Testing For Opens

PREPARATION
Before you begin your testing and diagnosis, you should sketch a schematic of the system. This will help you to logically walk through the diagnosis process. Drawing the sketch will also reinforce your working knowledge of the system. Refer to the attached drawing for a sample schematic

There are two types of tests you can use for tracking down an open circuit: the Continuity Check using an ohmmeter or Voltage Check using the voltmeter. Either test, properly done, will yield good results.

USING THE CONTINUITY CHECK METHOD

NOTE: Make sure to always start with the DMM at the highest resistance level.

1. Disconnect the battery negative cable.

2. Start at one end of the circuit and work your way to the other end. (At the fuse block in this example)

3. Connect one probe of the DMM to the fuse block terminal on the load side.

4. Connect the other probe to the fuse block (power) side of SW1. Little or no resistance will indicate that portion of the circuit has good continuity. If there were an open in the circuit, the DMM would indicate an over limit or infinite resistance condition. (point A)

5. Connect the probes between SW1 and the relay. Little or no resistance will indicate that portion of the circuit has good continuity. If there were an open in the circuit, the DMM would indicate an over limit or infinite resistance condition. (point B)

6. Connect the probes between the relay and the sensor. Little or no resistance will indicate that portion of the circuit has good continuity. If there were an open in the circuit, the DMM would indicate an over limit or infinite resistance condition. (point C)

USING THE VOLTAGE CHECK METHOD

1. Connect one probe of the DMM to a known good ground.

2. Begin probing at one end of the circuit and work your way to the other end.

3. With SW1 open, probe at SW1 to check for voltage.

If voltage: open is further down the circuit than SW1.
If no voltage: open is between fuse block and SW1 (point A).

4. Close SW1 and probe at relay.

If voltage: open is further down the circuit than the relay.
If no voltage: open is between SW1 and relay (point B).

5. Close the relay and probe at the sensor.

If voltage: open is further down the circuit than the sensor.
If no voltage: open is between relay and sensor (point C).