Tool Briefing: Charge Light Check-out

Some tools and tricks for charging system analysis

There are two different places to check alternator duty cycle; within the PCM and at the alternator field circuit connections. To see the duty cycle commanded by the PCM, you need a scan tool. Not all scan tools can access that data, but if yours does it probably shows up on the Live Data (Mode 1) screen.


To check the field circuit duty cycle manually, connect the DVOM’s ground lead to chassis ground and backprobe the positive lead to the alternator’s field circuit connecter. Set the meter to read duty cycle and check the reading at different engine speeds. If there’s no change, turn on some lights. If there’s still no change, backprobe the field circuit at the PCM connector. If you get a reading now, there’s probably something wrong with the wiring. You can also try testing this circuit with a low current probe.

Low Current Probe

When using an ammeter or DVOM to read current, the meter must be connected into the circuit. A low current probe allows you to measure current using a DVOM without disconnecting any wires. It’s the perfect tool for measuring battery current draw when the engine is not running, otherwise known as a battery drain test.

The probe uses a Hall Effect sensor, a flat, rectangular semiconductor chip with two circuits and a wire leading to each side; north, south, east and west. The north/south circuit carries an extremely low but constant DC current, and the east/west circuit measures voltage. Normally, current flows straight through the length of the chip, and no voltage is detected in the measuring circuit. When a magnetic field comes close to the chip, it “bends” the current flow towards the east or west side of the chip, and the measuring circuit detects voltage. The stronger the magnetic field, the greater the affect on current flow through the chip, and the higher the voltage in the measuring circuit. The measured voltage is extremely low level, but it’s accurate enough to measure the strength of that magnetic field.


When DC current flows through a wire, a magnetic field is generated around the wire. The low current probe we’re using to detect battery drain actually detects the magnetic field around the battery cable, and that current reading is reported. But to get an accurate reading, the probe must be used correctly. A low-current Hall Effect sensor is extremely sensitive to current flow direction and ambient electrical energy fields.

There are many opinions about what is considered normal key-off battery current draw. Most techs agree that when all of the vehicle’s control units are in ‘sleep mode’ (up to 30 minutes after turning the key off), 50 milliamps is OK, 75 milliamps is too high. However on the newest models those numbers may vary, and some car manufacturers actually publish a specification for key-off battery current draw.

Now that the PCM is in control of the charging system, there’s more information available about its performance. Since the alternator has already been replaced on this vehicle, damaged wiring or corroded connections are prime suspects. To find these, the only tools you need are time and your eyes, but only after you’ve confirmed the battery really is not charging.



Search our Online Product Guide for more tools:

We Recommend