Battery light on and car has power surges occasionally

  • 2.4L
  • 2WD
  • 220,000 MILES
1999 plymouth breeze. Battery light come on a week ago, made a few adjustment and it went off. Now it is back on. We brought the alternator in and had it test and was told that it was working properly. Not sure we're to go from here.
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have the same problem?
Thursday, January 9th, 2014 AT 12:10 PM

1 Reply

Three mistakes here. First of all, exactly what did you adjust? Second, you have an intermittent problem and any testing has to be done when that problem is occurring, otherwise of course everything will test good. Third, alternators need to be tested on the engine. Removing them for testing on a test-bench involves a real lot of work, and you will not get accurate results. The test-bench will tell you whether you have some output or no output, not how much.

Generators need about five to eight horsepower to run wide-open. Test benches usually have up to a one horsepower motor, so they can't test for maximum current. If one of the six internal diodes fails, you will only be able to get exactly one third of its rated output current. 30 amps from the common 90 amp generator isn't enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it runs down. That can take hours, days or weeks. Generators are also very inefficient at low speeds. All on-car professional testers involve raising engine speed to 2000 rpm for the current output test. Test benches run them at idle speed where output is going to be low.

By far the most common problem causing intermittent operation is worn brushes. Chrysler has always provided very convenient test points since they developed alternators for cars, (and copyrighted the term). Two voltage readings on the two small terminals on the back of the unit will tell you if the brushes are worn. If they are, as I recall, you can replace them without removing the alternator from the engine. The brush assembly costs around $12.00. I can describe how to replace them.

It is also possible to have a broken wire related to that brush circuit. Again, the voltage readings will tell that. One terminal on the back of the alternator will have full battery voltage, but only when the engine is running. The secret is the other terminal. It will normally have from four to eleven volts. If you find 0 volts, replace the brush assembly. If you find exactly the same voltage on both terminals, there's a break in that wire going to the voltage regulator inside the Engine Computer. The most common cause of a break is a corroded or stretched terminal in a connector, but even that is not real common unless the connector was damaged and is not sealed well.
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Thursday, January 9th, 2014 AT 2:29 PM

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