This isnt about current, this is about memory, ECU A is shot, in order for ECU B to learn faster you let ECU A drain its memory which essentially resets the whole system and prepares it for ECU B. There are a few more parts which have " memory" so to speak, Idle Air Controller is one, what happens when the ECU is disconnected and reset? It idles high and resets the IAC, but that is also one of the parts that the ECU doesnt learn from, the ECU sets the IAC by receiving data from other sources. Make sense?

 

I'm sorry, but with all due respect, you are wrong. The Idle Air Control is just a valve, controlled by a stepper motor. It doesn't have any "memory", it just stays put until the ECU(/PCM/ECM/computer/whatever you want to call it) tells it to move. This poor Idle Air Control gets blamed for way too many things - like not starting, bad fuel mileage, running rough, loss of power....the list goes on. It's just a valve. Does the same thing as opening the throttle a bit. It's just a valve, that the ECU adjusts to control idle speed. It's just a valve. Did I say it's just a valve?
The only other things in a jeep with "memory" are the radio (you'll have to reprogram all your presets), the airbag (which has a capacitor that acts like a mini-battery, to provide enough power to fire the airbag even if your battery is already crunched by the time it needs to fire), and maybe the automatic transmission controller.
None of which have anything to do with the alternator. Sure, you should disconnect your battery negative terminal while you disconnect/reconnect the ECU, but feel free to hook your battery right back up again. The ECU is well and truly reset by being in the junkyard for 6 months with no battery. 10 minutes more isn't going to make any difference. How does the ECU know it's plugged into your jeep instead of it's original home, if it's not powered up? (It can't do or know anything if it has no power)

(end rant)

 

It does indeed make sense, thats what I was getting at with the sensor memory thing. But the post after yours makes me unsure again.

 

Personally i dont think any auto parts store know hows to test alternators.. Heard way to many stories about they took it said it was good and still didnt work, replaced and now works..

Heres something for overchaging alts. :

Additionally, problems can occur if the alternator is overcharging the system. Some symptoms of overcharging include a swollen or seeping battery, both headlight bulbs failing at the same time, and other electrical component problems. To perform an alternator system check for this condition use a voltmeter on both positive and negative terminals while the engine is idling. If the voltmeter reading is over 15 volts this is an indication of an overcharging alternator. The alternator should be replaced with an OE (original equipment) unit. While an inferior rebuilt alternator is often much cheaper, it can fail prematurely causing another costly replacement.

All im saying is that you may still have a bad alternator.. They told you ok, but i believe not. I just reread an aritcle about how an alternator works. Yes the PCM controls voltage to the alt but it will not allow it to charge if bad.

 

ON-BOARD DIAGNOSTIC TEST FOR CHARGING
SYSTEM
The Powertrain Control Module (PCM) monitors
critical input and output circuits of the charging system,
making sure they are operational. A Diagnostic
Trouble Code (DTC) is assigned to each input and
output circuit monitored by the On-Board Diagnostic
(OBD) system. Some circuits are checked continuously
and some are checked only under certain conditions.

Do you have codes??

 

If you have a volt meter do this.. Check you Green Wire (Supply) See what voltage you get.. If its the same as mine then you have a bad alt..

 

here is info from Alldata.....

The Electronic Voltage Regulator (EVR) is not a separate component. It is actually a voltage regulating circuit located within the Powertrain Control Module (PCM). The ERR is not serviced separately. If replacement is necessary, the PCM must be replaced.

The amount of DC current produced by the generator is controlled by EVR circuitry contained within the PCM. This circuitry is connected in series with the generators second rotor field terminal and its ground.

Voltage is regulated by cycling the ground path to control the strength of the rotor magnetic field. The ERR circuitry monitors system line voltage (B+) and battery temperature (refer to Battery Temperature Sensor for more information). It then determines a target charging voltage. If sensed battery voltage is0.5 volts or lower than the target voltage, the PCM grounds the field winding until sensed battery voltage is 0.5 volts above target voltage. A circuit in the PCM cycles the ground side of the generator field up to 100 times per second (100Hz) , but has the capability to ground the field control wire 100% of the time (full field) to achieve the target voltage. If the charging rate cannot be monitored (limp-in), a duty cycle of 25% is used by the PCM in order to have some generator output. Also refer to Charging System Operation for additional information.






CHARGING SYSTEM
The charging system consists of:

• Generator
• Electronic Voltage Regulator (EVR) circuitry within the Powertrain Control Module (PCM)
• Ignition switch (refer to Ignition System for information)
• Battery (refer to Battery for information)
• Battery temperature sensor
• Generator Lamp (if equipped)
• Check Gauges Lamp (if equipped)
• Voltmeter (refer to Instrument Panel and Gauges for information)
• Wiring harness and connections (refer to Wiring for information)

The charging system is turned on and off with the ignition switch. The system is on when the engine is running and the ASD relay is energized. When the ASD relay is on, voltage is supplied to the ASD relay sense circuit at the PCM. This voltage is connected through the PCM and supplied to one of the generator field terminals (Gen. Source +) at the back of the generator.

The amount of DC current produced by the generator is controlled by the EVR (field control) circuitry contained within the PCM. This circuitry is connected in series with the second rotor field terminal and ground.

