Tachometer Swap : 280z into a 240z series 1 housing

[Zed Head]

18 minutes ago, Blitzed said:

Tach is still not functioning, all conversion steps have been confirmed and reconfirmed . Without a clear method to test the tach internal function, assume the 280z tach internals are the issue. Will re-install the 240Z internal and live with the tach as is. 

Thanks for all the info. 

I know that you posted some links to someone else's conversion procedure, and there was a lot of discussion that followed,  but you might describe exactly how your 280Z tach is wired up.  Power, grounds, and any added resistors.  It's not clear.  If I understand your posts correctly, you followed a conversion procedure but I don''t see the details on how the tach is connected to the car's wiring.  I didn't click through to the conversion links.

[Blitzed]

Hi Zed Head,

Hope all is well. The work on the Z is very fluid right now, work happening in between post. 

Purchased a used 280z tach. Followed the instruction in the post to swap into a 240Z can. No additional mods were made. Using the 240Z wiring harness and original 4 pin connector. Ran a wire from the 280z internal post to the negative side of the coil, per instructions have power in the wire. Tach will not operate. Turn signal lights do not function but hazard lights function (arrows on the tach). 

Moved one of the hazard switch wires (2) into the alternative input hole (3) in the can (in the dash). Now the headlights and brake lights all work. Only thing not functioning are the tach and turn signals. 

Progress.  

[duffymahoney]

I got my stock series 1 tach to work with coil on plug.  Basically it senses the pulses in the voltage, just like stock.  Works perfectly. 

[Zed Head]

Good luck.  I found that my 78 tach was sensitive to the signal on the coil negative.  After I switched to a GM HEI ignition module the tach needle would not track RPM it just quivered and bounced until I added a condenser/capacitor to the negative side of the coil.  I had a 78 tach in my 76 because the 76 tach would stop working when it got hot, in the summer months.

So the old tachs are wearing out and they are sensitive to the igntion coil pulses.  If you add more detail to your description there might be some clues.  "Doesn't work" for example, doesn't tell much.  My tach with the quivering needle didn't work, but the quiver was a clue.

p.s. I'll bet that a tach could be checked for proper operation from under the hood, right at the coil with some jumper wires.  Ground, power, coil signal, maybe an added resistor or condenser to be sure.  Use your car's engine as the test bed.

Edited May 19, 2020 by Zed Head

[Blitzed]

Hi Zed Head,

The 240z tach with the wire loop worked fine. The 280z tach internal swap into the 240 can will not work. Not sure how to clearly express it. Needle does not move or pulse when the car is running, turn signal lights in the tach do not work. The only thing operating on the tach are the dash lights (in the gauge in- line with the dimmer). 

Duffy Mohoney - please explain coil on the plug. Don't see this in the conversion post attached. Using the stock 4 pin connector with the white wire loop still intact in the connector (coming off the back of the tach)  In the connector coming off the tach (female), which pin is coil? The current black ground, white/red power and the white wire loop in the other positions.  

Thank you,  

 

[Zed Head]

5 minutes ago, Blitzed said:

Hi Zed Head,

The 240z tach with the wire loop worked fine. The 280z tach internal swap into the 240 can will not work. Not sure how to clearly express it. Needle does not move or pulse when the car is running, turn signal lights in the tach do not work. The only thing operating on the tach are the dash lights (in the gauge in- line with the dimmer). 

Have you used a meter to confirm that there is power to the proper places at the tach?  My first electrical problems on my car were when I just connected wires by color according to someone else's diagram.  The diagram and instructions are just a start.  You need to use a meter or test light to confirm that power and ground are correct.  I don't see that you've actually measured or confirmed anything, except for one post that says you confirmed "power in the wire".

I also see that you, apparently, did not try adding resistance in the circuit to the tach.  That would make it identical to the 280Z wiring.  The 240Z and 280Z tachs function based on different principles.

