We moved very quickly and motored or motor sailed the short hop from Norwalk to Stamford. We were doing hull speed and with the motor on that means around 2000 RPM. But the tach read 1200 or so. Engine sounded normal, we were doing hull speed and I suspected the tach was lying.

This spring I recommission the engine and again the tach was reading low. I got a hand held laser tach and confirmed that... yes 1200 RPM on the tach was actually 2000 or so.

Why was the tach reading low? The reading was consistently about 60% of the true RPMS... idle was now down around 500 or so.

Removed the transducer from the engine, cleaned it and remade the crimp connectors. No joy.

Tomorrow I will clean and possibly remake to wire connections at the tach. If the tach as means to adjust the reading I can make the adjustment. Don't know what I will find back there.

I will also test the resistance of the tach wiring which I think should be 0 ohms

If the wire clean up at the tach doesn't work... I think I am looking at a new transducer or a new tach.

Where did the RPMs go? Any suggestions?

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post #2 of 12 Old 05-05-2019
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You will not read a true 0 ohms on those circuits. Below 5 ohms should he good.

The sensor itself is just a magnetic rod with a coil of wire around it that reads notches on a reluctor wheel. Vibrations can cause this coil of wire to break and change the internal resistance which will lead to funky readings.

If your circuit checks out I would change the sensor.
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post #3 of 12 Old 05-05-2019
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Re: Missing RPMs

I'm interested in what you find out is the issue. Do you get appropriate readings up to a certain rpm, then they diverge? Or does yours consistently read 60% low across the board?

We also have a similar issue with one of our tachs. It reads correctly up to ~1400rpm, then increases slowly until it maxes at 2000rpm.

We are lucky for troubleshooting to have twin engines, so I have switched the sensors (no change), and switched the tachs - the problem followed the tach to the other engine. So must be a bad tach - but both tachs are almost new, having been installed in the past year.

I'm hoping you find some setting issue, or other simple thing that will keep me from having to buy a new tach.

Mark

Manta 40 Catamaran "Reach"

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Re: Missing RPMs

Thereís a place in fla that rebuilds tachs for short money. Do a good job. Lauderdale speedo. I had a turn around of less than a week.
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post #5 of 12 Old 05-05-2019 Thread Starter
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Re: Missing RPMs

It's a VDO and I will see if I can pin the culprit down tomorrow.

I found almost the identical one... for 125$ new in opened box... so I may be buying it. It's likely a newer version of the same series. New they are north of 400$

Reading is a consistent 60% of what it actually is.... doesn't flutter... just reads low.

Mark I still have not opened the box... stand by... thanks for your patience.

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Re: Missing RPMs

Sensors do fail, they even come DOA out of the new box. I'd also suspect the sensor to have failed, before the instrument itself would have "miscalibrated" itself. Checking the wiring at all points (and sometimes there are splices hiding in the harness is a good point. IIRC Volvo is like most everyone else, they didn't bother with tinned wire. Even if mice didn't get in and chew on it. (Yeah, unlikely, but sooner or later, unlikely happens.)
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post #7 of 12 Old 05-06-2019 Thread Starter
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Re: Missing RPMs

Cleaned the connections at the tach... I am ordering a new transducer/sender.... and will see what happens. Stay turned.

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post #8 of 12 Old 05-09-2019 Thread Starter
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Re: Missing RPMs

Replaced the sensor/sender... no joy.

Did not test resistance of the 2 sender wires...

Next up... a new gauge

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Re: Missing RPMs

Thatís too bad. Iíd want to know there was integrity in the wiring between the sender and gauge before I threw more money at it.


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post #10 of 12 Old 05-09-2019 Thread Starter
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Re: Missing RPMs

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Originally Posted by Minnewaska View Post
Thatís too bad. Iíd want to know there was integrity in the wiring between the sender and gauge before I threw more money at it.
The gauge reads but at 60% of correct reading. My thinking was the sender was not sending the correct voltage or pulses or whatever... but that was not true apparently... Now I am thinking the gauge is getting the voltage or pulses but is damaged... I don't think there is anything serviceable on it.

It was working Fall 17, boat not used in 18 except when I moved to winter storage when I discovered something was off. I got a laser tach and confirmed that... then winterized the boat... and after recommissioning this past March I am trying to solve this mystery.

Thanks!

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