Testing a Ground in a Truck Using an Ohmmeter

No ground equals no go, so test it before you make assumptions about anything else.

A truck's ground is the foundation of its electrical system. A basic electrical system is like a racetrack, with electrons going away from the power source on the positive wires and back toward the power source on the negative. A grounded chassis or engine block is essentially one big negative wire, a return track for electrons. Testing for excess resistance in the ground is one of the simplest things you can do with the electrical system, and an important first step in any troubleshooting procedure.

Things You'll Need

• Digital multimeter
• Wire brush

Instructions

1. Brush the top of the negative battery terminal by using the wire brush to remove any corrosion or contamination that might skew readings. Set the digital multimeter to read in ohms.

2. Touch one of the probe terminals to the battery's negative terminal, and the other to the point you wish to test. If you're testing the engine ground strap, touch the multimeter's probe to the bolt that attaches it to the engine.

3. Record the ohm measurement; typically it will be between 0.25 and 1.0 ohms of resistance. Higher numbers mean excess resistance in the circuit, likely a loose connector, corroded connection or damaged wire. A "0" reading means there is an open circuit -- a completely snapped wire or one not attached to anything.

4. Set the multimeter to read in volts, and touch the red and black probes to the positive and negative terminals, respectively. The reading should be between 12 and 12.6 volts. Touch the negative probe to the chassis; there should be a drop of no more than 0.25 to 0.50 volts between the battery reading and chassis reading.

References

• How to Diagnose and Repair Automotive Electrical Systems; Tracy Martin