non cable speedos
 

How is the spedometer triggered on a 1990 Chevy pick up truck?

They use an electonic "vehicle speed sensor" in place of the driven gear.

Instead of a mechanical cable, there's a signal generator driven by the transmission. The sig gen puts out a signal frequency that changes with transmission output shaft speed.

The same signal is used by the various electronic systems for road speed, cruise control, rear wheel speed sensing for ABS operation, etc.

George

If your asking because you have a tranny for that 454 truck thing in the other post??? some 80's cars came with a cable driven speedo and an inline VSS to send the signal to the ECM. and if you need to change the signal (tires or gears) you just chang jumpers in the DRAC