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Old 02-14-2016, 06:53 AM   #1
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Cummins Tachs Not Working

Hi Guys,

We have a 1996 Shannon with twin Cummins and since buying one tach has been erratic sometimes, generally at low RPM.
Just (unfortunately) had to have two new turbos installed and now both tachs are erratic at times and yesterday when running in the slip they were both normal then they both went to 0 RPM. Restart and both working although the one was jumping which seemed to be in time with one of the alternator dials which was jumping slightly.

Any thoughts would be appreciated,
Bill
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Old 02-14-2016, 07:34 AM   #2
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Which tachs? Faria? VDO? ???


Might be the tachs themselves, nothing to do with the engines. I had to replace both of our Faria tachs...


-Chris
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Old 02-14-2016, 02:25 PM   #3
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Thanks Ranger 42,
They are VDO and tomorrow I'll check the connections. Hopefully that's all it is.

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Old 02-14-2016, 04:21 PM   #4
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If the tachs are getting their info as an alternating current signal from the alternators this signal can drop out when the batteries are fully charged and the alternators no longer get a "demand" for charging voltage to be supplied. Putting a load on the batteries can get the alternator(s) to again begin to put out current and cause the tachs to again begin to operate normally.
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Old 02-14-2016, 05:31 PM   #5
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Thanks StoneJD,

That could be a possibility, the batteries are being charged by a new 40 AMP charger I installed.

Thanks,
Bill R.
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Old 02-15-2016, 07:49 AM   #6
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I suspect these are fed by a magnetic sender at the flywheels.

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Old 02-15-2016, 09:23 AM   #7
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Next time you run them, turn charger off and see if that makes a difference. Alts don't make a tach signal if they are not making output. Can also happen if both engines share a battery bank, one alt goes to sleep and the other does the work.

Some Cummins use flyweel sensors, some use the alt. I think alts were mostly used on old units.
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Old 02-15-2016, 10:40 AM   #8
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Because of their intermittent indications I would probably check all electrical connections first.

Why did you replace both turbo's? Corrosion on the exhaust side?
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