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Old Sep 14, 2023 | 09:09 AM
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Default Tachometer
Hello, was out last night in my 1980 and noticed two things. My tach is showing at least 1500 RPM's above what it should be showing. The other thing I noticed was the alternator light turned on a few times while it was idling. Any connection between the two? Thanks.

Update. I bypassed the Tachometer filter. No change.
Last edited by Rich80; Sep 14, 2023 at 12:41 PM.
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Old Sep 14, 2023 | 03:40 PM
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Check your alternator output with a DC voltmeter and check for AC current to see if you have a failing diode. Fix that first and see how the tachometer reacts.
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Old Sep 16, 2023 | 08:52 AM
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Originally Posted by MelWff
Check your alternator output with a DC voltmeter and check for AC current to see if you have a failing diode. Fix that first and see how the tachometer reacts.
I'm not an electrical guy, but just how do you check for AC = at what points ? untrained/unprofessional wants to know

Thanks
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Old Sep 16, 2023 | 10:39 AM
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Originally Posted by 1971CorvetteII
I'm not an electrical guy, but just how do you check for AC = at what points ? untrained/unprofessional wants to know

Thanks
same as DC, output terminal of alternator, set multimeter to AC
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Old Sep 17, 2023 | 01:31 PM
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[QUOTE=MelWff;1607050281]same as DC, output terminal of alternator, set multimeter to AC[/QUOTE)

Looks like I am showing AC voltage?
Battery measured 12.6. (Charged)
14.6 when running
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Old Sep 17, 2023 | 01:51 PM
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[QUOTE=Rich80;1607053383]
Originally Posted by MelWff
same as DC, output terminal of alternator, set multimeter to AC[/QUOTE)

Looks like I am showing AC voltage?
Battery measured 12.6. (Charged)
14.6 when running
Yes, you are showing AC
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Old Sep 17, 2023 | 01:59 PM
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Change out the alternator and see if the tachometer settles down.
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Old Sep 18, 2023 | 03:13 PM
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New alternator no change. Voltage spikes probably toasted the board. Ordering a pre calibrated board. Last thing I want to do is reset that needle.



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Old Sep 18, 2023 | 06:02 PM
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Resetting needle is very easy..should be zero with 12v

Is it zero now ignition on engine off?

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Old Sep 18, 2023 | 10:29 PM
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Ignition on Engine off tach goes down to 1500. When I start it the tach reads 2500 to 3000. The engine is actually running at around 1000 to 1200. When I shut it off the tach sits at 3000.
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Old Sep 19, 2023 | 09:23 AM
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Originally Posted by Rich80
Ignition on Engine off tach goes down to 1500. When I start it the tach reads 2500 to 3000. The engine is actually running at around 1000 to 1200. When I shut it off the tach sits at 3000.
i would reset to zero... and see..
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Old Sep 23, 2023 | 03:18 PM
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New board worked. I noticed the new board has a potentiometer for calibration if needed. Old board had a burn mark.

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Old Sep 23, 2023 | 03:20 PM
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Thanks for all the help! Much appreciated!
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