Alternator Charging - A Couple Of Preliminary Questions

[mbsl98]

I have had to jump my '95 Turbo (121k) twice in the past couple of weeks, after it sat for a day or two. I put a meter on it and found that the battery is showing only 12.2 volts after car being run for a good while. Alternator voltage, measured across the + and - battery terminals , engine running at around 1500, was 13.3, droppping to 13.1-13.2 whith full load of lights, heater, etc. running. Obviously, most likely issues seems to be that the alternator is not charging enough, or charge is getting lost along the way from alt. through starter post to battery. I will start sorting this out in a day or so, so just a couple of preliminary questions:

1. The battery is less than 2 years old, clean cable connections, but I will get it load tested before doing anything further - just don't expect this to be an actual battery issue due to the low charge rate. Is this a reasonable assumption, or can a failing battery result in low alternator output readings taken across the battery terminals?

2. The real question: I know that I need to test the output directly on the back of the alternator, from the B+ post, to see if there is actually good output that is being lost along the route to battery. Question is where to put the negative probe from the meter - to block near the alternator, back to battery negative post, etc? I assume if I went to the battery post, it could be a bad reading if the battery to ground cable has any flaws. Also, any trick to removing the plastic protective cap over the B+ post, or is it just a snap on fitting?

Any other quick thoughts? Thanks

[Dan A]

I have had to jump my '95 Turbo (121k) twice in the past couple of weeks, after it sat for a day or two. I put a meter on it and found that the battery is showing only 12.2 volts after car being run for a good while. Alternator voltage, measured across the + and - battery terminals , engine running at around 1500, was 13.3, droppping to 13.1-13.2 whith full load of lights, heater, etc. running. Obviously, most likely issues seems to be that the alternator is not charging enough, or charge is getting lost along the way from alt. through starter post to battery. I will start sorting this out in a day or so, so just a couple of preliminary questions:

1. The battery is less than 2 years old, clean cable connections, but I will get it load tested before doing anything further - just don't expect this to be an actual battery issue due to the low charge rate. Is this a reasonable assumption, or can a failing battery result in low alternator output readings taken across the battery terminals?

2. The real question: I know that I need to test the output directly on the back of the alternator, from the B+ post, to see if there is actually good output that is being lost along the route to battery. Question is where to put the negative probe from the meter - to block near the alternator, back to battery negative post, etc? I assume if I went to the battery post, it could be a bad reading if the battery to ground cable has any flaws. Also, any trick to removing the plastic protective cap over the B+ post, or is it just a snap on fitting?

Any other quick thoughts? Thanks

1. I would have the battery load tested. Probably not the issue, but good to at least rule it out. It should be sufficiently charged before you can test it, so hook it up to a charger till it's fully charged.

2. Any ground should work to test the voltage from the alternator.

If the alternator is putting out 14+ volts at around 2000 rpm, then you're alternator is probably good. More than likely, then it is a problem with the B cable. You should also do a drop voltage test. Connect your voltmeter to the b+ output on the alternator and the other end to the POSITIVE side to your batter. You should get a very low voltage reading. Like .2V or less. Anything more and you have a problem on that cable (basically you have too much resistence somewhere in the cable), either from the alt to starter or starter to battery. You can do the same test between each of those sections to find which one. You can also do the same on the ground side. I believe the ground on the alt is the housing itself, so connect the voltmeter to the alt housing and the other end to the - batt terminal. You should get again no more than .2 volts and I would expect probably less than .1V. Any more and you have a ground cable issue.

If all that checks out, you may want to check if you have something drawing power when the car is off.

Do NOT replace parts unless you do these tests.

[mbsl98]

thanks, Dan. That is very clearly set out. Just one last question - removing the plastic cap on the B+ alternator stud. I assume this just pries off, but don't like to break things. Is that right?

[Dan A]

thanks, Dan. That is very clearly set out. Just one last question - removing the plastic cap on the B+ alternator stud. I assume this just pries off, but don't like to break things. Is that right?

I don't recall mine having a cap. it could be my bad memory or it already being taken/broken off.