About a year ago I converted my hatchback to 5 speed and everything worked. Then I swiched my single cam for a twin cam and changed over all the parts and it runs real good. I changed over the rpm tach from one out of a twin cam and nothing. Everything works but I don't know what I'm reving at. There is no post that is clear or specific on this issue.
What gives signal to the rpm cluster the ecu or an ignition component?
Can the gauge be re wired directly to the distributor or place of signal without messing up?
I don't want to buy the huge 5" tach or something aftermarket, so is there a way to test the signal? Thanks to people who are brave enought to post and not just read and ask your self the same questions later on.

I think there is a guide somewhere that shows how to modify the gauge cluster to work after a swap.

Does the tach move at all?

No dosent move at all on off idle or rev nope never moves

I have always read that even when u do an sr swap u need to change out the rpm tach from a twin cam for it to work so I don't see how or wat do I do to wire it or fix it.

Hmm, yes that's what I'm coming up with.

I think there might be something unplugged or both your tachs are broken.

I heard just swapping the tach for a dual cam one is all you need to do.

Yeah I need to know the complete signal or circuit to the tach. Ok so the tach is powered by ? The signal comes from? I haven't found anything on this everything works but this. I hate it not working.

The tach gets a signal from the ECU, and I think the ECU gets the signal from the CAS in the distributor.

Then wat wire goes to the ecu from the cas? And it converts the signal or?

Then wat wire goes to the ecu from the cas? And it converts the signal or?

There should be a small harness that clips onto the distributor, that's probably what goes to the ECU.
If there is a problem there the engine probably won't run, so you should probably look at the wiring between the ECU and Tach.

Ok I will check those wires and trace them back to the ecu. U don't know wat wire or pin#? The engine runs great just no tach action lol. Is there a way to test the signal comming from the cas wire to the ecu. A specific one. And then the wire to the tack from the ecu? Any specifics on volts or circuit resistance?

Number "19" on that harness.

And according to the Pinout guide on this link:
http://www.midweststreets.com/240SX/ecupin.html

The Yellow/Red wire in the ECU is the tach wire.

I'm not sure on how to test all this though, I'm just looking through the factory service manual and trying to make sense of the wiring diagrams.

Thanks ill start with that. Is there a resistor in that circuit? Can the ecu not be sending that signal?

Just curious did your tach work when u had the single cam? When u did the sohc to dohc swap that yellow/red wire connects to the body harness.If you used a good harness when u did ur sohc to dohc it should have worked. (knowing u put it a dual cam tach and wired correctly). I would first check to see if that wire is hooked up where u joing the dohc harness to the sohc body harness. hope that helps

Yeah the soch worked good auto and 5 speed. Then I swaped over the de and no tach action(gigity gigity lol) yeah I wired the dash conecter cause the single cam plug/ connector is diffrent. So I cut the single cam one and solder the wires for it to work. But still no tach action. I need tofind the complete function and how to test it. Its the only thing that dosent work on the car. It buggs me not knowing wat I'm reving at.

Since you have an SOHC chassis you need to make sure you put the tach signal (orange wire I THINK?) in the right spot of the dash harness connector. There are many guides (the guy from 240sxTechDVDs has a 2 or 3 part video on Youtube on how to do it) around the net.

Also as far as the type of "signal" goes, I'm pretty sure it's just a voltage that leaves the ECU and goes to the gauge. You should be able to test that with a multimeter. If you get a clear reading, you very well might have a bad DOHC tach.