Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing a vehicle with an intermittent issue where the rear defroster does not activate. The technician uses a scan tool and observes that the Body Control Module (BCM) reports the defroster switch input as 'ON' when the button is pressed, but the defroster relay output status remains 'OFF'. The BCM also shows no fault codes related to the defroster circuit. What is the most likely cause of the defroster not activating?
- AA faulty rear defroster switch.
- BAn open circuit in the defroster grid element.
- CA short to ground in the defroster grid wiring.
- DA faulty defroster relay.
Show answer & explanationAnswer & explanation
Correct answer: D. A faulty defroster relay.
The scan tool indicates that the BCM is correctly receiving the 'ON' signal from the switch. However, if the BCM's output to the relay is 'OFF' or the relay itself isn't activating despite the BCM's command, the relay is the most likely point of failure. The BCM is aware of the input, but the output action isn't occurring.
Why the other options are wrong
- A. The BCM reports the switch input as 'ON', indicating the switch is functioning correctly.
- B. An open grid element would prevent defrosting but wouldn't cause the relay output status to be 'OFF' from the BCM's perspective if the relay itself was activating.
- C. A short to ground in the grid wiring would likely blow a fuse or cause continuous activation, not prevent the relay from turning on.
Relay Control Circuit Diagnosis
Troubleshooting the low-current side of a relay circuit to ensure the control module is properly commanding the relay to activate.
- Relays have a control side (coil) and a power side (contacts).
- Control module commands the relay by powering/grounding the coil.
- Scan tool can often monitor switch inputs and relay output commands.
- If input is good but output command fails, check module or control wiring/relay coil.
Memory trick: INPUT makes BCM COMMAND the RELAY.