You can keep swapping parts but you'll progress much faster with some good measurements.

[Blitzed]

Thanks Zed Head, all good stuff. Even if I owned a meter wouldn't know how to use it or understand the numbers. So this project is over my head. 

Dash is hung in the car, would like to leave it there moving forward. All previous electrical issues were all caused by the hazard switch. Reversed the wires in the small can in the dash and turn signals, brake lights are all functioning perfectly now. Didn't realize how critical this switch is to bunch of functions.

Tach is the only challenge. Understand your concerns about the wires. The dash wiring harness is untouched, zero splices, additional electrical tape or butt connectors anywhere, including both side of the firewall.  

Pic attached shows the tach wiring from the functioning 240Z tach (prior to removal and swap). So if it functioned with this set -up it's safe to say power is power and ground is ground.  Followed the instructions with the internal swapped and reverse the wires per the harness on the 280z tach and internals. The 280 tach I swapped is exactly like the tach pics in the instructions above (ground and power reversed). So let assume that's correct. Ran a wire from the post directly to the negative side of the coil, the only measurement I can give you, it's live. as I grab the connection while the car was running. 

So current could be the issue with the tach not working,  can anything be added in-line to the coil/post wire (the only wire I can touch with the dash installed) to reduce the current?. Sure this rambling drives electrical engineer's crazy. 

Thanks for all the advice. 

[Blitzed]

Sorry tach pic

[Seanw987]

Thanks for the tach conversion write-up.

It worked like a charm and went quickly once I managed to pull the 240z tach out from behind... pita.

The use of an MSD 8920 tach adapter was needed to get things working in my case. 

 

1971 240z  MSD 6AL and bluetooth 123tune distributor.

 

I now have my tach functional.

[Zed Head]

45 minutes ago, Seanw987 said:

The use of an MSD 8920 tach adapter was needed to get things working in my case. 

How did you know that you needed the 8920, and how did you wire it up?  Many people can't get the 280Z tach to work with the 6AL even with the 8920.

Give some wiring data and you'll be adding to the thread's usefulness.

[Seanw987]

3 hours ago, Zed Head said:

How did you know that you needed the 8920, and how did you wire it up?  Many people can't get the 280Z tach to work with the 6AL even with the 8920.

Give some wiring data and you'll be adding to the thread's usefulness.

I used the 240 harness car side and the 280 tach harness tach side and ran a seperate wire for the signal running from the tach feed on the MSD unit. Tach didnt work so I simply put in a 8920 and that did the trick. Just followed the directions and wired it between the msd signal and the tach as directed.

No fancy wiring at all in regard to the tach conversion. I did add a relay for the adapter but thats a no brainer. I followed the walkthrough and that was my experience.

Never got around to the passenger floor board resister check because I had results. Vehicle wiring harness is from a 72. 

My info may be more suited for the sub group of msd6al  users. 

I will check how my MSD is wired and inform here.

attached is the reference diagram i used. found  it online, possibly here, and it worked for me. 

 

Edited May 24, 2020 by Seanw987
elaboration

[Blitzed]

Hi Sean,

Thanks for the details. Too late for my project now, maybe in the future. Went back to the 240z tach using the stock harness. The dash restoration looks great back in the car.  

The swap of 280z parts into the 240z tach can is pretty easy, where the original post falls down is whats next?  Converting a 280z tach that uses negative coil post signal on to 240z harness receiving a signal from the positive post coil signal, doesn't work. Maybe why the post stops after the parts conversion, too many variables/changes and components to add to get the 280z tach to function with the stock 240z harness.

Started the tread with the comment, "converting the tach now for future ignition upgrades". Stock distributor (with pertronix upgrade) and stock coil, tough to accomplish the conversion without a adding and changing a bunch of the stock wiring and ignition components. Not as easy as running a wire the neg side of the coil. 

Engine is coming out next month for a rebuild, will revisit the tach. 

Thanks again for the details.