Battery Electrical Drain/Parasitic Load Test w/BCM

Tools Required

J 38758 Parasitic Draw Test Switch

If the vehicle exhibits a low or dead battery after an overnight period, or discharges over a period of 2 or 3 days, the electrical system should be checked for an excessive electrical drain. This is referred to as Parasitic Current Drain.

If a battery needs recharging and no cause is evident, check the vehicle for excessive parasitic current drain.

One or more on-board solid state control modules, such as the body control module (BCM) may at some time exhibit a failure mode that causes a high parasitic drain on the vehicle's battery. When the battery is disconnected to install an ammeter, etc., the excessive current drain may not occur once the circuit continuity is restored. Even though cycling the ignition key to the RUN and then to the OFF position may cause such a drain to recur, there may be drains that will not recur unless the vehicle systems are reactivated in a road test. Since the ignition switch must not be rotated to the ACCESSORY, RUN or START position with an ammeter installed between the battery terminal and the battery cable, a current drain test tool must be used as described in the following procedures.

Before starting this procedure, ensure that the ignition switch is in the LOCK position, all electrical accessories are turned OFF, the underhood lamp is disconnected, the door glass is open and the doors are closed.

Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.

Notice: Do not turn the parasitic draw test switch to the OFF position with the engine running. Damage will occur to the vehicle's electrical system.

Notice: The test switch must be in the ON position when removing the fuses in order to maintain continuity in the electrical system. This avoids damaging the digital multimeter due to accidental overloading, such as a door being opened to change a fuse.

1. Disconnect the battery negative cable. Refer to Battery Negative Cable Disconnection and Connection .



2. Install the male end of the J 38758 to the battery negative terminal.
3. Turn OFF the test switch.
4. Install the battery negative cable to the female end of the test switch.
5. Turn ON the test switch.
6. Road test the vehicle while activating all accessories, including the radio and the air conditioning.
7. Turn OFF the ignition switch. Remove the key.

Important: From this point on, electrical continuity must be maintained in the ground circuit of the battery through the J 38758 in the ON position or through the DMM. The BCM can draw several amps of current after the ignition is turned off. Refer to Body Control System Description and Operation .

8. Install the scan tool and perform the power down now feature from the BCM output menu. After performing this function the scan tool can be powered off or disconnected, then wait an additional 5 minutes before continue testing.
9. Set the DMM to the 10 A scale.

Important: If an ammeter other than the DMM is used, ensure that the vehicle does not have a high current drain that would damage the ammeter when connected to the circuit. This can be done using the following procedure:

9.1.	Connect a jumper wire with an in-line 10 A fuse J 36169-A to the terminals of the test switch.

9.2.	Turn the test switch to the OFF position.

9.3.	Wait 10 seconds.

9.4.	If the fuse does not blow, the current is less than 10 A and the ammeter can be used safely.

9.5.	Turn the test switch to the ON position before the fused jumper wire is removed and the multimeter is installed.

10. Connect the ammeter to the test switch terminals.
11. Turn OFF the test switch. This allows the current to flow through the ammeter.
12. Wait at least 60 seconds, then check the current reading.

•	When there is a current reading of 2 A or less, turn ON the test switch, this maintains continuity in the electrical system.

•	Then, switch the meter down to the 2 A scale, for a more accurate reading, when the test switch is reopened.

13. Take the reading in milli-amps.
14. Note the battery reserve capacity. Refer to Battery Usage .

•	Divide this number by 4.

•	Compare this to the multimeter reading.

•	The current drain should not exceed this number.

•	Example: If a battery has a reserve capacity of 100 minutes, the current drain should not exceed 25 milli-amps. If the vehicle has 2 batteries, add the reserve capacities together and divide this total by 4.

Notice: Always turn the test switch knob to the ON position before removing each fuse to maintain continuity in the electrical system and to avoid damaging the meter due to accidental overloading, such as opening a door to change a fuse.

15. When the current draw is too high, remove the electrical system fuses one at a time until the draw returns to a value less than or equal to specifications.
16. Repeat the parasitic current drain test procedure after any repair has been completed.
17. When the cause of the excessive current draw has been located and repaired, remove the meter and the parasitic draw test switch and terminal adapters.
18. Connect the negative cable to the battery negative terminal. Refer to Battery Negative Cable Disconnection and Connection .