Testing For Shorts

PREPARATION

Before you begin your testing and diagnosis, 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 a short circuit: the Resistance Check using an ohmmeter or Voltage Check using the voltmeter. Either test, properly done, will yield good results.


RESISTANCE CHECK METHOD

1. Disconnect the battery negative cable and remove the blown fuse.

2. Disconnect all loads (SW1 open, relay disconnected and sensor disconnected) powered through the fuse.

3. Connect one probe of the ohmmeter to the load side of the fuse terminal and the other probe to a known good ground.

4. With SW1 open, check for continuity.

If continuity: short is between fuse terminal and SW1 (point A).
If no continuity: short is further down the circuit than SW1.

5. With SW1 closed, relay disconnected and probes at the load side of fuse terminal and ground check for continuity.

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

6. With SW1 closed, relay contacts jumped with jumper wire and probes at the load side of fuse terminal and ground check for continuity.

If continuity: short is between relay and sensor (point C).
If no continuity: check sensor, retrace steps.

VOLTAGE CHECK METHOD

1. Remove the blown fuse and disconnect all loads (i.e. SW1 open, relay disconnected and sensor disconnected) powered through the fuse.

2. Turn the ignition key to the ON or START position and verify battery voltage at the B+ side of the fuse terminal (one lead on the B+ terminal side of the fuse block and one lead on a known good ground).

3. With SW1 open and the DMM leads across both fuse terminals, check for voltage.

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

4. With SW1 closed, relay and sensor disconnected and the DMM leads across both fuse terminals, check for voltage.

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

5. With SW1 closed, relay contacts jumped with fused jumper wire check for voltage.

If voltage: short is down the circuit of the relay or between the relay and the disconnected sensor (point C).
If no voltage: retrace steps and check power to fuse block.