I'm curious to how a vehicle speed sensor actually works. I found out that there is some kind of VSS Gear but could someone expand on that for me? Thanks.

 

The gear is on the diff and it drives the gear on the vss unit.. It pulses +12volts per revolution of the vss sensor.. I think its like 4000 pulses per mile on average..

 

Thanks

 

the same basic way a crank angle sensor, etc works..

 

it spins.

 

The VSS pulls the signal to ground as it rotates (and there is a resistor which pulls the signal back high inside the car electronics on the sensor line)

I have a 95 Accord and I tested the Speedo/Odo on the bench. (it requires only 3 connections, Ground, IGN @+12v, and the VSS/sense signal)
I connected a function generator providing a square wave to pull the Speedo/Odo sense signal to ground.
I measured the pulse at 15ms period (66.67Hz) when the speedo read 60MPH. (and at 10ms period, the speedo read 90MPH).

This corresponds to 66.67/sec* 60sec/min = 4000 pulses per minute.
That matches what was stated in this post "about 4000 pulses per mile", because 60MPH is one mile per minute.
The VSS pulls the signal to ground as it rotates.
In general, the Speedo MPH = approximately 900/(period of signal in milliseconds).

The Honda factory manual states that the speedo indicates 60mph@1026RPM of the VSS.
That would be 1026rev/min * (1min/60sec) = 17.1Hz or 58.45ms.
However, that does NOT seem to match what I measured.
(scaling my measurement, 58.45ms period would give about 15MPH).
However, assuming that the VSS produces 4 pulses per revolution, that explains the factor-of-4 difference.
So,
1026 rev/min * (1min/60sec) * (4 pulses/revolution) = 68.4 pulses/sec @ 60mph
1 / (68.4 pulse/sec) = 0.01462sec = 14.62ms period at 60MPH.
Since the period and MPH are inversely-related,
14.62 ms * 60MPH = 877.193 ms*MPH = Constant.
so, using Honda's 'exact' value:
mph = 877.193 ms * MPH / (signal period in mS)
The matches pretty closely to the measurements I took.
Period (ms) = 877.193 (ms*mph) / 60MPH = 14.62 ms (vs. measured value of 15ms)
Period (ms) = 877.193 (ms*mph) / 90MPH = 9.75 ms (vs. measured value of 9.92ms)

(My speedo reads a bit fast anyway!)

 

This thread is from 2006.

 

yes but I'm adding some info I feel might be useful to others. For example, if someone wants to test their speedometer off the car like I did,
to test accuracy or if it's not working.
These cars seem to have bad solder joints in the control board at the back of the speedometer
e.g. https://honda-tech.com/forums/honda-...epair-2087284/
and there are various videos and forums discussing resoldering the board and 'pin 10' on the IC#1.
So I wanted to be able to test my speedo without having to jack the car up and run it to actually test it.

 

Appreciate the additional tech info, @azchris

 

x2 - really appreciate the info @azchris