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Old 08-08-2013, 10:33   #1
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Tach does not Always Register

Tach goes to correct rpm as soon as engine starts. Only sometimes it will stick at that rpm after increasing the throttle to cruising speed (around 2000 rpm). After a few minutes it will usually move to correct rpm. 30 year old sailboat, 50 HP Perkins 4-108 motor. Any suggestions?
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Old 08-08-2013, 12:01   #2
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Re: tach does not always register

sticking meter?
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Old 08-08-2013, 17:30   #3
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Originally Posted by BozSail View Post
Tach goes to correct rpm as soon as engine starts. Only sometimes it will stick at that rpm after increasing the throttle to cruising speed (around 2000 rpm). After a few minutes it will usually move to correct rpm. 30 year old sailboat, 50 HP Perkins 4-108 motor. Any suggestions?
I have a 4108 and mine does it also. I would like the answer.
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Old 08-08-2013, 17:41   #4
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Tac is controlled by the alternater.
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Old 08-08-2013, 18:01   #5
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Re: Tach does not Always Register

Your tach quits when the alternator stops putting out if it's an electronic tach. Not enough information here to figure out why. Is there a second alternator? I've run into the problem of a Balmar kicking in at above idle, and leaking the voltage into the engine battery through the regulator via the ignition circuit. that caused the smaller alternator to shut off and kill the tach. The same can happen when plugged into the ac charger and it's putting higher voltage than the alternator. So you need to watch the output voltage of you alternator to see if it stops putting out when the tach goes out.
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Old 08-08-2013, 18:25   #6
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Re: Tach does not Always Register

I have the same issue with my Tach with my Perkins 4-108... Even after I replaced the alternator. The meter is about 8 years old... Probably time t replace it... I just have this feeling it's something else though. Checked all the ground connections. Took them off & remounted them.
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Old 08-08-2013, 18:35   #7
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Externally or internally regulated?

Fair odds it is an alternator or regulator issue. Ive seen this type of problem more w external regulators.
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Old 08-08-2013, 18:58   #8
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Tach goes to correct rpm as soon as engine starts. Only sometimes it will stick at that rpm after increasing the throttle to cruising speed (around 2000 rpm). After a few minutes it will usually move to correct rpm. 30 year old sailboat, 50 HP Perkins 4-108 motor. Any suggestions?

The ground wires should be serviced.
The ground from engine to battery and altenator to engine block. Weak grounds will hinder current flow and affect readings.
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Old 08-08-2013, 19:11   #9
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Re: Tach does not Always Register

Took one of the VDO tachs apart a week ago and removed two dead flies--it works a lot better now.
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