Tachometer shows wrong - Impala Tech
Electrical & Wiring Troubleshooting electrical problems

 
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post #1 of 5 (permalink) Old 06-12-2016, 02:10 PM Thread Starter
 
 
Join Date: Jun 2016
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Tachometer shows wrong

Hello everybody.
I have problem in my 67 Impala 327 . I am going to install a tachometer. All very simple: plus to plus, negative to ground and signal to negative coil. And the tacho shows double revs!! Tacho have a selector for number of cylinders. All set on 8, there is only one distributor - generaly can't be more simple. I checked all cylinder selections variants on tacho . Shows different numbers of revs but never proper. I even exchanged the tacho for another one and the same problem appears. Any ideas, please???
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post #2 of 5 (permalink) Old 06-12-2016, 02:33 PM
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Welcome to the Team!

By any chance do you have an ignition box? If so the plug is fired multiple times instead of just once. The tach is counting the number of times the coil fires, so with a CD ignition box it could be firing up to four times per cycle.

MSD and Accel offer a Tach Terminal on their ignition box to prevent this false reading. If you have a Petronix Flame Thrower III it acts like an ignition box but doesn't have a terminal for the tach. They use an electronic device that generates a square wave to activate the tach based on RPM.

Big Dave
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post #3 of 5 (permalink) Old 06-13-2016, 02:22 AM Thread Starter
 
 
Join Date: Jun 2016
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Hi Big Dave,

Thank you for a quick reply. There is no ignition box. This car is total analog. Simple coil and capacitor on it and distributor .Nothing else.
What course an extra signal?

Best regards
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post #4 of 5 (permalink) Old 06-13-2016, 10:18 AM
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If it is exactly double the RPM set the tach for a four cylinder.

Big Dave
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post #5 of 5 (permalink) Old 06-13-2016, 01:17 PM Thread Starter
 
 
Join Date: Jun 2016
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I tried already . What is interesting that shows the same double revs on 4 and 8 cylinders. I think I will have to get another Tacho and compare.
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