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Old 03-10-2015, 12:31   #31
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Re: Engine Tachometer shows lower RPMs than expected

Both of mine were off and not wanting to adjust 30 year old tachs, I made a cheat sheet.
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Old 03-10-2015, 16:58   #32
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Re: Engine Tachometer shows lower RPMs than expected

Believe this could be related to the different tooth set on the flywheels. 97/114. It is interesting just how long some of these issues remain after years!! If you can find someone with the skills to remove or replace the bessel then it is pos to adjust the reading to make them read correctly. Just do as Terry is the easy answer.

Very unlikely to be a faulty sender.
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Old 04-10-2015, 14:01   #33
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Re: Engine Tachometer shows lower RPMs than expected

Originally Posted by Pizzazz View Post
Bringing closure to this topic. I checked the RPMs with a laser tach ($14.99 from Amazon) and it is all according to the manual. Idle is at 900, WOT at 3,600 in neutral.

The Yanmar tach measures 500 at idle and 2,900 at WOT.

One way to go from here is to start replacing the sending unit, the dampening system of the tach or the entire tach. Another way is to consider upgrading the sender unit with some more modern device that can send NMEA/Wifi output.

For now, I just put a never exceed duct tape marker on the tach glass at 2,700 RPMs

Thank you.
Glad you got to the bottom of it! My Yanmar tach is accurate but both my temp and oil pressure gages are off. Temp always reads slightly into the red and oil pressure reads about 25 when it should be higher. I also wanted to get to the bottom of my gage issues and verify all was well with my engine so had my local diesel mechanic measure oil pressure and I used a heat gun to determine temps were actually normal. My diesel mechanic contacted Mack Boring to see about replacing gages or sending units and they recommended that we not bother. According to them, the Yanmar gages/sending units are notoriously inaccurate and even if we replaced either or both, they couldn't guarantee any improvement in accuracy from what we have now. That comes from the guys who sell the parts we'd need to buy! They said to verify that pressures/temps are within spec by independent means and note what gages say, and as long as the gages keep reading close to that amount, all is well.

You mention a "never exceed" line on your tach that's less than your actual maximum. It won't hurt anything but it's not really necessary on a diesel engine that's properly propped.
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