I have installed a 3 wire tach in my 71 240Z with stock L24, 280ZX ignition, and MSD Blaster 2 coil. I spliced the positive and ground wires into the original 4 wire tach harness, and ran a wire with a 2200 ohm resistor in line from the negative terminal on the coil to the tach.

When I start the car the tach indicates what appears to be the correct RPM for 3 or 4 seconds, then drops to zero.

Revving the engine results in a reading on the tach which seems to be too high (for example, about 2800 RPM at 60 with stock 4 speed and 3.36 axle).

Also, whenever an electrical load is applied (turn signals, 4 way flasher, heater blower, headlights, any or all) the tach needle jumps to a higher RPM and back continuosly.

Bad tach or did I miss someting in the installation?

I'd have to look at the wiring diagrams to be sure, but it sounds like a grounding issue to me.

I tried grounding directly to the negative battery terminal. No change in behavior!

SteveJ you were right! It took me awhile to figure it out, but the problem was with grounding.

I finally realized the tach signal comes from the negative terminal of the coil, which, since it is negative, must also be grounded. I cleaned the back if the ignition module and the dizzy body where it attachs. When it was making a good contact with no corrosion the tach works perfectly!

SteveJ you were right! It took me awhile to figure it out, but the problem was with grounding.

I finally realized the tach signal comes from the negative terminal of the coil, which, since it is negative, must also be grounded. I cleaned the back if the ignition module and the dizzy body where it attachs. When it was making a good contact with no corrosion the tach works perfectly!

I just pointed you in the right direction. You did the real problem solving.