Poor grounding point

Most circuits on the vehicle use a chassis ground, a ground which is fastened to any metal surface of the vehicle. These ground points tend to be more exposed to weathering than the B+ side of the circuit, with a high potential for corrosion.

Many chassis grounding points are located on painted areas. A poor connection could result if the "cutting" action of the terminal or lock washer does not sufficiently clear the paint from the surface.

Diagnosing High Resistance Problems

Determine if There is Current Flow in the Circuit

Isolate the Location

Because you are dealing with a series resistance, you can use the series circuit voltage principles to quickly determine if you have a high resistance problem and isolate its location.

You can usually determine if there is current flow by seeing if there are any visible signs of operation (dim light bulb, slow turning motor, relay contact "buzzing", etc.). However, there still can be some current flow in a circuit even if there is no external sign of operation.

A voltage drop measurement can verify if there is current flow or not. Since voltage drops occur only if there is current flow in a circuit, a voltage drop at the load, with confirmed continuity through the load, means that there is current flow in the circuit.

Measure for the voltage drop by connecting the voltmeter in parallel directly at the B+ and ground terminal of the load, with the circuit ON.

By taking this voltage drop, and comparing it to battery voltage, you will know how much voltage is being lost to resistance in the circuit. Remember that for most body electrical circuits, about 0.2 V per connection or about 0.5V for the entire circuit is allowed. For low current flow sensor circuits, or any circuit related to an ECU, up to about 0.1V loss in a circuit's wiring and connections is acceptable.

The exact location of a high resistance problem can be easily found. Any resistance in a series circuit causes a voltage drop. To isolate the problem, you just need to look for the voltage drop to "flag" the exact location:

1. Connect the voltmeter in parallel: Place one probe at the ground terminal at the load, and the other probe to a known good ground.

2. With the circuit ON, measure the voltage drop. If the voltage drop exceeds 0.5 V (about 0.2 V per connection) you have a problem on the ground side of the circuit. If the voltage drop is OK, the problem must be on the B+ side of the load.

If you want to measure in parallel to the B+ side of the circuit, you can connect one probe to the B+ terminal of the load, and the other probe to a fuse or other wiring that has a connection to the positive terminal of the battery.

3. When you know which side of the circuit has the problem, use the EWD to locate test points in the circuit (wire harness to wire harness connectors, junction or relay block connectors, etc.) that you can continue to make voltage drop measurements at. Remember that a near zero volt drop is normal if the wire/connection is OK. The voltage drop occurs only when there is resistance.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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