How to test a delco 10si alternator

Written by stephen benham
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How to test a delco 10si alternator
Test your Delco 10Si alternator with a multimeter. (digital multimeter image by dinostock from

The Delco 10Si alternator was made by Delco from early 1970 but has since been superseded, although you can still obtain refurbished models. Four different versions can be fitted to cars, depending on the amperage requirements of the vehicle. Alternators produce a steady 13 to 15 volts which is controlled by an internal voltage regulator while the ampere it produces varies on the amount of electrical equipment in use on the car. Use a multimeter to test if your Delco 10Si alternator is producing the correct voltage and amperes.

Skill level:
Moderately Easy

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Things you need

  • Protective gloves
  • Multimeter

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  1. 1

    Put on protective gloves before testing your Delco 10Si alternator. Alternators produce high amperes, so you may get an electric shock if you accidentally touch a live cable or a battery terminal.

  2. 2

    Check the label on the rear of the alternator to find the ampere rating -- it is clearly labelled. Delco 10Si alternators have ampere ratings of 37, 42, 55, 63 and 70, depending on type. Some later models produced up to 85 amperes. Make a note of the ampere rating.

  3. 3

    Set the multimeter to the voltage setting. Turn on the car engine and let it idle. Two wires extend from the multimeter: one is the positive, red wire, the other is the negative, black wire. On the ends of the two wires are metal pins. Place the metal pin located on the end of the black wire onto the negative battery terminal. You can tell it's the negative terminal as it is labelled "Neg". Place the other metal pin, which is attached to the end of the red wire, onto the positive battery terminal -- the terminal is labelled "Pos".

  4. 4

    Read the multimeter. You find it displays between 13 and 15 volts, if it's operating correctly. It the voltage is less than 12 volts, or more than 15.5 volts, it's sensible to get the Delco 10Si alternator checked professionally. Low voltage can lead to your battery not getting charged properly and high voltage can damage the electrical equipment in your car.

  5. 5

    Set the multimeter to the ampere setting. Turn on your car lights, fan heater and other energy consuming electrical equipment. The more equipment you turn on, the more amperes your alternator needs to produce.

  6. 6

    Put the negative metal pin from the multimeter onto the negative battery terminal and the positive metal pin onto the positive battery terminal. Be prepared for an electrical spark when you touch the terminals using the metal pins as the alternator is producing high energy to power the equipment you've turned on.

  7. 7

    Read the display on the multimeter. It reads close to the ampere rating you made a note of earlier, if your alternator is operating correctly. For example, if your Delco 10Si alternator is rated at 37 amperes expect to read between 30 and 37. If the reading is 20 per cent or more below the labelled rating, check to see if you can turn on more electrical equipment and then repeat the process. If all the electrical equipment is on and the reading remains 20 per cent below the labelled rating, get the alternator checked out professionally.

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