Either the charging system is not working or the battery is bad. The transmission has computer controls and will do strange things when the supply voltage is low. Don't fret over the low idle speed. Most people never figure out it will stay running when you hold your foot on the gas pedal. That has a real easy fix later.
You'll need a helper to hold the gas pedal for you or you can wedge a small piece of folded paper in the idle stop to keep the engine running for the following tests.
With the engine off, measure the voltage between the two battery terminals. Fully charged, it should be around 12.6 volts. 10 point-something volts suggests a shorted cell. Around 12.0 volts suggests a good battery that is run down. After starting the engine, measure the battery voltage again. It must be between 13.75 and 14.75 volts. If battery voltage is lower while the engine is running than when it is off, three voltage measurements on the back of the alternator will determine the direction to head.
The large black wire must have full battery voltage all the time. If it's missing, check the bullet connector near the battery for corrosion. Next, look at the two small terminals bolted on the back. They are attached to a black plastic block. If you find 0 volts on either terminal, scratch with the probe a little. There might be some rust on the nuts preventing a good reading. One of the small terminals must have full battery voltage. It is switched on by the Automatic Shutdown (ASD) relay for only two seconds after the ignition switch is turned on, then again when the engine is rotating, (cranking or running). You can't simply turn the ignition switch on and expect voltage to be there. While the engine is running, look for that 12 volts on one of the terminals. On the very small chance it's missing, there is only one small piece of wire to look at. The ASD relay is not the problem because it also sends voltage to the coil, injectors, and fuel pump or pump relay. The engine would not run if the ASD relay didn't turn on.
Assuming you find battery voltage on one small terminal, the big clue comes from the other terminal. It should have less than battery voltage, but typically not less than four volts. There are three possibilities on this second terminal; full battery voltage, 0 volts, or something in between.
If you find full battery voltage on the second terminal, there is an open circuit between the alternator and regulator which is in the engine computer. The most common cause is a corroded pin in an electrical connector. The least common possibility is a defective regulator.
If you find between four and ten volts on that second terminal, that entire part of the circuit is working. The cause of low voltage has to be related to open diodes in the alternator. Chrysler has always had very little trouble with diodes.
The most common problem by far results in 0 volts on the second small terminal. Remember, this assumes you found full battery voltage on the first terminal. Only two things can cause 0 volts on that second terminal. One, the wire to the regulator is grounded, but that would make the alternator charge wide open; not the problem you're having. Second, and more commonly, the brush assembly is open due to wear from high mileage. Your mileage isn't nearly high enough yet unless you drive on a lot of dusty roads. On cars and minivans, the brush assembly can be replaced without removing the alternator from the engine. On Dakotas, you'll have to take it off to remove the rear cover. Three screws hold the brush assembly on. Replacement should be obvious when you have the new one to look at.
Open brushes should have been detected by the engine computer as it watches the amount of current going through the voltage regulator circuit. It will set a diagnostic fault code "field circuit not switching properly", and turn on the "Check Engine" light. The light is turned on when any detected code will result in excessive tail pipe emissions. Low voltage causes weak spark, a slow running fuel pump, and delayed injector opening. Since the Check Engine light was not on, either the code got erased when the battery was dead, or the problem really is just a simple bad battery.
As for the failure to idle, once all the other repairs are done, you simply need to relearn minimum idle. Drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the brake or gas pedals. That will meet the conditions necessary for the engine computer to memorize the voltage coming from the throttle position sensor. Anytime it sees that same voltage, it knows it must be in control of idle speed.
Friday, December 11th, 2009 AT 3:06 AM