How to know altrnater is bad

Asked by Aug 26, 2013 at 04:53 PM

Question type: Maintenance & Repair

2 Answers


Hook up a volt meter to you battery, take a reading, it should be around 12.5 to 13.2 volts, if voltage is lower than recharge battery. Start engine and take another reading, at idle the volts should be 13.7 to 14.2 volts. This a good voltage test. To check the amperage output you really need either take the alternator off and take it to someone who can test it's amperage output or take the car to a shop that can do it.


You can purchase a 100 to 150 amp carbon load tester at most larger automotive parts stores. But the voltage check above is quick easy test. On most vehicles there is also extra wires besides the 4 - 10 gauge wire that charges your battery. One will be the "excite" lead. If this wire is faulty it will not allow the altenator to excite and start producing power. I will read 12 volts with the key in the on position. (Some altenators can self excite if you go over a certain rpm)

Your Answer

Add photo

Related Questions

  • How to know altrnater is bad 2 answers

    Maintenance & Repair

  • How to know if the ECU is bad on truck?

    How can you "sort of" know if the ECU is bad. I have a 2007 (07.5) Chevy Silverado 1500 and I'm having them problems: transmission is not wanting to shift, when trans does shift it it jerks, odomet...

    2007 Chevrolet Silverado 1500 LTZ Ext. Cab 4WD


    Maintenance & Repair

  • How to know if the water-pump is going bad? 1 answer

    The motor was over-reving for no reason, checked the water and it was bone dry, filled it back up with water, run it for a few mins and checked again and again no water. Still over-reving. No leak on ...

    2006 Kia Sorento LX


    Maintenance & Repair

  • how do i know my module is bad? 4 answers

    how do i know my mazda 323 Module is bad?

    Mazda 323


    Maintenance & Repair

  • how do i know if my computer is bad.. 4 answers

    my 1995 olds cutlass ciera with a 3.1 engine..hard to start and when does wont it wont start at all..if my computer is bad how can I check it..

    1994 Oldsmobile Cutlass Ciera 4 Dr S Sedan


    Maintenance & Repair


Search Maintenance & Repair Questions

Maintenance & Repair Experts

#1 Tom Demyan
Tom Demyan
Reputation 174,920
#2 dandyoun
Reputation 93,540
#3 tennisshoes
Reputation 76,120
View All

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use.