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How to test for a voltage drain

What’s the proper testing procedure to find the source of the voltage drain? Mark Burnside, senior product manager of engine starting products for Maxwell Technologies, walks through the steps.

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What’s the proper testing procedure to find the source of the voltage drain? Mark Burnside, senior product manager of engine starting products for Maxwell Technologies, explains:

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❶ Begin by collecting together all cables coming from the truck/cab to the positive terminals of the battery.

❷ Connect them together with a single bolt and nut and attach to the negative side of an ammeter.

❸  Join the positive end of the ammeter to a positive battery terminal.

❹  Note the current being drawn.

❺ Gain access to the fuse panel and remove one fuse at a time. Check the ammeter for a change in amps being drawn after the removal of each fuse.

❻  If there is no change, replace the fuse and remove the next fuse.

❼ When a lower reading is observed, the source of the drain has been identified. The diagram on the fuse panel cover will identify the load connected to the fuse. This load is very likely the cause of the voltage drain.

❽ If the reading on the ammeter remained constant through the removal of all fuses in the truck, it means that the source of the voltage drain is not connected through the fuse panel. The likely cause is an aftermarket option that was added after the truck was built and delivered. In this case, move on to the following sequence:

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  • Remove one cable at a time from the cables bundled together in Step 2.
  • Observe the ammeter reading after each cable is removed.
  • One of the cables will cause the ammeter reading to drop. This cable is connected to the voltage drain. Trace the cable back to the offending load.

❾ If none of the above steps results in identification of the voltage drain, it means that the source of the voltage drain is switching on and off automatically. In this case, it is necessary to install a data logger on the battery that monitors current and identify when and how the offending load switches on and off. It then becomes necessary to correlate the timing of the on/off events with a load on the truck.

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