A battery temperature sensor, located in the battery tray housing, is used to sense battery temperature. This temperature data, along with data from monitored line voltage, is used by the PCM to vary the battery charging rate. This is done by cycling the ground path to control the strength of the rotor magnetic field. The PCM then compensates and regulates generator current output accordingly.

All vehicles are equipped with On-Board Diagnostics (OBD). All OBD-sensed systems, including EVR (field control) circuitry, are monitored by the PCM. Each monitored circuit is assigned a Diagnostic Trouble Code (DTC). The PCM will store a DTC in electronic memory for certain failures it detects.
The Check Gauges Lamp (if equipped) monitors: charging system voltage, engine coolant temperature and engine oil pressure. If an extreme condition is indicated, the lamp will be illuminated. This is done as reminder to check the three gauges. The signal to activate the lamp is sent via the CCD bus circuits. The lamp is located on the instrument panel. Refer to Instrument Panel and Gauges for additional information.

GENERATOR
The generator is belt-driven by the engine using a serpentine type drive belt. It is serviced only as a complete assembly. If the generator fails for any reason, the entire assembly must be replaced.

As the energized rotor begins to rotate within the generator, the spinning magnetic field induces a current into the windings of the stator coil. Once the needed to energize the rotor.

The Y type stator winding connections deliver the induced AC current to 3 positive and 3 negative diodes for rectification. From the diodes, rectified DC current is delivered to the vehicle electrical system through the generator battery terminal.

Although the generators appear the same externally, different generators with different output ratings are used on this vehicle. Be certain that the replacement generator has the same output rating and part number as the original unit. Refer to generator Ratings in the Specifications for amperage ratings and part numbers.

Noise emitting from the generator may be caused by: worn, loose or defective bearings; a loose or defective drive pulley; incorrect, worn, damaged or misadjusted fan drive belt; loose mounting bolts; a misaligned drive pulley or a defective stator or diode.

BATTERY TEMPERATURE SENS0R
The Battery Temperature Sensor (BTS) is attached to the battery tray located under the battery.

The BTS is used to determine the battery temperature and control battery charging rate, This temperature data, along with data from monitored line voltage, is used by the PCM to vary the battery charging rate. System voltage will be higher at colder temperatures and is gradually reduced at warmer temperatures.

The PCM sends 5 volts to the sensor and is grounded through the sensor return line. As temperature increases, resistance in the sensor decreases and the detection voltage at the PCM increases.

The BTS is also used for OBD II diagnostics. Certain faults and OBD II monitors are either enabled or disabled, depending upon BTS input (for example, disable purge and enable Leak Detection Pump (LDP) and 02 sensor heater tests). Most OBD II monitors are disabled below 20 °F .

ELECTRONIC VOLTAGE REGULATOR
The Electronic Voltage Regulator (EVR) is not a separate component. It is actually a voltage regulating circuit located within the Powertrain Control Module (PCM). The ERR is not serviced separately. If replacement is necessary, the PCM must be replaced.

The amount of DC current produced by the generator is controlled by ERR circuitry contained within the PCM. This circuitry is connected in series with the generators second rotor field terminal and its ground.

Voltage is regulated by cycling the ground path to control the strength of the rotor magnetic field. The ERR circuitry monitors system line voltage (B+) and battery temperature (refer to Battery Temperature Sensor for more information). It then determines a target charging voltage. If sensed battery voltage is0.5 volts or lower than the target voltage, the PCM grounds the field winding until sensed battery voltage is 0.5 volts above target voltage. A circuit in the PCM cycles the ground side of the generator field up to 100 times per second (100Hz) , but has the capability to ground the field control wire 100% of the time (full field) to achieve the target voltage. If the charging rate cannot be monitored (limp-in), a duty cycle of 25% is used by the PCM in order to have some generator output. Also refer to Charging System Operation for additional information.

 

 

 

SYMPTOM
* CHARGING SYSTEM NO CODE

POSSIBLE CAUSES

• Trouble codes present (B)
• Trouble codes present (A)
• Generator field terminal intermittent defect
• Generator ground circuit high resistance
• B(+) circuit high resistance (0.4 volt )
• B(+) circuit high resistance (1.0 volt )
• Powertrain control module (charging system no code)

 

I went through 3 brand new atlernators before i got a good one that worked right in my 96 xj . Just something to think about.

 

I'm not ready to blame the alternator since the one that was on it acts the same way as the new one.

 

Holy crap that's a lot of info!

First off Thank you all for the help!!

Here is a follow up if some one develops this problem.

-My alt was dead. got a new one $70
-Batt dead (because of the alt I think) got a new one $125
After going through 3 alts thinking they were bad rebuilds...
-I listened to Diesel (Thanks) and replaced the PCM. -$90 j/y

Out come... she works!!!!!!!!!!!!

Got the PCM at a j/y from the same year and same Part # I didn't "Flash" it and everything works like it should.

the only thing is after the first few start ups she would idle low and sputter out but it hasn't done that in a couple runs so i think it was just the system trying to learn? let me know what you think about that??

 

knowledge is power! Glad to hear it's fixed.