Author Topic: tach question  (Read 232 times)

1sgtretired

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tach question
« on: October 09, 2018, 09:57:08 AM »
 72 TA, originally came with the unitized ignition.  When I purchased the car it had an MSD distributor set up installed.  Also, when I purchased the car the tach didn't work.  Long story short I replaced the MSD set with a HEI system.  Figuring out the wire mess I had I traced the brown wire from the tach into the engine compartment part of the wire was replaced with a different color wire.  Found the shaker wire taped up in a bundle.
The question does the tach have to be calibrated to the HEI distributor?  The tach currently shows idle at about 750 RPMs but when revving the engine the tach only will go to 2000 RPMs.
My next though was to hook up a tach and dwell meter and check it that way after 8 hours had to take a break.

Any thoughts would be appreciated. 
USAF 1st Sgt Retired

1970 RAM AIR III TA. White
1972 455 HO, Blue
1973 Formula, Functional Ram Air set up
1973 455 TA, Red
1976 400 TA sold
1978 Black W-72 TA
2006 Chevy 2500 HD Turbo Diesel
2011 3500 Chevy Turbo Diesel
2012 BMW X5

firebirdparts

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Re: tach question
« Reply #1 on: October 09, 2018, 01:33:16 PM »
I am not an expert on this, but I think it should work.  I think GM may have had a different tach for HEI vs points, but it seems like unitized ignition and HEI are very close to the same thing.  Since you have multiple cars, I might run a wire over to another one, and a common ground, and see if the tach works for a different car that doesn't have HEI, if you have one.

When we were parting out cars I tested all tachs on an F-150.  I wired it for Trans Am tachs under the hood.
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1sgtretired

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Re: tach question
« Reply #2 on: October 10, 2018, 01:05:58 PM »
Interesting I do have a couple cars I could try that with.
Thanks
USAF 1st Sgt Retired

1970 RAM AIR III TA. White
1972 455 HO, Blue
1973 Formula, Functional Ram Air set up
1973 455 TA, Red
1976 400 TA sold
1978 Black W-72 TA
2006 Chevy 2500 HD Turbo Diesel
2011 3500 Chevy Turbo Diesel
2012 BMW X5

Jack

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Re: tach question
« Reply #3 on: October 23, 2018, 06:22:10 PM »
George, just seeing this but thinking about it some more I'm convinced that the circuit and the wiring is not the issue. I still think that the tach circuit and coil is the issue especially since the tach is working fine and reading correctly up to 2,000 rpm (with 700-750 RPM at idle).

With both points and HEI the tach wire converts the engine pulse to a voltage that is sent the the tach circuit. The tach circuit is nothing more than a coil of wire wound around a core and the pointer is attached to it. When voltage is applied the coil creates a magnetic field which pulls the needle to the correct reading. Often dust or debris gets collected and it gets stuck at certain RPM. Newer tach (and after market tachs) use solid state circuits and therefore will not have similar issues.

The coil and circuits do go bad and often when that happens the tach will simply not work (or work intermittently). If the coil is weak most likely the needle will bounce around. But when the tach is working correctly to a certain RPM I would think that dust, debris or rust is the issue (not much clearance between the coil and core). As I have suggested before sometimes by gently moving the needle past the stuck point will free it up enough for the tach to work normally BUT you always run the risk of taking it out of calibration.   

Finally, whatever you do do not blow compressed air to clear an debris the circuit will simply fall apart.




Regards, Jack

Re: tach question
« Reply #3 on: October 23, 2018, 06:22:10 PM »
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