Thank you for your kind help. I've looked in the Haynes manuals, but the VSS output signal is not discussed in sufficient detail to build a mechanical speedometer drive. I'll try the oscilloscope today to read the signal if the rain doesn't spoil it!

I was hoping there was some clues... I've been lucky before with them manuals. I poked around on the internet as I'm sure you have and couldn't come up with any decent information. I'm guessing it's a hall effect and with any luck it will be a nice 5 volt square wave. The scope will let you know. Good luck!!

 

Why not, if your using the sensors and ECM of the donor car, adapt the dash speedo drive to your recipient cars speedo? Don't mean the whole dash just the electronics/stepper motor in place of the original mechanical driver. It's really no different than an engine swap, just on a smaller scale. A company called Dakota Digital got their start doing this, adding new "guts" to old school dashes.

I think you have a tiny problem: Your not measuring the speed of your motor and using feedback to control the RPM.

Think KISS was meaning that for your speedo motor to work correctly, the RPM of that motor has to be feed back into the electronics of it's drive, like in a servo motor drive. Just taking an out put from a sensor and converting to a voltage without feedback won't work on its own.

 

Think KISS was meaning that for your speedo motor to work correctly, the RPM of that motor has to be feed back into the electronics of it's drive, like in a servo motor drive. Just taking an out put from a sensor and converting to a voltage without feedback won't work on its own.

Indeed correct.

50% duty cycle does not translate to 50% max RPM. It might if there is no load. A permanent magnet DC motor basically follows V=Vm-I*Rm
Vm, but Vmotor you cant really measure. When I=0, the motor can act as a tachometer, since I=0.

I is generally proportional to torque and V is a function of RPM. Motor speed controls have been known to use just the V=Vm-I*Rm relationship.

 

I also should have said, that Vm is the motor voltage at the desired RPM, measured with no load. Vm= k*RPM