Really specific SR question (for electrical guru's) Fixing 89,90 240's w/SRs wo/tachs

[einherjar]

Hi,

I'm creating a way to deal with the fact that i do not have a working tach (i'm a 90 with SR). So i came to the conclusion that the SR is giving off either a voltage, or a current signal, while the KA24E gave off the opposite. I can build something to convert the the two signals, but i need some information from someone with a SR20DET installed into a 91+ 240.

- I need to know the type of signal it is sending. (I am at work, so i cant bust out my ammeter/voltmeter).
- I need to know the range of signal: for example 3000 RPM = X mA / V (the signal it recives).

If anyone knows off hand what is needed, that would be great! or if someone is willing to help out.

thanks

`Nik

[AKADriver]

Tachometers are pulse-driven instruments. I believe the SOHC creates a standard tachometer 12v pulse signal. That's one pulse per firing of the coil, and since there will be two coil firings per revolution in a 4cyl (half the cylinder count) that means it's just RPM = pulse rate/2.

The DOHC, as well as CA and SR just use a 5v square wave instead of the 12v. Same signal, RPM = pulse rate/2, just lower voltage so it can't drive your tach.

[einherjar]

but if that was true, wouldn't there be some showing of movement in the tach? if the max of the SR is 5v and SOHC is 12V, when its at 6000RPM the signal would be enough to generate some power to the move the SOHC tach, though not the the appropriate amount? all the tach is is a meter.

i'll bust out my ammeter / voltmeter tonight to double check this i guess.

[Bradman123]

check ebay for a cluster, make sure that its sr and not ca. the redline will start solid at 7500 with no dashes. the ca cluster redline starts at 7000 with dash's and then hits solid at 7500. or u could call around and see if phase2, enjuku, jspec, or any of those places could get you one. im sure jspec could hook u up. i also have a 90. good luck. if anyone wants a ca gauge cluster i have one i would sell.

[einherjar]

yeah but if my theory is right, i could build something to fix it with barely spending any money.

[AKADriver]

A stock DOHC 240SX cluster barely costs any money, either, and it'll definitely work.

Your multimeter really won't help here. The tach signal is definitely a pulse train, and I believe the peak-to-peak voltage difference is the incompatibility.

[kazuo]

Too much effort, get a SR or DOHC KA tach.

SR and CA tachs are not interchangeable. And don't tell me they are, cus they're not. I've tried it.

[Bradman123]

Too much effort, get a SR or DOHC KA tach.

SR and CA tachs are not interchangeable. And don't tell me they are, cus they're not. I've tried it.

deffinately not interchangable cuz i know that from experience too. bought a ca by accident and ended up not having a tach for a over a month till i could find another one that was sr.