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Discussion Starter · #1 ·
It all started with my battery dieing on me one day. I took that to Sears and got it tested and recharged. Then I did the negative cable battery disconnect test while it was running, and the engine died. So I replaced the alternator, and did the negative cable disconnect test again, and it was running great. As days go by, I notice my car begins to start with less power each time. SOOOOOOOOO I do the test again and the car engine dies. I got the alternator tested, and it passed. Does it have a short somewhere?? The dash "alt" light is on. The only thing I can think that it could be the little wire that you bolt down behind it, or the one you clip in. How can I locate the problem?? Any knowledge and advice would be appreciated.


2 Posts
You should NEVER pull the battery cable to test the alternator as it can blow the diodes. To check the charging system, check the voltage at the battery with the car running at a fast idle, should be 14-14.5 volts. If you are using a externally regulated system see below, post back if internally regulated.

This was written by another member, a very good write up on how it works and how to troubleshoot a externally regulated charging system.
Peter F said:
I dug this out from before, hopefully it help. If not, just post or send me your questions.

I'll try to provide a simplified explanation of how the externally regulated alternator and regulator work here. This explanation starts from a parked car with engine off state.

The regulator brown #4 wire is connected through the light bulb to 12V whenever the key is in the ON position. The other end goes through a set of points in the regulator and then out the F terminal to the field winding in the alternator. So some current begins flowing in the field winding from this connection. The little bit of current flowing produces a weak magnet field in the alternator and also lights the bulb.

When you then start the car, the weak magnetic field from above begins rotating which makes the alternator producing a little bit of voltage. This voltage is fed from the alternator to the regulator on the white regulator #2 wire. This wire connects to a coil in the regulator and turns on a contact when the voltage goes above 3.2V.

Once the contact turns on in the regulator the #3 and #4 terminals are connected together. Terminal #3 is battery power so this puts battery power on both sides of the bulb and turns it off. This means the brown #4 wire should go to +12V. Also, this connects the field terminal right to battery power but still going through the points.

When the battery voltage goes above a set level, the F terminal is disconnected from the battery power by the points I kept mentioning. Then the voltage drops and the points close again. This on-off cycling happens rapidly and is how the voltage gets regulated.

From the above (KOEO = key on engine off and KOER = key on engine running).
F or #1 terminal (field) - KOEO = #4 slight voltage, KOER = 9-12V typically
#2 terminal (sense) - KOEO = 0V, KOER = >3.2V
#3 terminal (Battery) - KOEO = 12V, KOER = 12V
#4 terminal (light) - KOEO = F slight voltage, KOER = 12V

By 12V above I mean battery voltage, which may be 12V when engine is off but could be up to 14.5V when engine is running.

Check the light: Ground the brown #4 terminal wire at the regulator. When grounded the light should come on. If you don't have a light then skip this test. Test this with the connector off the regulator.

Checking the alternator: Jumper the blue wire F terminal to the battery post on the back. This should make it easily crank out 16+ volts. Next, connect a troublelight between the battery post and the F terminal. The light should come on and you should measure > 3.2V at the other alternator terminal. If it passes these tests, then it's in the regulator or wiring.

Checking the wiring: At the regulator connector jumper the brown light wire (#4) to the F terminal (#1) and you should be able to measure >3.2 volts on the other alternator terminal or terminal #2 of the regulator connector. If these tests pass, it is in your regulator.

When doing these tests, make sure you turn off or pull the fuses for any added electronics that you can. The alternator test can produce enough voltage to damage stuff.

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