1993 Jeep Cherokee Check engine light and low voltage

1993 JEEP CHEROKEE
285,000 MILES • 6 CYL • 4WD • AUTOMATIC
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RUSTYMULE
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My 93 4.0L jeep 's check engine light comes on about every other time I start it.When this happens the volt meter reads low voltage and the alternator is not charging. Sometimes the alternator starts charging after 15-20 seconds but check engine light does not go out. I am using a clamp around amp meter on the battery wire on the alternator to check for output. The check enginge light will go out if I shut off the engine and start it again. The never comes on if the alternator is charging at start up. Is it the regulator in the alternator or the ECM. I have had to replace the ECM twice already for other reasons and don't want to do it again. Thanks Rusty Mule.
Oct 24, 2010 at 3:35 PM
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CARADIODOC
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Hi Rustymule. Welcome to the forum. What are you getting for alternator output? A better test is to measure battery voltage while the engine is running. The goal is to find 13.75 to 14.75 volts.

If you find less than 13.75 volts when the problem is occurring, the most common cause is worn brushes inside the alternator, especially if you have the little silver Nippendenso unit. Look for the two small nuts on the back. One of them must have full battery voltage but it will only be there when the engine is running, not just with the ignition switch turned on. The other one will have less but not 0 volts. If it has 0 volts, the brushes are open. If both terminals have exactly the same voltage, there is no current flow through the field winding, most likely due to a corroded pin in an electrical connector.

I can't say for sure on a Jeep, but on the minivans, the brush assembly can be replaced without removing the alternator from the engine.

If you find around 4 - 11 volts on the second terminal, but output voltage and current are low, suspect a shorted or open diode in the alternator's output circuit. I suspect that is not the problem because if the Check Engine light is on, the Engine Computer detected the problem and it monitors current flow through the field winding in the alternator / voltage regulator in the computer. If there is a break in that circuit, including open brushes, the diagnostic fault code will be "field circuit not switching properly". There will be at least one fault code stored once the Check Engine light turned on.

caradiodoc
Oct 24, 2010 at 4:44 PM
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