Tachometer Readings

[BlueZ31]

Hey all,

 

Just a quick question, I've got a MSD Blaster coil on my Z31 N/A and I've been fighting incorrect tachometer readings for quite awhile, both with the original ignition coil, an OEM replacement, and now with the MSD. On a cold start the tachometer reads correctly throughout the RPM range, but, once warmed up to operating temperatures the tach won't move past the 2,500 RPM mark (needle style guage). The MSD has less than 50 miles on it and I'd like to think it's not the problem.

 

Could this be a ignition control module (power transistor) going bad? It's the original unit, and the black plastic backing is cracked heavily, and I know that the semiconductor material in transistors degrade with prolonged heat exposure. I just need a few expert opinions before i spend the $120+ to replace it. The z31 FSM only shows a continuity test for the ICM, which doesn't always show that a component is still within working parameters.

 

Thanks for your time and happy driving.

Edited July 9, 2011 by BlueZ31

[NewZed]

Related problem by tach behavior only - I recently put a Z31 coil on my 76 and found that the tach would just bounce around between 500 and 1000 RPM indicated, no matter what the engine RPM. I guessed that the circuit from coil negative was getting extra noise so I put a condenser/capacitor on the coil negative and the problem went away. I assume that the tach circuit never saw the zero part of the square wave (my understanding of how the later tachs work - could be off) so couldn't count right.

 

Just one possibility, and an extra condenser/capacitor won't hurt anything. It might be a band-aid fix though, if the electrical noise is coming from an ignition module about to die.

[michaelp]

The Z31 has a 2.2k Ohm resistor between the tach and the negative of the coil from the factory, not sure how that affects using Z31 coils in other cars and vice versa.