Which procedure is used to locate a parasitic electrical draw on an EMS vehicle with the engine and accessories off?

• A. Disconnect the battery negative terminal and monitor draw.
• B. Measure current with an ammeter in series with the battery ground while removing fuses/circuits to isolate the source.
• C. Use a clamp-on meter around the positive cable while engine is off.
• D. Remove the battery from the vehicle and test with a bench supply.

Answer: (B)

The main idea is to measure actual current flowing from the battery when the vehicle is off, so you can pinpoint what is drawing power. The best way is to put an ammeter in series with the battery’s ground path, so every milliamp leaving the battery passes through the meter. With the engine off, you read the total parasitic current. If that current is higher than normal, you then isolate the source by removing fuses or disconnecting circuits one at a time and watching the meter. When the reading drops back to a normal parasitic level, you’ve identified the circuit responsible and can inspect that system for things like lights, modules, or aftermarket devices that shouldn’t be drawing power.

This approach is preferred because it directly measures the current and allows systematic isolation of the responsible circuit. Disconnecting the negative terminal and monitoring draw would break the circuit and not yield a meaningful measurement, since the current path is interrupted. A clamp-on meter around the positive cable can measure total current, but it’s less reliable for isolating a specific culprit in a complex harness and may not clearly show which branch is responsible. Removing the battery and testing with a bench supply takes the vehicle out of its normal operating context and does not reflect the parasitic draw present in the vehicle’s own electrical system.