Zed Head

Here's a guy who goes way deep on how to convert an older tach to an electronic style. He had a comment about a calibration tool that was interesting. Maybe that slot is a potentiometer used to calibrate the tach (mentioned in this article). Wouldn't hurt to give it a twist back and forth and see what happens. Maybe it's corroded and not passing low current well. The material in this article is way over my head but the guy seems to know what he's talking about. Edit - I used the Hz measurement function on my multimeter a few years ago to calculate RPM from the coil output using the frequency measurement and math like he shows to see if the tach was right. Just an aside. You'll get a different number than his 1800 for our six cylinder engines. My tach was off by 200 RPM, but I never tried to calibrate it. http://www.nonlintec.com/ http://www.nonlintec.com/sprite/Sprite_Electronic_Tach.pdf

Back to how it works. The signal from the coil is just a voltage pulse. The capacitor(s) just "collect" those pulses and convert them to some sort of steady voltage that creates a magnetic field. More pulses equals stronger field. So, what components fail in a way that makes them fail sporadically, to make the needle jump then fall back like yours does? Sometimes they work and sometimes they don't. I assume that there are diodes involved also. Not an electronics guy, just like to dig as deep as I can. The above is a simpleton's guess at how the tach works. Feel free to correct. The needle movement is similar to what happens with a funky ignition module. Is it possible that the problem is the ignition module and not the tachometer? Your point about some sort of test bench is relevant. I was watching Wheeler Dealer and the guy took a tach to a guy who had a pulse generator for testing them. So they aren't that uncommon.

Looks like an adjustment. I know that the voltmeter has a similar slot that I used to adjust the needle position.

I don't really know how tachs move the needle. But I think the sweep of the needle is a continuous function, not a set of discrete steps. I found a Google thing with a nice color drawing and some words. Not sure if it's right. But, digging in to what's supposed to make the needle move, the principles behind it, might offer a clue. https://mechanics.stackexchange.com/questions/25584/how-does-the-tachometer-on-my-dashboard-actually-work

You didn't show higher RPM like 3-4000. But it looks like a physical problem might be a possibility. Sticky needle maybe, something dragging on it. Worth taking a close look at the needle movement itself. Dust, crud, rust.

So the tach needle just sits at zero, completely still, not even a quiver until engine RPM gets to about 1500, then it jumps up and sits steady at ~1500 and works normally/smoothly up to the high end? Pretty sure that a 260Z already has the 2200 ohm resistor. It looks different than the later models but it's there. Since you have the tach out you could measure resistance back to the coil negative. You could make three or four jumper extensions and run them out from the tach plug to wherever you want to set the tach for testing. Still have to start the engine but not so tedious to run a test.

I think that there's a drawing at the front of the Engine Fuel chapter of the 280Z FSM. An exploded view of all of the EFI components.

1981 280ZX Turbo. Looks good. 13,000 miles shown. https://bringatrailer.com/listing/1981-datsun-280zx-turbo-6/

I scrolled down the comments and at least three different guys have bid over one million dollars. But people are still picking over the incorrect door handles.

@Dave WM knows about reverse problems, he has some parts, and has worked on trashed transmissions. They are surprisingly durable. Reverse is a simple mechanism and can be reached from the backside of the adapter plate. Shouldn't have to remove the front case although that would let you inspect the synchros and gears. If the drainplug is in it see if it has metal stuck on it. Most yards drain the fluids though before they put them out so it might have been wiped already.

Not sure but I think that I've read about tachometer schematics somewhere in the past. I found a couple of old threads but not sure they go in to that level of detail. If you have the right meter and the capacitor still works you could measure it. Or, rearranged words, if you have the right meter and you can measure it, it still works.

Thanks for following up on the switch. It's a good tool for somebody who might have one of the aftermarket FPR's set to 38 or 40, or an AFM that runs just a bit rich, and just can't get that last bit of leanness they need to pass emissions or get a clean exhaust smell. Just short that circuit and leave it. Very useful for tuning. You kind of reported the results in the wrong thread though. Here's the post just to tie things together.

Good catch by Dave, but have you gone back and tried again? You suggested that you had already tried the proper shifting motion, above. Right and down. Just saying...

This one? I wouldn't call that teal.

First gear is 3.321 in the Z and 3.062 in the ZX. The ZX is usually paired with a 3.9 diff, the Z a 3.54.

I did the right click trick on an image and found this one in MI. Seems fishy. Identical cars in MI and CO? Hmmm.... It's a scammer/dealer. Actually, it looks like the CO guy has the MI dealer advertising it for him. "**Vehicle Location is at our clients home and Not In Cadillac, Michigan." So there's a commission baked in to the asking price. Odd combo, CO and MI. https://www.oldcaronline.com/dealers/Classic-Car-Deals-Cadillac-MI https://www.oldcaronline.com/1972-Datsun-240Z-Cadillac-Michigan-for-sale-ID956188.htm

Looks interesting but the keywords list says he's either a professional salesman and it's not really "his" Z, or he's a scammer.

This seems to fit these crazy times. Make America Mike's Again. https://www.mmafighting.com/2020/11/27/21723193/tyson-vs-jones-jr-weigh-in-results-mike-tyson-outweighs-roy-jones-jr-by-over-10-pounds

Modified 72 with a recent tuneup! It's a Z sellathon. https://bringatrailer.com/listing/1972-datsun-240z-151/

$38,500 for a 108,000(?) mile refurbed 73.

This one is about to end if anyone wants entertainment.

The wiring is the same without the ballast resistor. If it ran for 6 months it was probably right. Check the play on the distributor shaft. That will affect the air gap. The screw is just as tight as you can get it with a hand screw driver. Sounds like yours just came loose. You're saying that you reset the gap and it runs now right? You're probably good to go.

That's the Remover. AK260's was the Converter. https://jenolite.com/about-us/faq/

If the engine's not running sometimes reverse will find a spot where the teeth hit and don't mesh. If you can find a way to turn the propeller shaft it might go in to reverse. Worn out shifter bushings can cause shifting problems also.

Check the inside of the distributor. The triggering mechanism can wear, there's an air gap to set. The magnetic pickup coils go bad also, there's a resistance measurement to take.