Tach Question: 280zx dist. in a 240z

[Hardway]

I know this has been covered a million times and I have spent the last 1.5 hours trying to find an answer but so far have not.

The PO installed a 280zx distributor Accel Super stock "yellow box" coil in my July '72 built 240z. The car ran and the tach worked. The resistor was also being utilized. Once I got all my projects on the car done I went to start it and had no spark even though I had not touched any of the underhood electrical components. With my multi-tester I determined I was not getting any power to the coil or the distributor. So I went about unwrapping the electrical tape from all the wiring. Everything was in pretty bad shape so I started over. I got the car running but now the tach does not work and I cannot figure out why. Below is the wiring I have.

B/W wire that gives 12V w/key turned run that is connected to pos. side of the coil.

B/W wire that gives nothing w/key turned to run and currently not connected to anything.

G/W wire that gives nothing w/key turned to run and currently not connected to anything.

B wire from the C terminal on the distributor connected to neg. side of the coil.

Y wire from the B terminal on the distributor connected to pos. side of the coil.

Does the tach need the resistor under the hood to operate? If so how do I hook it up? I used the diagram below and tried connecting the B/W and G/W wires together that are currently not connected to anything but no response from the tach. I also tried connect them to the pos. side of the coil and nothing either. If connect any of them to the neg. side of the coil it kills the engine. My fuses are good and all my dash lights and other gauges work fine. Any guidance is greatly appreciated.

[beermanpete]

You probably have the 2 B/W wires reversed. If the wiring to the tach and back is in good condition the G/W wire and the "other" B/W wire should have +12V when the key is in the START position. Those 2 wires should also have continuity with each other and measure very low resistance, < 1 ohm. If not there is another wiring problem to find and fix.

[SteveJ]

Your circuit drawing is correct. Let's go into that a little more. Attached is a simplified drawing of the stock circuit.

The White/Red wire is the 12VDC+ source from the battery. In the Start position, it connects to the Green/White wire on terminal 5 and goes through the tachometer to the coil.

In the Run position, it connects to the Black/White wire on terminal 2 which goes to the ballast resistor and over to the Green/White wire.

Technically, you could jumper terminals 2 and 5 together and that would bypass the ballast resistor. The bottom line is that with the ballast resistor bypassed, the B/W and G/W wires are electrically the same.

To test your wiring, look for continuity between the G/W wire at your ignition switch and the Green/White wire at the tach.

Look for continuity between the B/W at your ignition switch and the G/W at the ignition switch.

With the tach unplugged, look to make sure you DON'T have continuity between the B/W at the switch and the B/W going to the tach. If you do have continuity there, you managed to bypass your tach. The car will run, but the tach won't get a signal.

Edited August 10, 2012 by SteveJ

[Hardway]

Thank you Steve J. I followed your diagram and presto, working tach! Not only does it work but according to my dwell/tach box its pretty spot on.

[SteveJ]

Did you leave the ballast resistor in place or did you jumper it out? With the ZX distributor, all the ballast resistor does is reduce your spark voltage.

[Hardway]

I actually kept the ballast resistor. Since everything works now and sounds strong I will leave it as is. If I experience a drop in performance once I get to drive some I will take the resistor out. Just to confirm if I did so, would I connec the the 12V b/w wire to the g/w wire directly? Will this hurt the tach by doing so?

Edited August 10, 2012 by Hardway

[SteveJ]

You could just move the B/W wire to the other side of the ballast resistor, so they land on the same terminal.

[Hardway]

You could just move the B/W wire to the other side of the ballast resistor, so they land on the same terminal.

Okay, so I did a quick test, I dont think my resistor is resisting. I have 11.9v on each end of the resistor. In order to wire the resistor out would I take my switched run 12v b/w wire, connect it to the pos. side of the coil along with the g/w wire and the other b/w wire? In order to power the distributor would I also connect it to the pos. side of the coil since that is where my 12v source is connected to? I just want to make sure I don't mess up my tach.

[SteveJ]

My guess is that the wires are joined elsewhere. Resistors tend to fail open and not closed. If you have about 12VDC at the coil, you're fine.