keep in mind the battery charging system in the x type is not normal.. is has a very fancy system that keeps the voltage just right for the electonics.
I would consult with the Jag service manual about what voltage is normal before using any old rule of thunb.

I wonder if the car logs the reason for the light coming on? hate to have to go to the dealer.............

 

did you see what i put on roadfly? there is lots of info i cut and pasted from the JTIS.... too much to really help but the point is that this is not a simple alternator with a voltage regulator.. there are plenty of reasons to light the lamp and they need to be read like a fault code...

 

this will lead you to **** for sure.. since this is not a real benefit of the internet.. ahahhahahhaha

Charging System:

The charging system consists of a 120 amp output generator and regulator assembly which generates current to supply the vehicle electrical system with electricity when the engine is running and maintain the battery in a charged condition.The generator is belt driven by the accessory drive belt. For additional information, refer to . When the engine is started, the generator begins to generate alternating current (AC) which is converted to direct current (DC) internally. The DC current and voltage is controlled by the voltage regulator, (located inside the generator), and then supplied to the battery through the main battery positive cable.The generator is solidly mounted to the engine timing cover and is driven at 2.8 times the engine speed.Vehicles fitted with manual transmission have a one way clutch fitted to the drive pulley, which prevents torsional vibrations from the engine being transmitted to the generator.The engine control module (ECM) can switch the voltage regulator between two voltages to optimize the charging of the battery.The low voltage regulator setting is 13.6 volts and the high voltage regulator setting is 15.3 volts, measured with the generator at 25°C (77°F) and charging at a rate of 5 amps. These values decrease with a rise in temperature or current flow.The ECM determines the voltage setting of the voltage regulator. The high voltage setting is always selected by the ECM once the vehicle has started. The ECM determines the period of time that the high voltage setting is selected for.There are three different time periods selected by the ECM which is dependent upon the vehicle conditions when the vehicle is started:The longest time period is selected if the ECM determines that the vehicle has been 'soaking' for sufficient time to allow the engine coolant temperature (ECT) and the intake air temperature (IAT) to fall within 3°C (37°F) of each other.The intermediate time period is selected when the ECT and the IAT is below 5°C (41°F).The shortest time period is the default time and is used to provide a short period of boost charge.At the end of these time periods the voltage is always set to the low voltage setting to prevent the battery from being overcharged.The time periods are variable depending upon the temperature and the battery voltage. The target voltage of the battery varies between 14 volts and 15 volts depending upon the ambient temperature and the vehicle operating conditions. Once this target voltage has been achieved, providing the vehicle has been operating for at least the shortest time period, the ECM will reduce the voltage regulator to the minimum setting of 13.6 volts.There are three wires connected to the generator from the ECM by connector EN16:EN49-1; RG wire, is the voltage regulator request setting from the ECM to the generator.EN49-3; OG wire, is a pulse width modulated signal (PWM) from the generator to the ECM which enables the ECM to monitor the generator load on the engine.EN49-4; U wire, is the charge warning lamp signal wire from the generator to the ECM.If the voltage request signal wire (EN 49-1) is open circuit or short to battery positive, the generator will charge the battery at a setting of 15.3 volts. If the wire is short circuit to ground, the generator will charge the battery at a setting of 13.6 volts.A fault in the wiring or the connections from the generator to the ECM, will cause a fault code to be generated and stored in the ECM and the charge warning indicator lamp to be displayed in the instrument cluster (IC) after a short time.The charge warning indicator lamp is operated by the IC after receiving a signal from the ECM through the control area network (CAN).With the ignition switch in the RUN position the charge warning indicator lamp will be displayed in the IC when the generator is not generating power.If a fault is detected with the generator a fault code will be generated and stored by the ECM. The charge warning indicator lamp will also be displayed in the IC.Units should be repaired as an assembly and not dismantled for repair. For additional information, refer to

