Ok, so I am very confused. I've searched the forum and the general internet, and cannot find a consensus on what the proper voltages should be in this car. In a different thread, I was told that while the car is running with accessories on, the voltage should be above 13, and with engine off, battery should be above 12. Then someone else in the same thread said that engine running with accessories should be above 13.5, and with engine off, battery should be above 12.5.

Now I've found an older thread saying that battery voltage of 12.5 with engine off is too low, and that voltage should be above 14.5 with engine and accessories. The rest of the internet gives off a whole multitude of different numbers.

For the Jaguar X350, can anyone please give me a solid definitive answer?

 

A charged battery sitting at rest with less than an amp load should always read 12.6v...period. That is just the physics of a lead acid battery with 6 internal cells. If your battery reads higher, it has an internal open circuit or your voltmeter is out of calibration. If it reads lower, then it is under significant load, it is undercharged, it is shorted internally, or (again) your voltmeter is wrong.

Unlike the battery voltage, the charge voltage is more complicated. As noted above, if you are reading more than 13v with the engine running, then your battery is charging. If it reads less than 13 volts, then you have a charging issue. Immediately after starting the voltage regulator will normally put out between 13.7 to 14.4 volts. The varience is to optimize charging for the current ambient temperature. These cars actually raise idle speed if the alternator cannot keep up with load...meaning you should never see less than 13v, and rarely less than 13.7v.

Hope that clears it up...battery always 12.6v charged. Alternator output always above 13 volts.

Only one last thing warrants being addressed. It is possible for the voltage regulator to fail so the voltage rides too high. Any voltage above 14.5 is too much. Once again, make sure your voltmeter is calibrated before you get too freaked with a high reading. Common voltmeters are far less reliable than our Jags are.

 

Thanks very much, Cjd. I'll have to double check my numbers tonight just to make sure, but according to my first test, it would seem my alternator is fine, even though the car is exhibiting classic failing alternator symptoms. I also just found this in the Electrical Manual for 3.5L and 4.2L engines:

"The battery charging voltage is determined by the temperature of the generator. In cold conditions, starting the vehicle from cold the battery voltage will be between 14.2 volts and 15.1 volts and will reduce as the engine warms up. In hot conditions starting the vehicle when the engine is already warm the battery voltage will be between 13.5 volts and 14.3 volts."

 

I tested the battery again, and everything seems fine. 12.7 with engine off, 13.75-14.25 with engine on, idling, and revving. However, while revving or idling with accessories on, it read around 13.25. Does that seem low?

 

That sounds pretty normal to me...

What symptoms are you getting that concern you?

 

Hi XJ8JR. It might help if you can lead us a bit to a specific problem you think may be occurring. Sorry, this may be more than what you're looking for. Short answer is no, it doesn't seem low.

From the training manual:

Principles of Operation
The BMS module measures battery voltage and current
which provides information about the battery state of
charge (SOC) and state of health (SOH). SOH measurements
provide an indication of battery condition. The
BMS compares this information to new and used stored
battery values.
Battery information is then communicated to the rear
junction box (RJB) over a LIN bus connection. The RJB
transmits the battery information to the instrument cluster
via the MS CAN bus.
The instrument cluster displays battery charge warning
messages to indicate generator or BMS faults. The
instrument cluster also acts as a gateway between the
MS CAN and HS CAN bus networks to transmit battery
condition information to the ECM from other modules
(audio and climate control, for example).
Based on the information received from the BMS module,
the ECM will control the output from the generator
via LIN bus. The ECM can also request the switching
off or reducing of power to electrical loads if necessary
and override the BMS signals if a fault is detected.
The BMS also monitors battery status with the engine
switched off, sending a signal to switch off the infotainment
system if necessary to protect the battery condition.
Once triggered, the engine must be run for at least 5 minutes
to charge the battery before the infotainment system
will be allowed to operate with the engine switched off
for a second time.
Calibration
Periodically the BMS module will instigate a self-calibration
routine. To self-calibrate, the BMS first charges
the battery to its full condition.
Once the battery is fully charged, the BMS will discharge
the battery to approximately 75% of its full state
of charge, but never lower than 12.2 V. The time taken to
complete this part of the routine is dependent on the
electrical load on the vehicle and the length of time the
vehicle is used.
When the second part of the routine has been successfully
completed, the BMS will return the battery to its
optimum level of charge. The optimum level of charge
will be between 12.6 V and 15 V, depending on battery
condition, temperature and electrical loading. This process
is run approximately twice a year.
NOTE: If the vehicle is only driven for short periods the
self-calibration and charging process could take a number
of days to complete.
CAUTION:
Due to the self-calibration routine, it is recommended
that all power supply diagnostic testing
is carried out using IDS rather than a
digital multi-meter.

 

Thanks guys for responding.

I had initially stated my issues in a different thread:

https://www.jaguarforums.com/forum/x...issues-123201/

...but wasn't getting anywhere, so I'll restate them here. Basically, there are a number of things that were pointing me towards a failing alternator:

- rpms drop down to 500 for a moment, then rebound back to 600 either when coming to a stop or when already stopped at a light, especially if I've just come off the freeway
- rpms drop when turning steering wheel at slow speeds, like in a parking lot
- high-pitched whining sound upon acceleration, almost like a supercharger
- abnormal idle vibrations

These symptoms get worse if the AC is on.

 

Have we checked for a idler pulley going bad?

 

Yes, I replaced that, the tensioner pulley assembly, and the serpentine belt.

Last night I checked for vacuum leaks and couldn't find any. I cleaned out the air intake and tried to clean out the throttle body but it was too hot, so I'll finish that up later today after it cools down.

 

Nothing I've read in any of your posts sounds remotely like an alternator problem. Even a dead alternator won't cause engine idle speed to drop, at least until the battery goes flat and the engine stops.

I'd concentrate on determining why the idle speed drops abnormally. I've presume you've used a code reader and it revealed nothing?

 

Correct, no codes whatsoever.

 

In the 'good old days' cars had ammeters to show the charge/discharge on the battery (starter circuit excepted), but they vanished in the 1950s/70s. One would be a great help here.

Now, more practically ... there are such things as 'clip-on ammeters' (Google it) which measure the current in a wire from the magnetic field around it, outside the insulation. There are even DC ones ...

 

A simple voltage meter is all that's required. The tests the OP has done indicates that the charging system is just fine.

 

My battery is showing 12.18v.(engine off). I am currently having air suspension issues.could this be the fault. In addition to that Ive just tried it again after the car was stood overnight and Im now getting 11.99 volts. It does however go up to 14 volts with engine running

Thanks
steve

 

Well new battery fitted and it still rides like a roller skate

 

Roller skate ???

 

So this may be the reason sometimes my entertainment does not come up on the touchscreen??? Seems it only happened after parked on a really hot day with the boot facing the sun all day. HMMM?

 

Yep, or should I have said Go Kart. my suspension is virtually none existent. i can feel every bump on the road. I have had codes read and the following was revealed. C1430,C1427,C1424,C1419 and B1671(hence the battery change). The suspension does self level so it must be working but the ride is awful.

 

Find a jag indy with a clue.