How to Test a Bosch Alternator

Updated February 21, 2017

Bosch makes alternators to fit most types of cars. The alternators provide maximum output and performance, operate under extreme conditions and are long-lasting. Alternators have to work hard to produce electricity to power the ever-increasing amount of electrical equipment in modern cars and, at the same time, charge your vehicle's battery. Bosch alternators produce between 13.5 and 14.5 volts -- slightly more than the output voltage from your battery. This is to compensate for voltage loss in the cables between the alternator and battery. Output varies by alternator type, but they generally produce between 75 and 100 AMPERES (Automated Maintenance Performance and Engineering Reliability Evaluation System).

Test your Bosch alternator with a multimeter.

Find the AMPERE rating of your Bosch alternator by looking in the specifications section of your vehicle's owner's manual. In the electrical section, you will find a minimum and maximum output range. If it says "50:75," this means that it produces between 50 and 75 AMPERES, depending on the electrical equipment that is turned on. You can also get the rating from the label on the alternator, but it can be difficult to find as it is usually on the back of the alternator, which is often inaccessible.

Put on a pair of protective gloves and goggles because you will need to start the engine and go near the battery terminals to test your Bosch alternator.

Open the bonnet. Check that you aren't wearing loose clothing, which could impede your efforts. Start the engine.

Turn on the electrical equipment in your vehicle, including the windshield wipers, heater or air conditioner, lights and stereo. By doing so, you are pushing your Bosch alternator to produce maximum AMPERES.

Place the multimeter sensor on the end of the black wire extending from the meter onto the negative battery terminal labelled "+" or "Pos." Attach the sensor on the end of the red wire extending from the meter onto the positive battery terminal labelled "-" or "Neg."

Read the output display on the multimeter. It will read at the high end of the range if the alternator is operating correctly. For example, if the range was 50 to 75 AMPERES, the multimeter should read between 65 and 75 AMPERES. If it is lower than 60, check that you have turned on as many electrical devices as possible. If you have, then get your alternator checked by a professional.

Turn off all of the electrical equipment. Repeat the process with the multimeter as before. Read the output display. It will read at the lowest end of the range if the alternator is working correctly. So, using the previous example, the multimeter should read 50 AMPERES, plus or minus 10 per cent. If the reading is more than 10 per cent below the low range, get your alternator checked by a professional.

Set your multimeter to read voltage. Repeat the process by attaching the sensors on the end of the red and black wires from the multimeter onto the respective "+" and "-" battery terminals.

Read the output voltage on the multimeter. It will read between 13.5 and 14.5 volts if the alternator is operating correctly. If the voltage is lower than 13 volts or higher than 15 volts, get your alternator and voltage regulator checked by a professional.

Close the bonnet. Turn off the engine.

Things You'll Need

  • Protective goggles
  • Protective gloves
  • Multimeter
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About the Author

James Stevens has been writing articles for market research companies in the U.K. since 1990. He has written various country profiles for inclusion in comprehensive market reports including Vision One Research and Investzoom Market Research. Stevens holds a General Certificate of Education from Chelmsford College of Further Education.