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Chuck, ASE Certified Technician
Category: Ford
Satisfied Customers: 2101
Experience:  33 Years Experience,Ford Senior Master,ASE Master,L1 Advanced Engine Performance
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2004 expedition: / 5.4l..battery light..illuminated..charging system

Resolved Question:

2004 expedition w/ 5.4l arrived with battery light illuminated and message center with check charging system flashing. diag showed 12.2 volts at battery while running at idle. replaced alternator and tested battery. battery was good and new alternator output to battery is 13.9-14.2 once vehicle is started the message center shows charging system is ok but after 3-5 mins. battery light illuminates and message center reports to check charging system.voltage at battery remains constant at around 14
any ideas on what may cause this condition

Submitted: 7 years ago.
Category: Ford
Expert:  Chuck replied 7 years ago.

Did you replace the alternator with an aftermarket alternator or a ford ot motorcraft alternator?

Customer: replied 7 years ago.
aftermarket - oreillys
Expert:  Chuck replied 7 years ago.

The alternator on your vehicle is controlled by the PCM and we have seen many after market alternators not work with this system especially the Delco ones for some reason.


Basically how your system works is after the engine is started, voltage is applied through the I circuit 1931 (BN/PK) from the powertrain control module (PCM) to the generator. This turns the regulator on, allowing current to flow from the battery sense A circuit 35 (OG/LB) to the generator field coil. When the engine is started, the generator begins to generate alternating current (AC) which is internally converted to direct current (DC). This current is then supplied to the vehicle electrical system through the output (B+) terminal of the generator. Once the generator begins generating current, a voltage signal is taken from the generator stator and fed back to the regulator internally. This voltage feedback signal (typically half the battery voltage) is used to turn off the warning indicator.

With the system functioning normally, the generator output current is determined by the voltage of the A circuit 35 (OG/LB) which is compared to a set voltage internal to the regulator. The regulator controls the generator field current to maintain the current generator output.

The set voltage will vary with temperature and is typically higher in cold temperatures and lower in warm temperatures. This allows improved battery recharge in the winter and reduces the chance of overcharging in the summer.


Now the best way to test and see if your alternator is actually functioning correctly on this vehicle is to hook up your meter on the battery.Start vehicle and run it at about 1500 to 2000 rpm's for about 10 seconds to initialize the alternator then unplug the 3 wire connector on back of the alternator with engine running. You are looking for 13.5 volts plus or minus a couple of tenths. If you are reading say 14v or more or 13v or less your alternator is bad or will not work with this system.


If this reading is ok then check your connections good and check the voltages at the alternator connections. Here is a chart for this


Normal Charging System Voltages and Charging System Warning Indicator Operation

Key PositionA-line 35 (OG/LB)I-line 1931 (BN/PK)Generator B+ 38 (BK/OG)BatteryEngine to Battery GroundCharging System Warning Indicator Operation
Key OFF12 volts0 V12 volts12 volts0 voltsOff
RUN - engine OFF12 volts0 V12 volts12 volts0 voltsIlluminated
RUN - engine running13- 15V8- 12V13- 15V13- 15V0VOff



If all of this is ok you will need to have it checked by someone with a scan tool that can perform charging system test.

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