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Old 08-07-2011, 05:55 PM   #1
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Calibrating Tachs

This topic came up on my "overheating" post.* (Which was all resolved with the impeller replacement).

I picked up an inexpensive photo tach to*calibrate my teleflex/morse tachs.* I located the calibration adjustment screw(s) on the back of the gauges...problem is they were already "dialed down" as much as possible.

I chose to dial them "up" so that they would read exactly 2X what the actual engine RPM is.*

While I could easily live with this, I wonder if anyone knows how to reduce the signal the tachs get from the alternator?

Best as I can figure, the tachs were reading about 60% high (cruising at 7 knots @ "2500 RPM).* Engine now idles at 550-600 RPM.

Any thoughts?

Thanks!

Tom
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Old 08-07-2011, 06:34 PM   #2
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RE: Calibrating Tachs

I have calibrated scores of tachs....and I have no idea what you are trying to say.

What do you mean by dial them up 2x?

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Old 08-07-2011, 07:19 PM   #3
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RE: Calibrating Tachs

If I turned the adjustment all the way down the tachs would still read 1000 RPM at idle whiile the actual RPM was about 550.

Since I could not get them to read any lower I just calibrated them so they would read exactly twice (2X) the actual RPM.

So, I am looking for a way to reduce the signal to the tachs.

Thanks!
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Old 08-08-2011, 05:08 AM   #4
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RE: Calibrating Tachs

Most tachs will not be accurate for the full range , so cruise RPM would be my choice .

A calibration l table , similar to what you have for the compass deviation would work at other RPM.

Check the tach mfg site , sounds like the Tach dip switches are set up for wrong impulse count.
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Old 08-08-2011, 07:06 AM   #5
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RE: Calibrating Tachs

Diesel electronic tachs when set up correctly and matched to *the alternator, work well. My OEM*VDOs matched with OEM Prestolite alternators have done fine over the entire RPM range. As David, FF's 3rd line*and JayN suggest and state, something isn't right in your setup.

Lots of alternators, chargers, tachs and electronics get fried by experimenting. Be careful of internet advice too* - now may be the time for a accurate schematic to aide in troubleshooting. With dozens of*similar engines, alternators, chargers*and tachs*on this site, I'm sure posting a schematic would get you lots of feedback on not only tachs, but other issues too.
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Old 08-08-2011, 07:14 AM   #6
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RE: Calibrating Tachs

Tanks FF and David,

I'll check the tachs for DIP switches and also determine what type of alternator I have. Good tips!

Tom
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Old 08-08-2011, 07:22 AM   #7
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RE: Calibrating Tachs

I recently did a Neece Neville alternator upgrade (90 amp I think) and my tachs did not change.
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Old 08-08-2011, 10:29 AM   #8
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RE: Calibrating Tachs

My installation of a Power Line high output alternator drove my tach about 400 rpm high.

I found the trim screw on the back of the tach took care of it.
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