Hey Everyone,

I just replaced the stock distributor in my 11/70 240Z with a new 123Ignition unit along with a new Bosch coil.  The car now runs great, but the tachometer no longer works.  When the engine first turns on the tach jumps up a little bit, but then slams back down to 0 and stays there.  Anyone have any advice on what I screwed up?

 

Here's how things are hooked up currently (see image below for my wire numbering):

1. [Black] The black wire going to the 123Ignition unit is connected to the coil negative
2. [Black] There's another black mystery wire that was hooked up to the old coil negative, so I hooked it up to the same place on the new one
3. [Red] The red wire to the 123Ignition unit is connected to the coil positive
4. [Black/White] The green/white wire that used to connect "back" side of the resistor is now connected to coil positive
5. [Black/White] Another mystery black/white wire that used to be connected to the old coil positive so I connected it to the same place on the new one
6. [Green/White] This wire used to connect to the "front" side of the resistor - it's currently disconnected
7. [Black] This wire used to connect to the little spade terminal on the old distributor, which then connected to the little capacitor hanging off the side.  There's no such terminal on the 123Ignition unit, so this is also currently disconnected.
8. [Blue] (not shown) The ground wire to the 123Ignition is connected to the chassis

Someone in another forum had mentioned connecting wires 5 and 6 together, which I tried but no joy.

 

 

By the way @Captain Obvious your spring has served very well up to now - let me know if you want it back and I'll be happy to put it in the mail!

I don't really need the spring back. Keep your distributor together in the event that you need to put it back in for some reason.

As for the problem you're having with your tach, there are a couple people here on the forum who have done the 123 ignition thing... Hopefully some of them will chime in with some help? I've not messed with that unit myself, but if you can't seem to generate any help elsewhere, I'll do what I can.

My 123 worked perfectly in my 1/71.  Your tach should just sense pulses from the coil and should be separate from the 123.

44 minutes ago, duffymahoney said:

Your tach should just sense pulses from the coil and should be separate from the 123.

@duffymahoney Yeah, that was my hope but it doesn't seem to be sensing anything now.  I figure that wires ②, ⑤, ⑥, and ⑦ must have something to do with the tach and I just haven't hooked them up correctly.  Would you mind snapping a picture of the wires you have hooked up to your ignition coil?  What did you do with that little black wire that was hooked up to the capacitor hanging off the old distributor (mine is electrical taped along side the temperature probe wire)?

 

In better news, I went for the first real drive this morning with the new distributor and the engine feels really great.  I'm not sure if it's going to help my fuel economy, but my smiles/gallon has definitely increased.

Edited February 28, 20214 yr by rcv

Yours has the current sensing tach.  I can't remember the details but people have found that fiddling with the white wire loop on the back of the tach can solve those types of problems.  I think that CO has even had  comment or two about it, but it was years ago.

Which 123 are you using. and which coil?  I think that the latest ones would pass more current than the old stock setup.  Maybe the tach is seeing too much current.

Might help, I think there's a clearer version out there somewhere.

Here's an old discussion.  The guy had the same two first initials!  Solution about post #11.  The Pertronix and 123 modules work in a similar way.

 

17 hours ago, rcv said:

Hey Everyone,

I just replaced the stock distributor in my 11/70 240Z with a new 123Ignition unit along with a new Bosch coil.  The car now runs great, but the tachometer no longer works.  When the engine first turns on the tach jumps up a little bit, but then slams back down to 0 and stays there.  Anyone have any advice on what I screwed up?

Your Tach is expecting the stock 1.5 Ohm coil to still be there. If your new coil is a different Ohm value it will affect the Tach operation. I had the same issue here in the shop.

I used the stock coil or at least a similar to stock coil.  

He has replaced the coil with one made by Bosch.   Maybe temporarily put the original coil back in, see what the tach does. 

1 hour ago, S30Driver said:

Maybe temporarily put the original coil back in, see what the tach does. 

Yeah, I think that's my plan. I did some more searching, and came across this great diagram by @240260280z in this thread: 

 

 

I think that basically explains it - I removed my ballast resistor when I installed the new Bosch coil which I think was the wrong move.  I'm guessing the tach is acting like a voltage divider along with the ballast resistor, and removing that resistor screws up the formula.

 I measured that Bosch coil this morning, and it reads 1.5Ohm on my meter even though it's spec'd at 1.8.  I'm just going to replace it with the old 1.5 Ohm Flamethrower coil and the ballast resistor that I had in there before.  Here's hoping I didn't fry the loop in my tach when I was playing around.

Edited March 1, 20214 yr by rcv

More stuff is coming out of my brain on this topic.  It's not uncommon, I remember past discussions.  I think that in the past I suggested that a person actually divide the voltage, like you implied, purposely.  Run an extra wire around the tach circuit, parallel circuit.  Use a potentiometer to tune the current flow.  I don't know if it would work but I think that it should.

The whole point of using a current limiting ignition module is to get a stronger spark.  Using an old high resistance coil to get the tach to work defeats the purpose.  

p.s. this might be why Nissan went to a voltage spike/kickback (probably the wrong words but figurable) sensing tach for the 280Z electronic ignition instead of current sensing.

Edited March 1, 20214 yr by Zed Head

47 minutes ago, Zed Head said:

Run an extra wire around the tach circuit, parallel circuit.  Use a potentiometer to tune the current flow

Yeah that's a good idea. You might need to put a very small resistor in parallel with the potentiometer, otherwise the current through the pot will be super low and subject to whatever the impedance is in just the wiring.  The goal would be to make the current flowing through the pot+tach the same as it used to be, but to also get some extra juice by pulling in extra current through the low-ohm resistor that's in parallel.  I'm not sure when I'll have time to source the parts and try this, but it sounds like good experiment.