tach issues

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2 fast z28

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hay guys wondering if anyone had this problem I have a brand new Stewart warner digital tach and shift light when I'm doing a pull the tach will start to not read accurately at the top of a pull around 4600 or so what it does it will read 4600 then 5200 back to 4300 and just bounce around and the shift light will come on and off and back on the car doesn't miss of stall its smooth I have a recurved stock mopar dizzy I have it converted to a gm hei ignition module from Davis unified and and a msd blaster ss coil and I have the tach input hooked up to the negative side of the coil any help would be great and I also had this problem with a different tach I had in the car
 
2 fast z28 said:
........... brand new Stewart warner digital tach and shift light when I'm doing a pull the tach will start to not read accurately at the top of a pull around 4600 or so what it does it will read 4600 then 5200 back to 4300 and just bounce around and the shift light will come on and off .................recurved stock mopar dizzy I have it converted to a gm hei ignition module from Davis unified and and a msd blaster ss coil and I have the tach input hooked up to the negative side of the coil ...
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What is the model of the tach, link to destructions? I would think that would work fine, barring bad connections, and watch the tach wire routing in relation to plug wires, etc. Keep the tach wire "dressed" close to the firewall IE a metal ground plane. and away from the coil / plug wires
 
I would also check ground connection, from engine to chassis. Is tach wiring grounded at engine or chassis? How and where is the HEI grounded, and tach wiring grounded? The distributor body needs a ground too.

Tachs are triggered by the (-) coil terminal going low to high. If grounds are poor, there will be voltage such that the signal does not go low enough. It must also go high enough. It goes high, a few hundred volts when ignition happens. At higher RPM, the ignition interval is cut short, to alow more time to charge the coil. It could be the tach has a problem with that.

Using a high voltage probe and scope on coil (-) may be helpful to visualize the voltage.
 
hi guys update here added ground to the ignition module and now my tach issue is gone thanks for the input always helpful
 
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