The Speedometer should be driven from your transmission,& not require any input from your engine.
It will read with your engine off!
Even if the Nissan,& the Chev electrics use some more sophisticated circuitry to do this,why can't you keep the Nissan circuit just for the Speedo?
I can't keep the Nissan circuit for the speedo because along with the V8 I also replaced the transmission. The original Nissan cluster used to read from a gear-driven VSS in the Nissan transmission, now the Chevy transmission has a hall-effect based VSS that feeds into the Engine computer. The Nissan was originally designed such that the cluster would read the VSS signal and then "clean it" and pass it along to the Nissan Engine computer (along with other systems such as ABS and cruise control) The Chevy system is designed such that the computer reads the VSS directly and then outputs a "clean" signal for the cluster and other system. I theoretically could convert the signal from the Chevy VSS but it's just a dirtier version of the output I can get from the engine computer, it's still wrong for the Nissan cluster and I have no opportunity to multiply the signal using the computer (I'd be forced to do it externally).
Back in the days of first generation Electronic speedos,the sensors on quite a few cars were mounted in a cylindrical housing with a drive gear on it,which replaced the old speedo cables.
After market vendors would sell an adaptor to drive the speedo at the correct rate when different engine +transmissions were fitted.
I'm pretty sure the same people who marketed these have a product for sale which addresses the OPs problem,so the easy way might be to bite the bullet & buy one of them.
Fairly obviously (though not to me--duhh! ) he is getting the right stuff from the transmission sensor anyway.
yes there is such a product, the only compatible one I know about is made from a company called Dakota Digital, however the unit costs $80 and from what I've heard from people who have used it on setups similar to mine, it does a very poor job at low speeds (below 20MPH the signal becomes jumpy and the needle bounces around). I figured I would take the opportunity to learn something new and possibly build something that operates smoothly throughout the speed range.
They do. There's is a Silicon chip kit sold as a "speedo corrector" that offered a moderately popular way of mating diverse hardware. I'm not sure it will solve this problem though, but could be worth a look. http://www.siliconchip.com.au/cms/A_108157/article.html
thats a pretty slick circuit... it may be useful and I think I have most of the parts necessary to build it, even if it's not something that will work for this car it would be useful in my truck, in which the speedo reads about 15% off due to the dramatic difference in tire size.
Here is what i meant,
http://goo.gl/ZTj8N
an op amp used as a comparitor, and a 10K liftup for the computer output, though a much higher value could be used, 10K is good for noise immunity,
and you would run the op amp off which ever voltage you wish to use (i reccomend 5 or 12V) and its inverted compliment, you can pick up inverting regulators for a few dollars and hide this all away on a bit of perf or vero board,
that's pretty useful, while I haven't found much regarding the actual output of the original Nissan VSS I have found that most similar VSS from other manufacturers typically only output 1V on the peak of the wave and -1V in the valley.
... The early sensors made to fit existing locations in the transmission.
this is absolutely true, that truck of mine (a 1989 Toyota Pickup) originally had a mechanical, cable driven speedometer, the cable broke and I discovered that newer models (1992 and newer) had the same transfer case but used an electronic speedo... I bought a cluster from a newer model as well as a gear driven VSS that fit right in the same spot as the old cable adapter. All I had to do was run 3 wires to the dash and I was able to "upgrade" to an electronic speedometer.
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Thanks again for all the comments, it looks like I've still got some more work to do