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Old 05-24-2014, 01:07 PM   #1
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Tach reading low on bridge

I have one engine tach on bridge showing 1000 rpm readout and engine is running at 1800 according to tach in lower helm. Any ideas on what the problem is?
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Old 05-24-2014, 01:47 PM   #2
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Calibration would be my first thought. Most tachs are adjustable. Check with a hand held tachometer. If you can't borrow one, they're available on Amazon or eBay for less than $15.
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Old 05-24-2014, 01:52 PM   #3
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Thanks for your reply! It was working fine for the past few hours then suddenly stopped.
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Old 05-24-2014, 02:09 PM   #4
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You might have a lose or resistive connection. I would try taping on the dash or on the upper tach also try wiggling the wires associated with that tach circuit. It doesn't sound like a calibration issue from your description.
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Old 05-24-2014, 05:24 PM   #5
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Tachometer prob

Thanks for replies. I'll check for loose connection.
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Old 05-25-2014, 07:29 AM   #6
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I'll check for loose connection.

LOOSE means electrically lousy , not usually mechanically loose.

This requires removing the end and cleaning the end and its attachment spot. SS toothbrush works fine.

One trick is to obtain brass star washers of the stud or screw size.

Then install the washer after cleaning.Inside or outside teeth , does not matter.

It will help ID the ones you have cleaned , and 10x more important it will keep the terminals in good contact after a high current heating episode.
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Old 05-26-2014, 11:58 AM   #7
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I checked and cleaned connections at the bridge tach but that did not rectify. Running this morning on way home to Galveston, unit went from 300 rom low to normal and then back to 300 low. My thought is to check the tach readout unit with jumpers when I get back to port. Can you tell me if GB has a single or dual transmitters at engine. I'd think single with the two racha daisy chained. Is that right?
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Old 05-26-2014, 12:00 PM   #8
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Not racha, meant tachs.
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Old 05-27-2014, 07:46 AM   #9
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Switch it with the other engine. If the problem moves, its the tach itself.
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Old 05-27-2014, 08:59 AM   #10
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Most tachs, nowadays, don't have senders, but work off the alternator. (They even change slightly when you change or adjust a belt.)

Since one tach works fine, I would trace the offending wire back to the alternator checking and cleaning each connection as you go. Also the ground to the offending tach may need attention. I doubt if it is more than that since the other tach is fine.
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