Lazy Tach with 280ZX Distributor

[kcpope]

 

Onto the next thing…

Yes, I know I’ve got several other threads that aren’t resolved yet, but I need to solve this issue before resolving those issues🤦🏼‍♂️

First of all, thanks to everyone whose been so helpful to me in sorting out all these gremlins in my Z. I drove it for the first time today after slowly building it over the past year. Seemed to run well, but not quite all sorted out yet…

1972 240z, L28 with SUs.

My tach isn’t working right. I have done the 280zx dizzy upgrade, and wired it according to this diagram…

I do not know much about the coil, but I suspect it’s from a 280Z because the engine was L28 swapped at some point. Car starts up very easily and runs well. The issue is, the tach is reading about half the RPMs that the engine is actually spinning, and seems to become even more “lazy” as the revs increase. 

What might cause this? Where should I begin?

One thought…the wiring harness at the coil has (2) B/W wires. One gets spliced together with the G/W wire. The other gets connected to the + terminal on the coil. How do I know I didn’t accidentally reverse these? How would I test that?

Here’s my current wiring setup…

 

 

Edited September 8 by kcpope

[siteunseen]

My tach acted up a little here and there but overall it was okay with the stock coil. I read a good bit about them and ended up with the Crane PS20. It's 1.4 ohms which a little higher than the fsm specs, .06 to 1.0. I figured the bigger distributor cap and button could take the extra .04. 

It has been rock solid since day one. I have a 10 lb-ish aluminum flywheel and that tach needle pops up and down like the stock market.

https://www.amazon.com/Crane-Cams-730-0020-PS20-Performance/dp/B000CIO1F0/ref=sr_1_1?s=automotive&ie=UTF8&qid=1401630162&sr=1-1&keywords=crane+ps20+coil#productDetails

 

 

 

 

Edited September 8 by siteunseen

[kcpope]

14 minutes ago, siteunseen said:

My tach acted up a little here and there but overall it was okay with the stock coil. I read a good bit about them and ended up with the Crane PS20. It's 1.4 ohms which a little higher than the fsm specs, .06 to 1.0. I figured the bigger distributor cap and button could take the extra .04. 

It has been rock solid since day one. I have a 10 lb-ish aluminum flywheel and that tach needle pops up and down like the stock market.

https://www.amazon.com/Crane-Cams-730-0020-PS20-Performance/dp/B000CIO1F0/ref=sr_1_1?s=automotive&ie=UTF8&qid=1401630162&sr=1-1&keywords=crane+ps20+coil#productDetails

 

 

 

 

Cool, thanks for the info, I’ll consider buying that.

But before I do, is there anything I can test to confirm the coil is the problem and not the wiring?

[ollie]

Interesting, I'm experiencing a tach that works great with my 280zx distributor until 4,000 rpms then stalls out and won't rev any further.  I'm leaning towards the coil as well and am going to try this one out. 

 

Just to clarify, the engine runs great to redline, it's just the tach that stalls out at around 4,000 rpms.  I confirmed it reads correctly until then with a timing light.

Edited September 9 by ollie

[cgsheen1]

11 hours ago, kcpope said:

...

One thought…the wiring harness at the coil has (2) B/W wires. One gets spliced together with the G/W wire. The other gets connected to the + terminal on the coil. How do I know I didn’t accidentally reverse these? How would I test that?

 

Firstly, the fact that it's an L28 or has SU's has no bearing on this distributor swap. None of the Tach rewiring was actually necessary. You really just needed to install the distributor and provide it's wiring. The 240Z Tach is "sensing" the current drawn by the coil. It would make sense to use a stock coil or equivalent. The 280ZX "matchbox" doesn't care about the coil. (It's operation isn't affected by the coil). It also may make sense to leave the stock ignition wiring in place as well - as the Tach thinks it is...

Secondly: You can indeed test the ignition wiring. Remove both B/W wires and the G/W wire from each other and the coil. Isolate them. Turn the ignition switch to ON. Test those three wires with a multimeter. ONE of the B/W wires should show BATTERY VOLTAGE. The other two should show ZERO voltage. (Originally the B/W with voltage at IGN ON would be connected to one side of the ballast resistor, the G/W to the other side of the ballast resistor. (which THEN goes through the Tach) The B/W showing no voltage (at the time of test) should connect to the "+" on the coil (this actually comes FROM the Tach). (The "start signal" (+12v) originally would bypass the ballast to deliver un-resisted battery voltage to the coil for starting))

It's your call whether you put the ballast back and wire it up stock OR just connect the B/W that gets power at IGN ON to the G/W wire and put the other B/W on the coil... I prefer making things as simple as possible by NOT introducing EXTRA troubleshooting steps SO I would make things stock and troubleshoot from there.

Edited September 8 by cgsheen1

[SteveJ]

12 hours ago, kcpope said:

One thought…the wiring harness at the coil has (2) B/W wires. One gets spliced together with the G/W wire. The other gets connected to the + terminal on the coil. How do I know I didn’t accidentally reverse these? How would I test that?

Here’s my current wiring setup…

 

There's a video for that.

How it works on the wiring diagram:

https://youtu.be/5eUMImOnfHc

[kcpope]

@SteveJ @cgsheen1

Ok, I followed what both of you said and watched the video. I tested everything with my multimeter and can confirm that my wiring was correct. My B/W wire with 12V (key on) is connected to the G/W wire. The B/W with no voltage (key on) is connected to coil positive. No changes made.

Thoughts on what to check next?

[w3wilkes]

On 9/8/2024 at 1:11 AM, kcpope said:

 

My tach isn’t working right. I have done the 280zx dizzy upgrade, and wired it according to this diagram…

I do not know much about the coil, but I suspect it’s from a 280Z because the engine was L28 swapped at some point. Car starts up very easily and runs well. The issue is, the tach is reading about half the RPMs that the engine is actually spinning, and seems to become even more “lazy” as the revs increase. 

 

I have this exact same issue. ZX dizzy wired just like this. Originally the tach worked fine then one day about 20 years ago I was driving and the tach slowly went all the way pegged. Shut the car off and restarted. Ever since then it's been "lazy" as described here by KC and Ollie. Idle shows about 1,000 RPM and it tends to not go above about 3,500 RPM. Car runs great and I just shift by ear.

[Zed Head]

1972 electronics technology and parts inside these current sensing devices. I found this interesting page on the internet.  Might have something for the guys with electronics expertise.  Seems like a tachometer rebuild might be in order.

The page is a good example of how wrong things can get when a person tries to be clever with page color and font.  Easier to read if you highlight the text with your cursor.  Not sure of the quality of the information but it's an example.

https://www.sw-em.com/Smith's Tachometer.htm

[kcpope]

Good info Zed, thanks. That said, I really want to eliminate all other possible causes before having to pull the dash to get to the tach.

Like I said above, I confirmed my wiring at the coil is correct according to the video SteveJ shared. So what's left to test?

I decided to check my coil and here's what I got:

***edited because I wasn't reading my multimeter correctly the first time***

Primary: 1.0 ohms

Secondary: 12400 ohms

Any smoking gun with that info?

Edited September 9 by kcpope

[Zed Head]

Here's what the matchbox expects.  Don't know if it will help the tach.  From the 1982 FSM.

[SteveJ]

19 minutes ago, Zed Head said:

Here's what the matchbox expects.  Don't know if it will help the tach.  From the 1982 FSM.

But @kcpope jumpered out the ballast resistor, so maybe a higher impedance coil might help.