A technician is troubleshooting a vehicle where the rear defroster does not activate. Using a wiring diagram, the technician identifies the defroster relay. With the ignition on, the technician checks for voltage at the relay's coil power supply terminal (Pin 86) and finds 12.0V. The technician then checks the ground-side control terminal (Pin 85) and observes 0V when the defroster switch is activated. What do these readings indicate?
- AThere is an open circuit in the power supply to the relay coil.
- BThe relay coil is faulty and needs to be replaced.
- CThere is an open circuit in the ground-side control wire to the relay coil.
- DThe control module or switch is successfully providing a ground to activate the relay.
Show answer & explanationAnswer & explanation
Correct answer: D. The control module or switch is successfully providing a ground to activate the relay.
In a ground-side controlled relay, the control module or switch provides a ground path to activate the relay coil. If the coil power supply (Pin 86) has 12.0V and the ground-side control terminal (Pin 85) shows 0V (meaning it's at ground potential) when the switch is activated, it indicates that the control module or switch is successfully completing the circuit, allowing current to flow through the relay coil and activate it. The problem lies elsewhere, likely the relay itself or the load side of the circuit.
Why the other options are wrong
- A. 12.0V at Pin 86 indicates the power supply to the relay coil is good, so there is no open circuit there.
- B. If the relay coil were faulty (open), even with proper power and ground, it would not click or activate. However, the voltage readings alone don't confirm a faulty coil; they confirm the control circuit is working.
- C. If there were an open circuit in the ground-side control wire, Pin 85 would show 12.0V (floating voltage through the coil) when the switch is activated, not 0V.
Ground-Side Relay Control
In ground-side controlled relays, the control module or switch completes the relay coil circuit by providing a ground path. A voltage reading of 0V at the ground-side control terminal (Pin 85) indicates successful activation of the control circuit.
- Relay coil receives constant power on one side (e.g., Pin 86).
- Control module/switch provides ground on the other side (e.g., Pin 85).
- 0V at Pin 85 (when activated) confirms ground path is complete.
Memory trick: 0V on ground side? Control's supplied!