The speedo won't be too hard to deal with. For the pulse count, I'm basing some of this on the given picture. It sets the scale as 20 ms per block, and 8 pulses is around 140 ms. This estimates to around a 175 kHz frequency. I tried to find a similar pulse count for the SR VSS, but no dice. The old method was to check the terminal voltage when you spin the gear by hand (0.5 V). However, it looks like the newer sensors use battery voltage since the signal amplitude in the FSM is around 12 V.
Now for how easy it is to deal with that.
http://www.dakotadigital.com/index.cfm/ ... ct_id=126/
SR guys have been using these boxes for years to convert ABS sensor outputs into speed signal inputs to run S15 SRs in their older gen s-chassis. The Max does something similar in the factory setup. It takes an ABS sensor output over the CAN comm, sends that to the meter control unit (unified meter A/C amp), and converts it to the 175 kHz signal output the speedo needs.
The tech is going to be a bit more difficult. There's no information on the signal from the ECM to the meter control unit. It receives it over the CAN comm, but doesn't specify anything else about it. The S14 EUDM manual I have states that there's a linear relationship with the voltage along the tach signal wire from the ECM and the tach readout (higher voltage equals higher RPM read on the tach). Looking in my JDM S13 FSM, I get a bit of signal information. The CPS reports 1* and 180* pulses to the ECM. The ECM then carries the 180* pulses to the tach, ATCU, and HICAS units. Along with a frequency change, it also carries a rising voltage that is indeed RPM dependent. At idle, it should be around 0.5 V and at 2500 RPM, it should read around 1.5 V.
The Max uses signal frequency rather than a change in voltage potential from what I can tell since it transmits the engine speed over the CAN system. on EC-369, it reports signal pulse data for idle and 2000 RPM, but it states that the voltage is 10 V average. This is where you're going to run into some issues I think.
Dakota makes a tach box as well.
http://www.dakotadigital.com/index.cfm/ ... prd127.htm It's designed to change pulse rates when dealing with a change in cylinders. This will probably be a best fit for your application since the Max uses 6 cylinders versus the SR's 4.
Now, I just saw Dakota had a neat combo unit that allows adjustment of both tach and speed in one package.
http://www.dakotadigital.com/index.cfm/ ... prd825.htm It's meant for OBD-II cars, but it's a possible solution for the datsun.
Sorry I couldn't be a bit more definitive, but most people just say "LOL, make sure the tach is the right one" rather than try to come up with unique solutions, so nobody really has any additional information on the signals for the tachs available online. What I might do is pull my gauge cluster and do some voltage readings for you when I have time in the near future to give you an idea of voltage ranges across the RPM band. If I had an oscilloscope, I could easily get the signal frequencies you'd need as well, but alas, I am scope-less. So for now, all I have to work with is what the service manuals say.
I'll have to take it one step at a time to make sure it doesn't fly over my head 