[font=arial][size=2]Charging SystemInspection and VerificationVerify the customer concern by operating the system.Visually inspect for obvious signs of mechanical and electrical damage.MechanicalElectricalDrive beltDrive belt tensionerGeneratorGenerator pulley (vehicles fitted with manual transmission)BatteryGeneratorFuseCircuitWiring harness for damage or corrosionElectrical connectorsWarning lamp operationIf an obvious cause for an observed or reported concern is found, correct the cause (if possible) before proceeding to the next step.If the cause is not visually evident, verify the symptom and refer to the Jaguar Approved Diagnostic System.Symptom chartCharging system faults/codes and related causesDTC P1146 Low input from the Engine control module (ECM) on the voltage regulator request signal wireCircuit.Generator.ECM.GO to Pinpoint Test B.DTC P1244 Generator overchargingGenerator.Circuit.Battery.GO to Pinpoint Test C.DTC P1629 Generator load monitoring signal failureCircuit.Generator.ECM.GO to Pinpoint Test D.DTC P1632 Charging system failureFuse 82 (20A).Circuit.Generator.Battery.ECM.GO to Pinpoint Test A.The charging system warning indicator is on, intermittent or flickers with the engine runningAccessory drive belt.Fuse 82 (20A).Circuit.Generator.Generator pulley (vehicles fitted with manual transmission).Battery.ECM.Instrument cluster (IC).Control area network (CAN).CHECK the accessory drive belt tension. For additional information, refer to .GO to Pinpoint Test A.Generator not charging, battery keeps going flatAccessory drive belt.Fuse 82 (20A).Circuit.Generator.Generator pulley (vehicles fitted with manual transmission).Battery.CHECK drive belt tension. For additional information, refer to .GO to Pinpoint Test A.Generator noisyAccessory drive belt.Accessory drive belt tensioner.Generator pulley (vehicles fitted with manual transmission).CHECK the accessory drive belt and tensioner. For additional information, refer to .GO to Pinpoint Test G.Radio interferenceCircuit.Generator.GO to Pinpoint Test H. Generator Electrical Connector (EN49, Harness Side)Pin NumberCircuit FunctionCircuit Color1Voltage Request Input Signal from ECMRed/Green2Power Supply (12 volts) from Fuse 82 of the Passenger Compartment Fuse BoxGreen/Red3PWM Output Signal to ECMOrange/Green4Generator Charge Warning Indicator Lamp Output Signal to ECMBlue Engine Control Module (ECM) Electrical Connector (EN16, Harness Side)Pin NumberCircuit FunctionCircuit Color53Voltage Request Output Signal to GeneratorRed/Green65PWM Input Signal from GeneratorOrange/Green79Generator Charge Warning Indicator Lamp Input Signal from GeneratorBlueLOW INPUT ON THE VOLTAGE REGULATOR REQUEST SIGNAL WIRE FROM THE ECM (P1146)CHECK THE VOLTAGE REGULATOR REQUEST SIGNAL WIRE FOR A SHORT TO GROUNDDisconnect the ECM electrical connector EN16 and the generator electrical connector EN49.Measure the resistance between electrical connector EN49-1 and ground.Is the resistance less than 10,000 ohms? REPAIR short to ground between the generator and the ECM. CLEAR the DTC. TEST the system for normal operation. .CHECK THE VOLTAGE REGULATOR REQUEST SIGNAL WIRE FOR OPEN CIRCUITMeasure the resistance between electrical connector EN49-1 and the ECM electrical connector EN16-53.Is the resistance less than 5 ohms? REPAIR the circuit from the ECM to the generator. CLEAR the DTC. TEST the system for normal operation. .CHECK THE OPERATION OF THE GENERATOR CHARGE WARNING INDICATOR LAMPConnect the ECM electrical connector EN16 and the generator electrical connector EN49.Clear the DTC. START and RUN the engine at 1500 RPM with no electrical