Recently, my amp needle has been very low while driving, around 10-11 volts, where normally it is 13-14. It was always low at idle because I have underive pulleys, but I never had a problem with charging at speed. Now it seems to be low almost all the time unless I really get on the gas; i.e. high rpms. I suspect the altenator but want a confirmation before I go out and start replacing parts. Can anyone give me some opinions?
Thanks,

Dave M
88 GTA

 

You can check the battery by simply getting a voltmeter to test the voltage between + and -. It should read around 12 volts.
With the car running, do the same test...your reading should range from 11 to 14 volts (a tech manual might give a more accurate range). It should fluctuate a bit...this shows that your voltage regulator is working properly. The sure-fire way to check the alternator is to disconnect the positive lead of the battery while the engine is running....if the engine sputters or dies, then your alternator needs to be replaced.

 

Dave,

I think what you meant to say was "low VOLTAGE (vice "amperage") while driving..." since you provided only voltage (not amperage) information in your post.

First of all, I would NEVER recommend disconnecting the positive cable from the battery while the alternator is operating because you'll risk damaging the ECM or other electronic devices in the vehicle. This is because the alternator needs the battery to act as a filter. The battery is needed essentially to only start the vehicle, and once the engine is running, the battery is not providing current as long as the alternator is. (When the alternator is unable to provide the current, then the battery takes over, as when idleling…) However, because the output of the alternator is pulsed DC (at a frequency determined by rotor (or engine) speed, usually several thousand hertz, or cycles per second), a battery is required to "smooth" this output. Disconnecting the battery positive cable is asking for problems. It may damage an electronic component in the ECM immediately, or it could cause a failure later by damaging a part only partly, and then, over time, it’ll get worse until it completely fails. Do you want to risk it? Your choice.

I'll try to answer your question as simply as I can. It appears it is a bad rectifier or regulator in the alternator, or a bad electrical connection.

1. Ensure the alternator electrical connections are good. Check the wire from the alternator output at the stud terminal to the battery. Confirm that this wire is not "open", that you have continunity and that it is clean and soundly connected. Depending upon the year/engine size, it may or may not be a fusible link.

2. It is common for CS130 alternators to fail due to overheating. Normally the rectifier (diodes) assembly will go bad. A replacement rectifier assembly can be purchased (about $50 from NAPA), but the leads will need to be soldered in place. If you don’t know how to solder, you’ll have to replace it another way (rebuilt, etc.). Some places (NAPA, Pep Boys) will check your alternator free for you (you’ll have to bring the alternator in to the store for checking on their tester).

Since it is common for the CS130 alternators to fail (heat…), I’d guess that your rectifier is bad. If you never get a voltage over 12.8 VDC (12.6 is the maximum battery float voltage), then you can safely presume that the alternator is not outputting anything.

If you can get the alternator to put out over 13.0 volts, then it is at least partially working, and could be a bad battery. Can you swap a known good battery in its place and see if the output returns to normal? If not, can you charge your battery from a battery charger (will it accept charge?) and see if the output returns to normal?

[This message has been edited by Stuart Moss (edited July 27, 2000).]