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Tech Tip: Measuring Current Draw

Oftentimes, school buses are outfitted with air conditioning units, wheelchair lifts and many other accessories after they are sold to the school district. Eventually, these buses start experiencing battery issues because the originally installed alternator doesn’t provide enough output to handle the new electrical demands. If the alternator is not upgraded, this will result in constant discharging of the batteries, premature alternator failure, and starting issues due to low voltage.

Checking Sufficiency of Alternator Output

Measuring the current draw for the bus is the best way to ensure the alternator has sufficient output for all of the vehicle’s electrical demands. This will help you determine if you have the correct alternator for your application.

1.  Fully charge the batteries before beginning. Partially charged batteries will result in inaccurate readings.

2.  Attach a clamp-on ammeter around negative battery cables.
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3.  With the ignition key on and engine off, turn on all accessories, including the heater, defroster blower fan, lights, wipers, radio, video monitor system, stop flashers and warning lights.

4.  With the clamp still in place, take and record a reading. Then turn off all the accessories and leave the ammeter in place.
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5.  Record the amperage draw of the special needs lift if the bus has one. With the key on and engine off, place a load on the lift and raise it. Take a reading and record it.

6.  Measure the amperage for all accessories that automatically turn on when the engine is started. Record a reading with the engine on, and at high RPM with the accessories off.

7.  Calculate the recommended alternator amperage rating by adding all the readings together and then multiplying that total by 1.2. As a rule of thumb, the total vehicle amperage demands should be 20 to 30 percent under your alternator’s rated output.

Watch our Tech Tip video to find out how you can determine if your alternator has sufficient output for your vehicle.

Visit the BorgWarner YouTube channel today!

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