Electronic Speedometer. How does it work?

[rscam]

I have put an NV3500 in my truck and it has an electronic speedometer. I have seen numerous posts about conversion units. From what I've seen you can only buy a converter to go from Mech. to Elec. Since the output on the trans. would be electronic. I would have to switch to an electronic speedo. My ? is if I get an Electronic speedo. out of an newer car do I have to have the ECM from the car or does the transmission speed sensor send its signal directly to the speedo? If I am correct the earlier cars and trucks say early 90's that were electronic speedo. The ECM was only used for emissions and fuel delivery. I imagine the newer cars have the ECM involved in everything.

[Longhorn Man]

a real quick google search netted this;
http://www.abbott-tach.com/cablex.htm

http://www.classicchevy.com/product....1&dept_id=1849 (photo looks the same, might be)

http://www.bowtieoverdrives.com/hot/

[rscam]

Anyone know if I can get just an electronic speedometer or would I have to get the ECM also?

[68gmsee]

From the internet "How Stuff Works."

The Electronic Speedometer
An electronic speedometer receives its data from a vehicle speed sensor (VSS), not a drive cable. The VSS is mounted to the transmission output shaft or to the crankshaft and consists of a toothed metal disk and a stationary detector that covers a magnetic coil. As the teeth move past the coil, they "interrupt" the magnetic field, creating a series of pulses that are sent to a computer. For each 40,000 pulses from the VSS, the trip and total odometers increase by one mile. Speed is also determined from the input pulse frequency. Circuit electronics in the car are designed to display the speed either on a digital screen or on a typical analog system with a needle and dial.