J 38758 Parasitic Draw Test Switch Tool

Notice: The parasitic draw test switch should never be turned to the OFF position with the engine running, or damage could occur to the attached ammeter or the vehicle electrical system.

Notice: Always turn the parasitic draw test switch knob to the ON position before removing each fuse in order to maintain continuity in the electrical system and to avoid damaging the ammeter due to accidental overloading (I.E. opening door to change fuse, etc.).

Important: The switch knob on the J 38758 is marked ON and OFF. When the switch knob is in the ON position, the circuit is closed and electrical current will pass through the switch. When the switch knob is in the OFF position, the circuit is open and electrical current will not pass through the switch.

Caution: Before servicing any electrical component, the ignition and start switch must be in the OFF or LOCK position and all electrical loads must be OFF, unless instructed otherwise in these procedures. If a tool or equipment could easily come in contact with a live exposed electrical terminal, also disconnect the negative battery cable. Failure to follow these precautions may cause personal injury and/or damage to the vehicle or its components.

1. Disconnect the negative battery cable from the battery.



2. Install the male end of the J 38758 to the negative battery terminal.
3. Turn the J 38758 knob to the OFF position.
4. Install the negative battery cable to the female end of the J 38758 .
5. Turn the J 38758 knob to the ON position.
6. Road test the vehicle and activate all of the accessories, including the radio and the air conditioning.
7. Park the vehicle.
8. Turn the ignition switch to the OFF position.
9. Remove the ignition switch key.
10. Set a digital multimeter (DMM) to the 10 A scale.
11. Connect the DMM to the test switch tool terminals.
12. Turn the J 38758 knob to the OFF position. The current to flows through the DMM.
13. Wait one minute. Inspect and record the current reading.

•	When there is a current reading of 2 A or less, turn the J 38758 knob to the ON position. Continuity is maintained in the electrical system.

•	Then, switch the digital multimeter down to the 2 A scale for a more accurate reading when the J 38758 knob is turned OFF.

14. Turn the J 38758 knob to the OFF position. Wait 10 minutes.
15. Inspect and record the current reading.
16. Note the battery reserve capacity. Refer to Battery Usage .

•	Divide the reserve capacity by 4.

•	Compare this to the multimeter milliampere reading.

•	The parasitic 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 milliamperes.

17. Inspect the charging system if the vehicle has an acceptable amount of current drain and the vehicle battery was weak or failed a load test. Refer to Charging System Check or Battery Is Undercharged or Overcharged .

Notice: Always turn the parasitic draw test switch knob to the ON position before removing each fuse in order to maintain continuity in the electrical system and to avoid damaging the ammeter due to accidental overloading (I.E. opening door to change fuse, etc.).

18. When the vehicle has an unacceptable amount of parasitic current drain, remove each fuse one at a time until the current drain falls to an acceptable level. This will indicate which circuit is causing the drain. Refer to Fuse Block Schematics in Wiring Systems in order to diagnose exactly which part of the suspect circuit is causing the parasitic drain. In some cases a non-fused circuit or component, such as a generator, is the cause of excessive parasitic current drain.
19. Repeat this procedure after any repair has been completed in order to make sure that the parasitic current drain is at an acceptable level.
20. Locate the cause of the excessive current draw. Repair and remove the J 38758 .
21. Connect the negative battery cable to the battery negative terminal.