I thought it was worth bumping this thread to post a follow up.
I was curious about the Vehicle Speed Sensor [VSS] output and the speedometer speed output so I put the car on the lift and attached an oscilloscope to the back of the instrument cluster.
I attached CH1 to the output for the VSS, which are pins 2M and 1M.
I attached CH2 to the output of the cluster speedo signal, which is 2L and used the cluster ground 3J.
Here's what I saw:
As you can see, the VSS on CH1 is a variable reluctance [VR] sensor
that outputs an ugly 0-centered sawtooth-esque signal. Both the frequency and amplitude of the signal depends on the speed, as is common with a VR sensor. I wouldn't feed this into anything expecting a logic-level signal without something like a MAX9924 in front of it. At high speeds I'd bet it would output north of +50V/-50V.
However, the speed output signal is a nice 5V square wave at half the frequency of the VSS, perfect for triggering just about anything.
Another question I had was the confusion between how many pulses per mile various sensors output and the PCM expected. Online it was reported at both 8000 and 4000 pulses per mile, but people were confusing the various signals and outputs.
At ~10mph, the frequency of the speed output signal is 9.76Hz. This gives 3513 pulses/mile. I suspect its actually closer to 4000 pulses/mile but the low end of the speedo is poorly scaled.
At ~20mph, the frequency of the speed output signal is 22.32Hz. This comes out to 4014 pulses, per mile, so exactly what I had expected.
Since this is half the frequency of the VSS, the instrument cluster
expects the VSS output to be 8000 pulses/mile. The VSS goes directly to the cluster and no other components interact with it.
The instrument cluster modifies the dirty VSS signal and outputs a square wave at 4000 pulses/mile that the PCM, Cruise control module, and Bose head unit can read.
Hope this helps someone else!