Reference Voltage - Review

Reference Voltage Review



Next month, S/N will feature an in-depth article describing the automatic shoulder belt system, its operation, and some troubleshooting tips. Since this system includes many switches that the control unit monitors with "reference" voltage, we thought a review of reference voltage wouldn't hurt.





To help understand how reference voltage works, look at the simple light bulb circuit illustration. When the switch is open, the circuit is not complete; the voltmeter indicates 12V all the way up to the open switch. When the switch is closed, it completes the circuit; the voltmeter indicates OV after the light bulb. The light bulb is the load, and the voltage is "used up" across the filament.






When we exchange a control unit for the light bulb as shown, the battery sends 12V to the control unit, then the control unit sends a reference voltage (for example, 5V) to the switch. When the switch is open, the voltmeter indicates the reference voltage between the control unit and the switch.






When the switch is closed, it completes the circuit. Just like the light bulb example, the voltmeter indicates OV after the control unit because the control unit is the load.



The control unit monitors this change in voltage to determine whether the switch is open or closed. When troubleshooting, you also monitor the reference voltage and, just like the control unit, your voltmeter will pick up any excessive resistance, an open, or a short in the circuit.





For example, if the reference voltage is 2.2V with the switch closed, there must be some unwanted resistance in the circuit. The resistance is a second load in the circuit, and the 2.2V is the voltage drop across the resistance. (The higher the resistance, the higher the reference voltage.) The control unit is probably "confused" because it only "knows" that 5V indicates the switch is open and OV indicates the switch is closed.






If the reference voltage is 5V with the switch closed, then there must be an open in the circuit. Though the switch is closed, the circuit is not complete. Since the reference voltage is the same as if the switch were open, the control unit "thinks" the switch is open.






If the reference voltage is OV with the switch open, there must be a short to ground in the circuit. The short completes the circuit even though the switch is open. However, the control unit thinks the switch is closed because the reference voltage is OV (If no short to ground can be found, the control unit is bad.)