I have insalled a 3 wire tach into a 71 240Z, stock L24 with 280ZX ignition and distributor and MSD Blaster 2 Coil. I spliced into the original 4 wire tach harness for positive and ground, 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 engine, the tach indicates what seems to be the correct idle speed for 3 or 4 seconds, then drops to zero. Revving the engine gets a reading, but my seat of the pants fell is that the reading is too high (indicates about 2800 rpm at 60 with stock 4 speed and 3.36 axle). Also, whenever any electrical load is turned on, such as turn signal, 4 way flasher, or blower motor, the tach needle bounces to a higher RPM and drops back to whatever it read before applying the electrical load.

Bad tach or is something else needed in the installation?

All thoughts wil be appreciated!

Oops! Should have posted this in the electrical forum.

Can anyone advise how to delete this?

Wrong forum notwithstanding, you've a 3-wire tach in a 4-wire car. The 2800 @ 60 mph is almost dead right... Should be 3000 @ 60. Repost in the elect. forum.

I had a similar problem with a Z31 coil. Installed a typical automotive capacitor/condenser on the negative side of the coil and fixed it. If you're sure the three wires you connected are right for a 280Z tach operation (and they appear to be since you're getting RPM readings), it might just be electrical noise, which the capacitor will quench.

If you have a voltmeter that reads DC frequency (DC Hz), you can measure the frequency from coil to ground and do the math to check accuracy.

Posting here in case a moderator decides to just move the thread.

Frank you are not quite correct. The car has a 280ZX breakerless electronic ignition, which uses a 3 wire tach. Got the problem resolved by cleaning the back of the ignition module and the distributor body where the module mounts. The completed a good ground and the tach works fine now.

Zed Head, see above for what fixed my problem. I also posted the question in the right forum once I realized my error in posting here.

Thanks to both for your thoughts!

Glad you resolved your problem.