How to find an electrical drain

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by: Alittle1, Cobalt327, Crashfarmer, Crosley, DarboCmonl, Garyroushkolb, Jon, Justacertifiedtech, Powerrodsmike
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Contents

[edit] Overview

If your engine fails to crank over and start after sitting overnight or longer but will start and run when "jumped", you may be experiencing an electrical drain. The test procedures below will help you to find the component(s) responsible if you are experiencing an electrical drain.

[edit] Electrical drain

This occurs when a component continues to use power from your battery when the vehicle is not in use. Although anything that uses power can potentially go bad and cause a current draw, the most common causes are user controlled devices that can be turned on with the key out of the ignition. Some of the more common items are phone chargers left plugged in to a power outlet, lights (interior or exterior) left turned on, like lighted vanity mirrors, glove compartment light, trunk or underhood lights; and aftermarket accessories (radio, amp, CB, etc.) left powered up.

[edit] Acceptable amperage draw

Exact maximum allowable draw is determined by calculating several factors. But as a general rule, the maximum draw should not exceed 50mA or 0.050 amps.

[edit] Testing

[edit] Tools and equipment

  • Battery tester
  • Battery charger
  • Digital volt/ohm meter (DVOM), set to DC amperage
  • 12 volt test light
  • Proper hand tools to disconnect the negative battery cable
  • Suitable fuse puller (a small pair of needle nose pliers or hemostats will work for blade-type fuses, glass fuses use a plastic puller or can be pried out from the ends using a pick)
  • Wiring diagram of related circuits

[edit] Preliminary inspection

  1. Test your battery and verify that it is capable of holding a charge. Charge if needed.
  2. Using the test light, check all fuses and replace any that are blown.
  3. Do a visual inspection of common problems (missing or loose ground wires, aftermarket radio or lights on a circuit always powered ON and not a switched circuit).
    1. On older vehicles you can get in the vehicle with all doors closed and not worry about a time out interval. Perform an under dash visual inspection.
    2. On most late model vehicles equipped with a Vehicle Control Module (VCM) and/or Body Control Module (BCM) there may be a time out interval (a certain length of time has to pass in order for the VCM and/or BCM to enter sleep mode where the computers will use less power. These are vehicle-specific and you will need to consult a proper service manual).
  4. It is suggested that you consult the owners manual in order to familiarize yourself with the location of all fuse boxes. You will also need to verify that you have a legend for the fuses telling you what amp fuse to use in each circuit and what components the fuse feeds power to. At this time you will also need to determine which fuse feeds power to the interior lights; often it is labeled "courtesy".
  5. Testing
    1. Ignition key off and all doors closed, and, all aftermarket accessories turned off. If the vehicle is equipped with an under hood light you will need to disconnect the wires going to it or remove the bulb.
    2. Disconnect the negative battery cable.
    3. Connecting DVOM
      1. Make sure the DVOM cables stay connected to their correct locations (and not touching any other part of the vehicle) throughout the entire testing procedure, except when instructed otherwise. If they become disconnected you will need to perform steps 3.2 through 3.4 again. Unless you have alligator clips or can secure your test probes you may need another person to help you perform these tests.
      2. Connect the DVOM black cable (connected to the com port of the DVOM) to the battery negative post
      3. Connect the DVOM red cable (connected to the port labeled A or Amps) to the negative cable
      4. Touch the negative cable end to the negative post of the battery for at least 15 seconds, then break the connection and let the meter reading become stable- record the reading.
        1. Was the reading 0.050 A or less?
          1. Yes- You are not experiencing an excessive power draw .
          2. No- It was higher- you have confirmed there is a power draw. Continue with the next step.
    4. Disconnect either one of the DVOM cables. It is now safe for you to open the doors in order to gain access to the fuses. You will now need to remove the "courtesy" fuse, and perform step 3.4 again.
      1. Did the reading remain the same or very close?
        1. Yes- it remained the same (it is still too high) - there is NOT a problem in this courtesy light circuit. Continue testing.
        2. No- the amperage reading is LESS than before - there WAS an excessive power draw in this circuit. This is the problem circuit.
  6. If readings are still in the unacceptable range after checking this circuit you will now begin the fuse panel isolation portion of this procedure.
    1. Begin by pulling the fuses one at a time while monitoring DVOM readings after each fuse is removed. When the amperage reading drops to an acceptable range, note the circuit it dropped on, set this fuse aside, and reinstall the fuses you already removed. Once all other fuses are installed verify the amperage is still below specification. If so, now you will reinstall the fuse in the suspect circuit and continue to the accessory/device isolation portion of this test. The amperage will increase (there IS an amperage draw) when you reinstall this last fuse.

