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

[zclocks]

You have two different problems. The back lights could be a ground to the lights or you lost pwr for the lights . Turn signal could be the same kind of problem . 

[duffymahoney]

Is the white current look important? 

[240260280]

On the 240z, the impulse signal from the coil flowing current for each spark passes through the white loop.  The early tach has a circuit that detects this impulse and uses it to turn the rpm needle.  If the loop is disconnected, the 240z circuit will not work... however, this is usually not an issue as the distributor wiring would have been reworked with the ZX swap to cause the need to change the tach guts.

[Blitzed]

Hi All,

Hope all are well. Pulled the dash to restore the heater box / heater core, replace heater hoses, clean and replace all vents. Clock was not working, sent to ZClocks.

While the dash is out like to updated the tach for future ignition changes/upgrades. Purchased a 77 280Z tach and replaced the internals into the 240Z housing, per instruction on in this post. 

Has anyone performed this resister jump on the 280Z tach internals? https://www.youtube.com/watch?v=Q5plL3aZbzA

First question, once you add the jump wire do you remove the 5 ohm resister? Limited electronic board experience, assume if the resister is still on the board wouldn't it still draw current?

Second question assuming the jump wire takes the 5 ohm resister out of the coil signal, what potential issues can this cause with the stock set-up listed below?

Reason for doing this now with the dash out,  never want to remove the dash again or try and remove the tach on my back. Understand some of the ignition upgrades have internal signal reducers which impact the tach's performance. 

73 -240Z, L28 N42 head and block, round top SU's. 

Current - stock distributor, Pertronix upgrade, stock coil. 

Future - MSD, matchbox or other. 

Any other upgrades / advice to other dash components are welcome. As mentioned this will be a one and done dash removal. 

Stay safe and well. 

[Captain Obvious]

Once you have shorted out the 5 Ohm resistor, it really doesn't matter if you leave it there or clip it out.

However, I have no idea as to the validity of that "fix". Without doing some reverse engineering I cannot provide any input into what that modification does or if it's really a good idea or not. The guy in that video seemed confident in it, but I'm not confident in his confidence.

He said something about "running the numbers" and "that resistor in there reduces the voltage"... Well, without seeing a schematic, I'm not sure at all what that means. Might be harmless, might not.

Is this a common thing that a lot of people mess with? I might have a spare tach around here somewhere...

 

[zKars]

I'm going to ad a detail about the 280z tach swap that I don't see mentioned here.

There is a 2200 ohm resistor in line with the line between the negative coil terminal and the tach signal pin. Here it is in the 76 FSM.

 

Now it would appear from the above discussions that many have not had a problem with getting the tach to work without it, so maybe it doesn't matter, but in the case where you have trouble, adding it might help.

 

 

[Blitzed]

Hi All,

Thanks for the responses. Sounds like this is uncharted waters with the 280Z tach internals. Was hoping for confirmation. 

Again, the goal is to make all the changes / upgrades under the dash while it's out of the car. Yes, this is not the norm, but the norm appears to be a tach that is very unpredictable in function when adding ignition mods. Adding components to the tach lead often is the fix. 

Any other suggestion / upgrades to the other gauges or switches? All are original to the car, including the radio.  

Thanks again. 

[Captain Obvious]

I'm a little skeptical about changes like that from someone who may or may not know what they are really doing. It's admittedly unfair for me to question his fix while I have nothing else to propose, but his explanation certainly didn't satisfy me.

As zKars pointed out above, there is a 2200 Ohm resistor in series with the tach on the 280's.  Another 5 Ohms added to that 2200 won't do squat. Makes me question the mod.

When I get a chance, I'll look to see if I have a loose tach around here.

[zclocks]

I'm with you all the way Captain. I know I had a schematic of the 280 Tach , but can't find it. Curiosity has the best of me and I went back and looked at the U-tube of the mod on the 5 ohm resistor. I can't be 100% positive , but what I see is a 22k  resistor that  was shorted by the jumper. That would make a lot more sense that eliminating a 5 ohm resistor and the tack started working. 

I also didn't realize that there was a resistor in line on the 280 tach . I pulled my 75 schematic and it shows the resistor , but no value. Now I'm going to have get under the dash and locate that elusive resistor.

[Zed Head]

It's a lump in the tape of the harness.  It's shaped just like the picture.  It's in the vicinity of the ignition module.

If too much current flows through the tachometer the coil might not discharge.  Make then break.  No break, no spark.  Maybe why it's 2200 ohms.

[Captain Obvious]

On ‎5‎/‎13‎/‎2020 at 11:13 PM, zclocks said:

I had a schematic of the 280 Tach , but can't find it.

Same here. I thought I had a schematic around here somewhere, but I can't find anything.

I'll keep looking.

[Blitzed]

Hi All,

So I'll be the test rat, replacement 280 tach internals can be obtained.

If the jump wire is installed (with the tach and dash out), is there a method of testing the tachs operating function to ensure it will work before painfully re-installing all components turning the key and praying the needle will move?

ZCLOCKS - Your 20 mins from my home. I can send the tach to you with the jump wire change made (complete in the 240z housing). Can you test the function/compare to unchanged 280 internal? Or are there to many other variables without the ignition circuit in-line to consider a function test successful? 

Goal - make all these little changes under the dash before re installation to support future ignition upgrade.

Thank you for the advice.