26SI Series Alternator – T1 Alternator Output – Test

  1. Ensure that the batteries are NOT fully charged.
    1. Fully charged batteries have open circuit voltage above 12.5 V on 12 V systems or 25 V on 24 V systems.
    1. If the batteries are fully charged, then crank the engine for 30 seconds. This action reduces the battery voltage. Operate the lights for 10 minutes while the engine is off as an alternative.
  1. Connect the 9U-5795 Current Probe to a DMM (digital multimeter) or use the 225-8266 Ammeter Tool Gp . The multimeter must have a peak hold feature. Clamp the probe around alternator output wire “B+”. Before you clamp the probe around the wire, ensure that the probe is “zeroed” .
  1. Set the digital multimeter to “peak hold” or “max mode” on the “mV” scale.
  1. Turn on all electrical accessories: lights, air conditioning and radio.
  1. Start the machine, and immediately set the throttle to at least 75%. The peak current will appear on the voltmeter in “peak hold” or “max” mode.

    Expected Result: This current reading should be at least 90% of the specified peak output.


    • OK – The current is at least 90%. For 85 amp alternators, the current should be over 76 A. For 75 amp alternators, the current should be over 67 A. For 50 amp alternators, the current should be over 45 A. Go to Testing and Adjusting, “T2 Electrical System Current – Test”.

    • NOT OK – The current is less than 90%. Go to Testing and Adjusting, “T4 Alternator Drive System – Check”.