[edit] Isolating the faulty part

Now that you know which circuit the draw is on, obtain a wiring diagram if needed to determine what devices are powered by that circuit.

  1. Begin the process of unplugging accessories/devices (just like with the fuses, one at a time) until your meter drops into the acceptable range. When it does, you have found your culprit.
  2. Replace that part and perform step 3.4 again to verify repair; test should now shows o.050 amp draw or less.

[edit] Parasitic draw test switch

Here is an alternative way to check for electrical drains using a parasitic draw test switch.

Parasitic draw diag.gif

[edit] Tools required

J 39200 Digital multimeter J 38758 Parasitic Draw Test Switch

Warning Warning: 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.

The switch knob on the J38758 is marked "ON" and "OFF". When the switch knob is in the "ON" position, the circuit is closed and electrical current may pass through the switch. When the switch knob is in the "OFF" position, the circuit is open and electrical current may not pass through the switch.

1. Remove the battery ground (negative) cable from the battery negative terminal. (Vehicles with dual batteries, isolate main battery by disconnecting negative battery cable from secondary battery while performing parasitic draw test).

2. Install the J 38758 male end to the battery negative terminal.

3. Turn the J 38758 knob to the "OFF" position.

4. Install the battery ground (negative) 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 (radio, A/C, etc.).

7. Stop the vehicle. Turn the ignition switch to the "OFF" position and remove the ignition key.

8. SET the J 39200 to the 10 ampere scale and connect it to the terminals on test switch tool.

9. Turn the J 38758 knob to the "OFF" position to allow the current to flow through the J 39200.

10. Wait at least 20 minutes. Check and record the current reading. If the current reading is at or below 2 amperes, turn the J 38758 knob to the "ON" position (to maintain continuity in the electrical system) and switch the J 39200 down to the 2 ampere scale for a more accurate reading when the J 38758 knob is turned "OFF"

11. Note the reserve capacity rating for the vehicle's battery. If the battery does not specify the reserve capacity, 30 milliamps can be used for a maximum parasitic draw on most vehicles. However some vehicles can have up to 50 milliamp parasitic draw.

  1. Divide the reserve capacity number by four.
  2. Compare this number to the ampere reading taken in step ten.
  3. The parasitic current drain should not exceed this number.
  4. Example: If the battery has a reserve capacity rating of 100 minutes, the current drain should not exceed 25 milliamps.

12. If the vehicle has an acceptable amount of current drain and the vehicle battery was weak or failed a load test, check the charging system for proper operation.

13. If the vehicle has an unacceptable amount of current drain, remove each fuse one at a time until the current drain is at an acceptable level (this will indicate which circuit is causing the drain). Then diagnose exactly which part of the suspect circuit is causing the drain. In some cases, a fusible link protected circuit or component such as a generator, may be the cause of excessive current drain. This should be investigated if nothing shows up doing the procedures outlined here.

Warning Note: Always turn the parasitic draw test switch knob to the "ON" position before removing each fuse 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.).

98 chev par draw test.gif

  • Repeat the parasitic current drain procedure after any repair has been completed to insure that the current drain has returned to an acceptable level.

14. When the cause of the excessive current drain has been located and repaired, remove the J 387S8 and connect the battery ground (negative) cable to the battery.

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