How does the VSS work with the speedometer to give you your speed? Looking for the theory here.

Thanks
Bill

 

There are 2 ways that the ECM recieves a signal from the VSS. First as a sine wave, the frequency of which increases as the speed of the vehicle increases.
The second way is as a square wave. This is on both trans. mounted ans speedo head mounted applications that use a buffer circuit. On these the ECM feeds 12 volts into the buffer. The buffer then grounds and ungrounds this voltage at a frequency proportional to the speed of the vehicle.
The VSS affects the operation of the lockup conveer clutch, idle air control stepper motor and ignition timing (on some applications).

 

VSS uses the Hall effect principle. Essentially what happens is a fixed magnet attached to a shaft inside the speedo rotates past a pickup coil. As the magnet passes past the coil a voltage is induced (Hall effect) and fed to a buffer(amplifier) module where it is amplified and the pulse is reshaped for input to the vehicle ECM. The input to the VSS will look as DB said like an AC waveform whose frequency will change with varying vehicle speed. Output of the buffer will be a 5v square wave with the same varying frequency so the ECM can interpret speed. Later models use a VSS mounted at the trans where the cable attached on earlier models, however the theory is the same.

 

Thanks for the replays so far. how would one go about building a circuit to change the speed shown on the dash? So my 91 has both signals or just one? Is the signal that goes to the speedo the same one that goes to the ECM?

 

The VSS also outputs 4,000 pulses per mile of travel, at whatever rate you might happen to be travelling. The analog speedometers use the raw signal, and the ECM gets the 5V P-P buffered signal.

How do you know the calibration is off? You would have to monitor the ECM serial data output to determine what the VSS circuit was reporting to the ECM. If that didn't match the actual vehicle speed or speedometer, I'd suspect the mechanical speedometer before the VSS buffer. It is posible that the VSS is not generating pulses of sufficient amplitude to trigger an output state switch in the VSS buffer, thereby causing "missed pulses" and incorrect ECM VSS data.

 

Here is what I know and the reason for my questions. I bought the car in 96. Since the day I got it the speedometer has never been correct. The dealership said that they put a different t-5 in in before they sold it. I just kinda assumed that what ever tranny they grabbed didn't have the same gear ratios. Then las weekend I changed the rear out for one in a V6 camaro. Gear ratio of 3.42. I haven't really driven it much since but if the speedo is further off I would like to be able to correct it and wanted to know how things worked. I would rather not have to purchase new drive or drived gears. I was wondering if I could just change the signal going into the speedometer.

Thanks
Bill

 

Why don't you want to buy the gears?

The easiest way to change the signal to the speedometer is to change the gears.

Last time I bought speedo gears it was about $14 altogether from a dealership.

 

Just go into the tech section or some place like TCI's website which has gear calculators and find out what the correct speedo gears are for your combination.

T5
3.42 gears
26" tire diameter (most likely)

 

I was hoping not to have to go back under the car for a while. We did a rear end sway in my driveway last weekend with a high of about 40. We got a total of about 5 hours of daylight and when the sun went down it was pretty miserable.