Parasitic Load Testing

Use Of Parasitic Draw Test Switch:

TOOL REQUIRED:
J 38758 Parasitic Draw Test Switch

If the battery discharges while the vehicle sits for a few hours or days, the vehicle should be diagnosed for parasitic loads. Simple connection of an ammeter in series with the battery may not find the cause as the condition may not show up when power is restored unless that particular component is repeatedly cycled. Parasitic loads can be diagnosed using either of the procedures below, but use of the special tool as described in the first procedure is recommended.

NOTICE: Parasitic draw test switch should never be turned to "OFF" position with engine running or damage could occur to attached ammeter or vehicle electrical system.

1. Turn knob on J 38758 to "OFF" position.
CAUTION: Refer to "Service Precaution".

2. Negative battery cable.
3. Install parasitic draw test switch male (cable) end to negative battery terminal.
4. Install negative battery cable to female end of test switch tool.
5. Turn knob on J 38758 to "ON" position.
6. Road test vehicle while activating all accessories (radio, A/C, etc.).
7. Turn ignition switch to "LOCK" position and remove ignition key.
8. Set ammeter to 10-amp scale and connect to terminals on test switch tool with alligator clips. Use of 10-amp scale may prevent damage to meter.
9. Turn knob on J 38758 to "OFF" position to allow current to flow through ammeter.

Component Parasitic Load:

10. Check current reading. Allow time for reading to stabilize. Verify that retained accessory power (RAP) is off. If current reading is at or below two amps, turn test tool knob to "ON" position (to maintain continuity in electrical system) and switch to a lower scale for a more accurate reading, or install a more accurate meter, if necessary. Turn knob on J 38758 to "OFF" If current reading is above 25 milliamps, refer to components parasitic load chart for specific current loads and micro-processor "time-outs" for modules in "AWAKE" and "ASLEEP" states. Total reading below 20 milliamps is normal, and a reading below 25 milliamps is acceptable. A zero reading indicates a problem with the meter, scale being used, or connection.
11. If the current draw is above specifications listed in component parasitic load chart, remove system maxi-fuses and circuit breakers one at a time until current draw returns to a value less than or equal to specifications listed. This should be done while monitoring ammeter so that a disruption of drain will be evident. Perform steps 9 and 10 each time a fuse is removed. Total readings in the hundreds of milliamps may suggest grounded wires, sticking relays, or small lamps remaining lit.
Important
- Always turn knob on J 38758 to "ON" position before removing each fuse to maintain continuity in electrical system and to avoid damaging ammeter due to accidental overloading (i.e., opening door). Do not turn knob to "OFF" while engine is running.

12. When cause of excessive current draw has been located and repaired, repeat steps 9 and 10 to verify the repair. Then turn J 38758 to "OFF," remove tool, and reconnect negative battery cable to terminal.
Tighten
- Negative battery cable to 15 N.m (11 lb. ft.).