Voltage Drop Testing

How do I do a voltage drop test?

With the circuit operating, put the meter leads at both ends of one side of the circuit, power side or ground side; the meter reads the voltage lost in that wire, connection or switch.

How do I test a starter circuit with voltage drop?

Disable the engine from starting, crank it, and measure voltage drop on the positive cable, the ground cable and the solenoid circuit; high drop on either side points to cables or connections, not the starter.

How much voltage drop is too much?

Short answer: It depends on the circuit and the current flowing; service information sometimes lists limits, and in general the drop across a connection or wire should be very small compared with the drop across the load.

Voltage drop measures how much voltage is lost across a wire, connection or switch while current flows. Any loss means less voltage reaches the load. Heavy circuits like starter cables are more sensitive, as small resistance causes large drops at high current.

Check service information for specific limits when they are available. Otherwise, compare drops on similar circuits and look for connections showing much more drop than others. Testing while the circuit is loaded is essential, since a resistance problem may not show when no current flows.

Measure across each section to find where the loss happens. Keep clear of belts and fans when testing running circuits, and wear safety glasses around batteries.

Why should I test voltage drop on the ground side?

Short answer: Bad grounds cause as many problems as bad power feeds, and a ground voltage drop test under load finds corroded or loose ground connections that a resistance check often misses.

Current must return to the battery through ground paths: wires, straps, the engine block and body. Corrosion, loose bolts or paint under connections add resistance, reducing voltage available to the component.

Place one meter lead on the component's ground and the other on the battery negative post, then operate the circuit. The reading shows voltage lost on the ground path. Engine and body ground straps are common problems.

Clean and tighten grounds and replace damaged straps. Check service information for ground locations. Keep hands clear of moving parts while operating circuits, and wear safety glasses.

How do I check a sensor's 5 volt reference and ground for voltage drop?

Short answer: With the circuit operating, measure reference voltage at the sensor connector and compare it to the module output, then measure the drop on the sensor ground back to the module ground.

Many sensors run on a 5 volt reference and a signal ground from the module. Resistance in either circuit changes the signal the module reads, leading to wrong readings or codes.

Back probe the sensor connector with the key on and compare reference voltage with service information. Then measure between the sensor ground and the module ground, or battery negative as service information directs; a significant reading means resistance in the ground path.

A shared reference shorted by one sensor can pull down others, so check all sensors on that reference. Back probe gently without piercing seals, and follow service information.