Speed Wire

Does anyone have an idea how the pruple/white "speed wire" transmits speed info? Is it a voltage proportional to vehicle speed or some kind of a pulse train? I hooked an oscilloscope up to pin#5 of connector 1E on the driver's side fuse box and just got random noise regardless of whether I was stopped or moving up to 35 mph. Not really sure what's going on. I can pull accurate speed data off the OBDii so I know that speed IS being properly measured and communicated to the ECU.. I just don't know how.

..I was wanting to use it as in input to a custom DVR/Data Logger I've been tinkering with..

2010 Access Cab MT

 

The vehicle speed signal is a square wave (approx 0-11v, aprox 4 pulses per wheel revolution), all the info I have on the vehicle speed signal system can be found in the post below (scroll down to P0500).
https://www.tacomaworld.com/threads/2nd-gen-information-diagnostics.784008/#post-28042157

A scope capture of it can be found here.
https://www.tacomaworld.com/threads/4-0l-scope-captures.759406/

 

Thank you. That’s just what I was looking for. I’ll have to do some digging to find out why I’m only seeing random noise.

 

I’ll have to wait till I’m back from a business trip to verify but I think this explains it. “Each module on the Speed circuit outputs aprox 5v or 12v on it's SPD terminal which the Combination Meter pulses to ground to indicate vehicle speed, voltage on the SPD circuit will vary depending on what modules the truck is equipped with but should be about 4.7-12v (note the voltage will be 0v key on with the Combination Meter connected).”
I was expecting the combo meter to drive the line high—rather it is driven high by its peripherals and pulled low by the combo meter. Is this correct?

 

Correct, each module on the SPD circuit outputs a bias voltage, the Combination Meter pulses the SPD terminal to ground to provide the speed signal.