Test the relay circuit for voltage with the relay removed. Use a voltmeter to inspect for system voltage at cavities B and 30. Cavity 30 is hot at all times. Cavity B is energized by the engine control switch.
Use an ohmmeter to test cavities A and 31. Cavity A is the ground side for the relay. Cavity 31 is the load side, glow plugs, of the relay.
The relays load side can be inspected for high resistance with an ohmmeter. The reading should be 0.1 ohms or less. If the resistance is higher, replace the relay.
Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.