SteveJ

The closest I could find with a quick search was here: https://zcarsource.com/seat-upholstery-set-240z-70-73-butterscotch-vinyl-new/

For the IAC, if you need a switched source, the black/white at the voltage regulator is a better choice. That wire is electrically the same as the black/white that goes to the ballast resistor. The only thing is that the wire at the VR goes through the fuse box before going out to the VR. If the insulation ever gets damaged, the fuse should give some protection to the rest of the wiring.

Who asked about the IAC current draw? The fuse is to protect the wiring. If you have a short in an unfused circuit, you could have an electrical fire unless you are lucky and take out the fusible link first. I gave you a safer alternative because the wiring is fused on that branch.

No, the green/white is not a ground. Why do you think it might be? You should not dismantle this circuit unless you have another source for a tachometer signal and are not using the 240Z tachometer. I haven't followed your build thread, so I don't know if you are planning on using coil packs. Otherwise, you also need this circuit to provide 12VDC to your coil. If you don't want your wiring to burn up, I would suggest you use a fused source for the IAC, such as the black/white at the voltage regulator connector. The ignition circuit you are thinking of using is a poor choice for the IAC.

Now that you have isolated the wire, disconnect the connectors between the dash harness and engine harness that have a black/white wire and re-test. If you have taken the dash out of the car during the build, I would wonder if maybe pinched a wire or the wire harness in the dash harness. Also look for previous owner modifications like a relay for a thermostat-controlled fan.

Both black/white and the green white wires show up in wiring diagrams. See attached for the highlighted path. One black/white goes from the ignition switch to the ballast resistor. The green/white lands on the other side of the ballast resistor and goes to the tachometer. The other black/white comes out of the tachometer and goes to the coil positive. If you think you have a short in the circuit, unplug the 4 wire connector from the back of the tachometer and lift one of the wires off the ballast resistor. You have isolated the wires from one another and can measure resistance to ground at each. Ignition Circuit.pdf

After a little research, I would concur with your assessment.

This AN603, @Captain Obvious? https://shop.ectransistors.com/PANASONI-AN603-CDIP-14-.html

Apparently DQ also has an article on grafting in the tach from an S110 200SX. There are several NOS tachs on ebay now. It's not cheap, but it would probably be a fix Cody could enjoy for a long time.

@Terrapin Zis correct. Those connectors are also color coded. C1 is blue as indicated on the wiring diagram. Another convention, knowing which side has the male pins: So in the wiring diagram: The male pins should be on the engine harness side, and the female pins should be on the body harness. If you see how the connector is shown in the wiring diagram, there are no wires going to where the Xs are.

Can you tell us the year FSM?

Not surprisingly, there was no change. But I did take the time to mark the polarity on the tachometer wires to make it easier to identify which wire goes where in the future. I couldn't find my red electrical tape (that had been sitting in the same place in the garage for about 10 years until I reorganized last year), but I did find some red heat shrink that fit over the spade connector. At least I had fun testing. I'm interested in Bruce's analysis. I could do a visual on the discrete components, but I think Bruce has better skills in that realm.

I'll try wiring it like it suggests on Dime Quarterly, and if it doesn't behave better, I can send it to you.

This article on Dime Quarterly is interesting: http://dimequarterly.blogspot.com/2011/08/tech-how-to-510-factory-tachometer.html It says the sensing should be done on the negative side of the coil, and it does mention polarity for the sensing. I'm not sure if I will have time to run another test today. We'll see.

I doubt it's resistance in the ignition circuit. It would have to be something internal to the tachometer. I don't know enough about inductive loop tachometers to say what, though.

Also a friend who used to own a 260Z had his tachometer repaired at Bob's Speedometer. https://www.bobsspeedometer.com/ I think you have choices.

I ran a couple of more tests. Reversed polarity - Tachometer did not register at all. Original polarity - Tachometer in car registers just as it should. Tachometer on multimeter put in 4 cylinder mode - 510 tachometer seems to be reading about 1/10 of the multimeter. The needle did appear to indicate proportionally.

They were in series. That would be the same current since neither should be a load. I might be able verify that with an oscilloscope.

Yes. One thing I didn't experiment with was polarity. I can run more tests tomorrow.

I'll have to see if she's up for an overnight. She's working again, so she has been a little tired...okay, a lot tired. She did seem tempted by the chance to be around dogs, though.

In looking at the 510 wiring diagram, for a manual transmission wagon, I think this is how the wiring is without the tachometer. BW comes off the ignition switch and goes to the fuse box. While one branch goes to a fuse holder, another branch goes to the ballast resistor via a fusible link. From the ballast resistor, the wire is BL. The BL wire goes to coil positive. When starting, the BL wire is hot from the ignition switch. With the tachometer, I think this might be how the wiring should be BW comes off the ignition switch and goes to the fuse box. While one branch goes to a fuse holder, another branch goes to the ballast resistor via a fusible link. From the ballast resistor, the wire is BG (maybe?). The BG wire should go to the dash for the tach and come out BL. The BL wire goes to coil positive. This may be backwards. The challenge is that I have yet to find a 510 wiring diagram that shows the tachometer and is very clear on the wiring. The one I used to find the BG wire color leaves much to be desired. It was created by a 3rd party. What looked to be a Datsun FSM BE section didn't even show the ballast resistor. It might require that I pay you a visit to work that out, @Patcon. B - Black L - Blue G - Green W - White @Patcon There is a place not too far from me that does car instrument repair. http://www.hampspeedometer.com/ You might want to see if they can test/fix the tachometer. Datsun_510_Wiring_Diagram_1971_Wagon.pdf

While I said the wrong thing in the video, off camera I put the meter in 4 cylinder mode and found out that the 510 tach should have registered higher.

A while back, @Patcon sent me a 510 tachometer to test before he and Cody attempt to install it in the car. It's similar to the 240Z tachometer with the inductive loop and the power. I finally got the testing up to the top of my list, and today was a great day to be in the garage with the summer heat not as strong as usual. I wired the tach into the ignition of the 240Z. I deleted the ballast resistor when I installed the Pertronix last year, so I could unplug the bullet connector joining the black/white wire to the green/white wire. I tapped that circuit for tachometer power, too, grounding the tach on the strut tower with a clip lead. The tachometer does respond to changes in engine speed but not much nor rapidly. I'm not sure if it was an impedance issue for that tachometer since the coil is only 1 ohm. I neglected to look at the tachometer in the car while testing since I had a meter connected to give me a reading. In normal operation, the tachometer in the 240Z works fine. So I can see that the tachometer works, but it doesn't work great. I'm not sure if it was something I did or something with the tachometer that caused the results. What do you think, @Captain Obvious? @Zed Head, do you see anything? Back of tach Connected to the Z Wiring at the ignition And here's the video

Yep. That's the kinky part of engine porn.

An air pocket would reduce heat transfer, and the reading would go down. The sender works by decreasing resistance in proportion to the increase in temperature. Do what Steve @madkaw said.