I recently bought a Bluetooth ODBC2 device and plugged it into my 2004 3.0. It seems to work great on this car and my other cars I own. What I have noticed is that the voltage is showing as 12.9 or so. I thought it should be higher than that while running at idle. My Explorer with V8 shows 13.9-14. What is healthy level of alternator output for our 3.0 Jag engine while running?

 

Jaguar explain the system thus:

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. 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 - Red/Green wire, is the voltage regulator request setting from the ECM to the generator. Orange/Green 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. Blue wire, is the charge warning lamp signal wire from the generator to the ECM.If the voltage request signal wire 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.

 

What product is it? A link would help in case it's something to do with your device drawing power.

 

12.9 is lower than I'd expect so maybe there's a voltage drop or some other problem.

 

Voltage reading by OBD2 device is lower than measured at battery terminals. At least in my case.
Use multimeter to check your voltage (and remember that voltage regulator has 2 modes, so at least 2 measurements will be needed).