Warning lights and loss of power

Hold on. Before we assume the worst, lets look at the most common suspect. Much of what you listed has nothing to do with the Engine Computer so put that way at the bottom of the list of suspects. Pulling the battery cable off to magically reset something is a throwback to the late 1980's and early 1990's when GM had a huge computer failure rate. If doing this was standard practice, they would have built in a switch somewhere. While it is true disconnecting the battery can clear up some things, that is not fixing the problem. It is making the problem not show up right now, and that can make it even harder to solve later. What defect do you expect to find when everything is working properly?

As for the circuits and systems that are run by the Engine Computer or the Body Computer, there are separate circuits for each one. A failure in a computer might involve two or three systems, but then not another one, and another one, etc. That is like saying one light bulb in your kitchen stopped working yesterday, then another one today, and another one tomorrow, therefore it must be a bad switch. (In case you did not get my comparison, if the switch was bad, all of the bulbs would have been out right away).

Every symptom you described can have one cause in common, and that is the voltage in the electrical system, meaning the charging system. If the alternator fails while driving, the car's electrical system will run on the battery until it slowly runs down. The most you will get is about an hour if you have the head lights and heater fan turned off. As system voltage drops, computers, which are real sensitive to supply voltage, will start to become confused and do weird things, including shutting down. Part of them will operate just enough to turn on their warning lamps to let you know their system isn't operating. As battery voltage drops lower and lower, more computers will shut down or do strange things.

The mileage you listed is rather low for this problem, but the most common cause is worn brushes in the alternator. To test for this, measure battery voltage with the engine off first, then with it running. With the engine off, it will be 12.6 volts if it's good and fully-charged. If it is closer to 12.2 volts, it's good but discharged. Charge it at a slow rate for an hour, then recheck it. If you find 11 volts, the battery has a shorted cell and must be replaced.

Next, measure the battery's voltage with the engine running. For this test to be valid, it must be done while the problem is occurring. It must be between 13.75 and 14.75 volts. If it is low, suspect the alternator. If it is okay, that only means it is okay to perform the second part of the test, and that requires a professional load tester. Your mechanic will test for full-load output current and "ripple" voltage. If one of the six internal diodes has failed, all you'll be able to get is exactly one third of the alternator's maximum rated current and ripple voltage will be "high". 30 amps from the common 90 amp alternator is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.

Diodes almost always fail permanently and can be identified during testing. Intermittent problems like you described are typical of worn brushes and those always start out that way and get gradually worse over the next few months. Most people replace the alternator for this problem, but it is possible to replace the brushes. On some Chrysler engines that can be done without even taking the alternator off. The brush assembly costs around 12 bucks.

I agree with caradiodoc. One other thing to check is belt and tensioner as well as recommended replacement is 60,000 miles for both so it may have been slipping at low rpm and at higher rpm like it was working, but not as efficient.

Hi HMAC300.

Yup, what you're describing is exactly what a failing alternator can cause. The most common failure on Chrysler alternators is worn brushes, and those always start out by making intermittent contact. When the problem occurs, it can take a few minutes for the battery's voltage to drop enough for the first computer to act up. You have dozens of computers, and one by one more of them will start to shut down and / or do weird things. The brushes can suddenly start to make contact again making the alternator start working again and recharging the battery. Typically the brushes will wear enough for this to become a permanent failure within the next month or two.

I've included a photo from my web site of the Nippendenso alternator brush assembly. The pretty blue arrow shows the worn brushes. The nifty red arrow shows what they look like on the new assembly.

You aren't following my logic. The alternator almost always starts out being intermittent, and that's how this problem is acting. That means it will work fine at times and not at other times. Any testing has to be done while the problem is occurring. If your mechanic tested the charging system while it was working, naturally everything will test good. My money is on the alternator or the wiring associated with it.