How To Bench Test An Alternator? | Mercedes-Benz Forum

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vtgreaser V Hello, I have a problem with my charging system, and it's not the new battery. The tester at Advanced Auto read low voltage from alternator. Should I pull it or try to test it in the car? The belt tensioner rod on alternator is broken, but the pulley spins and the belt doesn't seem too loose. Is there a pin on the alternator to test for voltage while it's running, or should I test for a short in the wires to the battery (block heater is tied in)? I could pull alternator and bring it somewhere, but can they bench test it out of the car? Thanks in advance #1 · Aug 15, 2011 Hello, I have a problem with my charging system, and it's not the new battery. The tester at Advanced Auto read low voltage from alternator. Should I pull it or try to test it in the car? The belt tensioner rod on alternator is broken, but the pulley spins and the belt doesn't seem too loose. Is there a pin on the alternator to test for voltage while it's running, or should I test for a short in the wires to the battery (block heater is tied in)? I could pull alternator and bring it somewhere, but can they bench test it out of the car? Thanks in advance Sort by Oldest first Oldest first Newest first Most reactions #2 · Aug 15, 2011 If the engine is running, you can test the alternator voltage at the battery. It should read somewhere between 13.8 to 14.5 V. #3 · Aug 15, 2011 Since you never mentioned what specific car you are you are referring to (and its not in your info/profile) so I'll assume you're referring to a 300D :rolleyes:

Our alternators may read "low" due to the fact they only output 55-65 amps, which would be low compared to modern cars with 80-110+ amps. What alerted you it being problematic? To me it may just be easier to get a new one/rebuild it... #4 · Aug 15, 2011 Hi, Sorry about insufficient info (a rookie). The car is a 240d. There isn't any fuse involvement in that line is there? I just don't want to go out and buy a new alternator only to find that it wasn't the problem (like I did with the battery after the test readings showed fail, though it still was taking a charge). Thanks for the reply #5 · Aug 15, 2011 Ok- I'll look for 13.8-14.5 at the terminal, but I know it's not there because it already tested low at the parts place (not sure how low or if none). If none, maybe a connection? If low, maybe voltage regulator or alternator. How does one go about testing a voltage regulator without removing it? Thanks #6 · Aug 16, 2011 If the alternator is good, but the voltage regulator is not, you're likely to get a voltage above 14.5 V. If the alternator is bad, the voltage will not reach 13.8–14.5V. The regulator can fail so it doesn't regulate anymore, or the brushes are worn out. The regulator can simply be checked by removing it from the back of the alternator. It is a white box held by two screws. When you feel inside the alternator (engine not running of course) you can feel whether the two rings of the alternator are not worn out. Insert Quotes Post Reply

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