3 hours ago, Zed Head said:

So, what, in detail, does the tach do and not do?  Does the needle twitch at all when you turn the key on or start the engine?  These old tachometers fail often and in many different ways.  

Here is a video of the car running/Erving and the tach not working. 
 

 

I did not watch SteveJ's video (no offense SteveJ) but if you have tested everything and confirmed that the wiring through the loop on the back of the tach is correct then you end up with "tach is bad".   They do crap out after 40+ years.  I had a bad one in my 76.

I think that there is a place or two that can rebuild them.  Have you confirmed that the needle is not stuck?  If you take it out you can take it apart and do some cleaning.  People have had success doing that.  

It's a common problem.

Just watched your three videos.  The tach is probably not destroyed.  It's just not working.  It's fixable.  You might post another thread titled "Who can rebuild/fix my tach?".  I can't remember who knows but they are out there.

I had this same issue on my 1971 240Z, albeit after a complete rewire of the entire car. The car would start and run fine but the tach was completely dead, turns out I had the two B/W wires backwards. Skimming through the thread I believe some of the others have already addressed the diagnostics really well, and if I remember correctly how the circuit works the ballast resistor gets switched out of the circuit when the key is in the Start position so that the coil can be connected to the full battery voltage for starting the engine. It gets switched back into the circuit in the run position to keep the coil operating at a reduced voltage, since it would get burned out operating at full voltage for an extended period of time. Someone correct me if I am wrong, but the ballast resistor is actually there to protect the coil, and the points condenser on the distributor is for protecting the points themselves. 

Anyway, sounds like it could be an issue with the Tach itself but might be worth swapping the two B/W wires just for kicks. Pretty quick way to rule out the issue I had. 

That's interesting.  It implies that 2 and 5 are both powered at Start, otherwise there would be no power to the coil at Start if the wires were switched.  I thought that 2 switched off at Start.  The diagram seems to show that every pin is powered on the bottom position which should be Start, which doesn't seem right.  Usually accessories are disconnected for Start.  5 is shown as bypassing the resistor which should be the Start circuit.  Kind of confusing.

Switching the wires would bypass the tach and the resistor at Run.  Makes sense that the engine would run but the tach would not work.

Edit - looking more closely at the grainy drawing I think I see small circles at 1, 3, and 5.  Matching AnvilZ's post below.  Doesn't answer the question though.

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Edited August 21, 20213 yr by Zed Head

8 minutes ago, Zed Head said:

That's interesting.  It implies that 2 and 5 are both powered at Start, otherwise there would be no power to the coil at Start if the wires were switched.  I thought that 2 switched off at Start.  The diagram seems to show that every pin is powered on the bottom position which should be Start, which doesn't seem right.  Usually accessories are disconnected for Start.  5 is shown as bypassing the resistor which should be the Start circuit.  Kind of confusing.

Switching the wires would bypass the tach and the resistor at Run.  Makes sense that the engine would run but the tach would not work.

 

Yes, I have my doubts about the swapping of the wires. I'll have to test that this weekend. Anyway, I believe the OP has his car wired correctly, so I hope he didn't swap wires.

Here's the 1976 scheme just for comparison.  Accessories off for Start.  R must be the resistor bypass.  Never really looked at it when I had mine.  It's EFI though so IG has to maintain power.

Here's a 72 switch from a diagram that EuroDat created.  Doesn't show B but it can be assumed, I guess.  Start and On/Run are powered separately.

I will verify this evening. It will only take a few seconds with a meter. 
I still have my doubts about being able to start the car with the black/white wires swapped.

It might start as soon as the key is let back from Start to On, wired that way.

I recall some old posts about early ignition switches being different.  There was discussion about losing power during starting, to radios or AFR gauges or or Megasquirt controllers or something.

I agree that it doesn't make sense from a schematic standpoint that the car would start with the wires swapped, and I seem to recall being confused by what the original schematic was saying compared to what I was actually seeing on the car. Sorry I can't remember all the details this was almost a year ago, but I specifically recall switching the two and that fixing my tach issue. Take it with a grain of salt, I completely rewired my car myself so good chance something got messed up along the way. Still works perfectly for now, fingers crossed it stays that way. 

Like I said it is most likely an issue with the tach itself in this case, but I would just be curious to see if swapping the B/W fixes the problem on this car as it did on mine (or even allows the car to start and run).

Edited August 20, 20213 yr by AnvilZ

12 minutes ago, AnvilZ said:

I agree that it doesn't make sense from a schematic standpoint that the car would start with the wires swapped, and I seem to recall being confused by what the original schematic was saying compared to what I was actually seeing on the car. Sorry I can't remember all the details this was almost a year ago, but I specifically recall switching the two and that fixing my tach issue. Take it with a grain of salt, I completely rewired my car myself so good chance something got messed up along the way. Still works perfectly for now, fingers crossed it stays that way. 

Like I said it is most likely an issue with the tach itself in this case, but I would just be curious to see if swapping the B/W fixes the problem on this car as it did on mine (or even allows the car to start and run).

I will swap them when I get home in about and hour and report what I find via video on youtube

You can just use your meter and disconnect the starter solenoid wire so the engine doesn't turn (for the quiet).  See which wires have power at each key position.