Wheelchair Lift Interlock and Door Ajar Fault
A bus equipped with a wheelchair lift reports that the lift will not deploy. The lift is powered from the body electrical system and has an interlock that prevents deployment unless the parking brake is set and the transmission is in park. The driver says the parking brake indicator is on and the shifter is in park. The lift control panel shows no power. The bus also has a door ajar warning that stays on intermittently. The lift has its own inline fuse at the battery and a relay controlled by the interlock module. Measured at the lift relay coil, there is no voltage with the brake set and in park. The interlock module receives a park signal and a brake signal.
1. What is the most likely reason the lift relay coil has no voltage?
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The interlock module is not commanding the relay, or the relay coil circuit is open. Since the driver reports park and brake set, verify the module actually receives both signals. The intermittent door ajar warning suggests a shared ground or a door switch issue that may affect the interlock logic. Check the module inputs with a scan tool or meter, check the relay coil ground, and check the inline fuse. Do not bypass the interlock; diagnose the missing signal.
2. How does the intermittent door ajar warning relate to the lift fault?
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The lift interlock may require the door to be open or closed depending on design, and it may share a ground or signal with the door ajar circuit. An intermittent door switch can cause the module to see an invalid state, inhibiting lift power. Inspect the door switch and its connector, and verify the module sees a clean state change. Repair the door circuit first, then retest the lift.
3. What must be verified before the bus is returned to service with a working lift?
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Verify the lift deploys only with park and brake set, verify the interlock prevents deployment in gear, test the lift with a simulated load, and confirm the door ajar warning is repaired. Check the inline fuse and relay, and document all tests. Confirm the emergency stop and manual backup operate.