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Old 11-24-2017, 10:09 AM   #1
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Tach

Why is my tach bouncing every so often ?
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Old 11-24-2017, 10:23 AM   #2
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One of my tach's will do this too. I have noticed this happens when I am cruising with the generator on, and the battery voltage gets to high. I will turn off the inverter battery charger, the voltage will drop back to a lower value and things are good to go.
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Old 11-24-2017, 10:37 AM   #3
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I️ am cruising the icw with the generator on .. will turn off and see what happens .. thanks for the suggestion
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Old 11-24-2017, 11:15 AM   #4
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The tach is driven by the alternator. When the loads on the alt drop to nothing the voltage regulator drives excitation voltage down to nothing and you lose the tachometer output.
Turn a small dc load on like a light or dc refrigerator and that should fix the problem.
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Old 11-24-2017, 12:21 PM   #5
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I️ turned the inverter on and the tv, turned off the generator and that did seem help ...
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Old 11-24-2017, 12:35 PM   #6
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Has anyone tried the “Tiny Tach” that senses the pressure pulses in an injector pipe? Looks like a reliable and accurate possibility.
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Old 11-26-2017, 06:46 AM   #7
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We have a circa 1975 engine panel with Motorola tachs. A couple of questions since the subject is on the table:

1) All four of the tach's read slightly differently. Is there an adjustment screw somewhere?

2) The tachs do not move when the engines are started. It is necessary to rev the engines up to about 1500 rpm in neutral when the needles suddenly jump up. After that function is normal even down to minimum idle. Is the normal?
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Old 11-26-2017, 06:54 AM   #8
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Quote:
Originally Posted by Russell Clifton View Post
I have noticed this happens when I am cruising with the generator on, and the battery voltage gets to high. I will turn off the inverter battery charger, the voltage will drop back to a lower value and things are good to go.
The first item on my mental checklist for engine start is to turn the battery charger off. I can't see any good coming from either the alternators or the charger having voltage fed back to them. Battery charger is only on when engines are off and generator is running or on shore power. I think this is good practice.
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Old 11-26-2017, 10:18 AM   #9
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Quote:
Originally Posted by Roger Long View Post
We have a circa 1975 engine panel with Motorola tachs. A couple of questions since the subject is on the table:

1) All four of the tach's read slightly differently. Is there an adjustment screw somewhere?

2) The tachs do not move when the engines are started. It is necessary to rev the engines up to about 1500 rpm in neutral when the needles suddenly jump up. After that function is normal even down to minimum idle. Is the normal?
Our tachs do not read upon starting as the alternator does not start producing till the oil pressure comes up, I always thought it was to stop any load on the engine till pressure is up.

Our tachs have a screw on the back that will step the rpm's up or down.
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Old 11-26-2017, 06:07 PM   #10
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We have a circa 1975 engine panel with Motorola alt

2) The tachs do not move when the engines are started. It is necessary to rev the engines up to about 1500 rpm in neutral when the needles suddenly jump up. After that function is normal even down to minimum idle. Is the normal?

This seems odd to me. My old MOTs had an excite wire with a tab terminal. Do yours have this? Has anyone seen an oil pressure connection to a alt regulator??
It sounds to me that the MOTs are having to work in a self-excited mode like the later “one wire” alts.
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Old 11-29-2017, 12:59 AM   #11
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I recently re-calibrated my circa 1979 Motorola tacs. There is a small white plastic plug on the back of the tac that needs to be removed. Screw driver adjust thru the hole. I recently went to electronic readout on the MFD using Actisense. So now the Motorola tacs are more for back up but I still wanted them to be accurate.
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Old 11-29-2017, 06:00 AM   #12
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"So now the Motorola tacs are more for back up but I still wanted them to be accurate. "

Impossible as the belt slippage or pulley diameter changes on alt. driven units.

Only with a mechanical device , usually points in a housing , can a tach be close to accurate.

The new electric injection by computer is best chance at accuracy.
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Old 12-01-2017, 04:01 PM   #13
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Possibly a bad ground to the tach? My port gauge will either bounce or start reading high every now and then. It’s a bad ground, going to make a new connection tomorrow.
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Old 12-01-2017, 06:04 PM   #14
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Quote:
Originally Posted by Roger Long View Post
We have a circa 1975 engine panel with Motorola tachs. A couple of questions since the subject is on the table:

1) All four of the tach's read slightly differently. Is there an adjustment screw somewhere?

2) The tachs do not move when the engines are started. It is necessary to rev the engines up to about 1500 rpm in neutral when the needles suddenly jump up. After that function is normal even down to minimum idle. Is the normal?
Do the alternators show any charge indication before you rev the engines? if not and you have alternator driven tachs that's your answer - unfortunately that leads you do another question, why do you have to rev the engines to get the charging system working...
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Old 12-01-2017, 06:28 PM   #15
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My tach runs off of a flywheel sensor, but it can refuse to read on startup, especially if the batteries are down a bit. A little nudge on the throttle sets things in motion.
So it isn’t a problem that’s limited to alternator driven tachs, it has more to do with the alternators exciter, and possibly weak grounding.
I can’t fathom it being related to oil pressure other than just coincidence.
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Old 12-04-2017, 03:49 PM   #16
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I had the same problem sometimes good sometimes it bounced all over.

Had my mechanic take to a shop in Greenville, NC rebuild it and no more bounce.

Sorry I don't know the name of the shop,
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Old 12-04-2017, 04:38 PM   #17
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Originally Posted by DW1979OA View Post
I recently re-calibrated my circa 1979 Motorola tacs. There is a small white plastic plug on the back of the tac that needs to be removed. Screw driver adjust thru the hole. I recently went to electronic readout on the MFD using Actisense. So now the Motorola tacs are more for back up but I still wanted them to be accurate.
Where did you get actisense in US? Is it only metric? What engine do you have? Do you think it would work with Ford Lehman engine and lastly how much did it cost? I would love to input my engine data into my Garmin 7612, thanks
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Old 12-09-2017, 11:29 AM   #18
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Bouncing tach can be due to alternator belt worn, wrong tension and worn alternator bearings.
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Old 12-13-2017, 01:19 PM   #19
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Quote:
Originally Posted by Roger Long View Post
We have a circa 1975 engine panel with Motorola tachs. A couple of questions since the subject is on the table:

1) All four of the tach's read slightly differently. Is there an adjustment screw somewhere?

2) The tachs do not move when the engines are started. It is necessary to rev the engines up to about 1500 rpm in neutral when the needles suddenly jump up. After that function is normal even down to minimum idle. Is the normal?
They don't read immediately because your voltage regulator probably has a delay before it "turns on" the alternator. Mine is about a minute or two after the engine starts.
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