alternator problem?

[ol_ben]

I've got a 93 xlt that is showing the "battery" light. It appears to be NOT charging.

I measure the battery voltage while not running and it drops by about a volt while running. I find 12 volts at the stud where the large wire attaches to the alternator. I find 12 volts at the y/w wire on the 3 pin alternator plug. I find 12 volts at the 15 amp (both sides) fuse in the engine compartment.

I have removed the alternator and replaced the brush/regulator assembly. This did not help.

Are there other voltages that should or should not be present on the other wires in the 3 prong plug? How about the single plug with the b/w wire?

This alternator is about a year and a half old. The brushes looked good. It turns smoothly and does not make noise while running.

Is there a complete diognostic procedure for checking the charging systen, alterantor, diode/brush assembly??

Can the alternator be tested independently from the rest of the charging system?

If there is a complete procedure it should be posted as a "sticky" in the tech section(s).

Thanks,
Ben

 

[jayhawkexplorer]

The alternator can be tested separately from the rest of the system. Remove it and take it to a parts store, one of them near you should have a tester. I don't know where you're from but if you have an O'Reilly Auto Parts nearby, they should test it for free. The voltage your alternator is producing sounds a bit low, I would definitely get it checked out. Good luck..

 

[spta97]

What is the running voltage at the battery? It should be about 14. Have you fully tested the battery? Sears will test them for free by putting the battery on a diagnostic machine.

 

[91_XLT]

Few Things to Check

There are a few ways to check if your alternator is working ok. First get a volt meter and check the with the motor off and all the electrical off and mesure the voltage across the battery by putting the negitive to negetive and positive to positive of the battery terminals. It should read 12-13 volts give or take in that range. Then start the vehicle and do the same test and the reading should be higher between 14-15 volts. If it is, then the alternator is charging ok. To confirm for sure it is working losen the positive terminal on the battery and start the vehicle again, while running remove the positive cable from the battery terminal completely and the vehicle should still run. If it doesn't then there is an issue with the alternator.

Hope that helps.

 

[ol_ben]

Thanks to all,

I called the local parts store and they said they will test it. As I was removing it it began to rain - so hard I had to quit 'till it lessens.

The battery is strong - measures about 13 volts, after starting the engine the measurement across the battery posts drops about 1 volt. it appears as if no current is geting to the battery from the alternator. Every thing I could check (listed in my original post) leads me to think it must be the alternator.
This is disappointing since it is not very old. My experience tells me that alternators fail from worn bad diodes in the regulator or worn out brushes (brushes were a problem with the original alternator). I had replaced the brush/regulator assembly on the original alternator and in two weeks the bearing failed. That is when I bought the current alternator. I saved the new brush/regulator assembly when I traded in the old core. I used that assembly in the current alternator to no avail.

When the rain lets up I will test the alternator at the local parts store and will proceed based on the results of that test.

Thanks,
Ben

 

[Firefightermike]

Just run the engine and pull off the positive termnal connecter...if it stalls you need a new alternator..if not id start checking your grounds.

 

[cjbarron5]

Just run the engine and pull off the positive termnal connecter...if it stalls you need a new alternator..if not id start checking your grounds.

no no no this can cause MAJOR problems with the electrical system not to mention the computer and if the alternater was not bad it will be when you hook it back up this causes a power spike not to mention the fact that if the battery still has juice then this wont "kill" the engine a dvom is the best home test for an alternator across the battery posts or a ground and the positive post on the back of the alternator

 

[ol_ben]

I took the alternator to get tested. Result was bad diode/triode? - Bought reman unit with lifetime guarantee. All working fine now.

-Ben