In the Porsche 993, vehicle speed is detected by the sensor on the left-rear wheel. The sensor generates a pulse train that passes through the ABS Control Unit and Central Informer before reaching pin 4 of the speedometer as a 12-volt square wave. The frequency of this signal corresponds to the vehicle's speed. The speedometer then applies pulse-width modulation (PWM) to the signal and sends the modified output, known as "Signal A," to the connected devices shown. 

To test the speedometer:

1. Remove speedometer from car
2. Apply 12 VDC to pin 1 and Ground to pin 3  
3. Connect a 0 to 12V square wave to pin 4.

The needle should indicate speed in mph equal to one-half the square wave frequency (e.g., 70 Hz = 35 mph).

To test PWM “A” signal:

1. Add a 10k-Ohm resistor from pin 2 to 12V.
2. Set the input to 70 Hz.
3. Observe the signal on pin 2. It should resemble the bottom trace shown below.

Note that the frequency of the "A" signal in the lower trace matches the input frequency in the upper trace, but the high and low durations differ. As vehicle speed increases, the duty cycle—the percentage of time the pulse stays high increases.  

By introducing Pulse Width Modulation (PWM), vehicle speed can now be determined in two ways: by counting the signal frequency or by measuring the AC voltage of the signal. This allows for a quick check using a basic multimeter. While not as precise as a frequency measurement, it's a simple "go/no-go" test anyone can perform:

1. Set your multimeter to AC volts.
2. Probe pin #13 of the OBDII port with the multimeter.

The chart below shows the relationship between AC voltage and speed for a typical Porsche 993. Since multimeters vary in how they measure AC voltage, and because each speedometer will be slightly different, use the chart as a general reference.