Alternator Test

[sbolejack]

Is there an easy way to test an alternator without taking it off the vehicle??

 

[Dan74Bronc]

Check the voltage at the battery with the engine off, start the motor then check it again. Should be a little higher, somewhere 13 to 14 volts.

There are a few people on this board who are experts. I'm sure they'll chime in soon with some exact figures.

Dan

 

[NMWILLY]

Start the engine, turn on the head lights and put a screw driver against the bushing cover on the back of the alt (where the shaft rides in the center). It will be magnetized if the alt is getting current. This won't tell you how much current it's producing but it's a good quick test to see if there is current to the coils for the stator to cut and produce power.

 

[garberz]

Start the engine, turn on the head lights and put a screw driver against the bushing cover on the back of the alt (where the shaft rides in the center). It will be magnetized if the alt is getting current. This won't tell you how much current it's producing but it's a good quick test to see if there is current to the coils for the stator to cut and produce power.

That is simple yet clever, and I learned something new.

 

[ransil]

Get a voltmeter, digital one harbor freight should have one for under $20.
Start Bronco.
Reading should be Better than 13.5 volts
Rev motor should be the same or higher.
Turn on heater & headlights should drop from previous readings but not by much.

 

[draz]

start your engine and pull the pos. battery cable from the battery if the alt. is not working then your engine will die if it is working then it will stay running.this test will not give you voltage but will tell you if it is working or not the more you can trun on (lights ,radio,ect. ) befor killing the engine the more the alt. is putting out. just a word of cauation dont ground out you pos. battery cable or let it get into you fan or other moveing parts during test

 

[RRRAAAYYY2]

start your engine and pull the pos. battery cable from the battery if the alt. is not working then your engine will die if it is working then it will stay running.this test will not give you voltage but will tell you if it is working or not the more you can trun on (lights ,radio,ect. ) befor killing the engine the more the alt. is putting out. just a word of cauation dont ground out you pos. battery cable or let it get into you fan or other moveing parts during test

Please dont do this. This is a test for a generator. When you do this to an alternator you severly damage it diodes.

 

[PaulS]

When you pull the battery cable while an alternator is running it no longer has any way to regulate the voltage. I have seen voltage go as high as 90 volts on an alternator. The diodes "should" hold up to 180 volts but if they are weak at all this is a great way to kill the weak diodes. Also, if the alternator is fresh it may not be able to hold enough magnetism to keep the car running. The rotor may not be magnatized sufficiently to maintain current at a high enough level to power the ignition and accessories.

 

[draz]

so I learned something new, I use to work in a truck shop and that was the frist test we did if we had a chargeing or battery problem in the fleet and never had any addverse affect testing that way I know of .But if I ever try it again the alt will blow up in my face and every bit of the elect. system will melt to the ground . you know God protects the ignorant tell we learn better


Thanks guys

 

[RRRAAAYYY2]

LOL, great way of looking at it.

What blows the diodes is they have more amperage trying to go through them than they can handle. That is why if the alternator is only putting our 30-40 amps when you pop the cable, it only damages the diodes and not blows them right up.

On a modern alternator, they are almost always over the 50amps, and pops them.

Another thing of interest, the little blue spark you see when you pop the cable, it is said electrcity has to be over 25,000 volts before you can see it. If it goes to the battery, no big deal, if it goes through the system first, it blows everything it sees. In modern cars, that includes your computer systems.

This also applies to booster cables. 1000 times in a row it will go in the battery, and the next 5 might be through your computer.