found out yester day that i am only getting 13.2v
looking on youtube, i found you dont have to take the engine out to remove the alternator

 

What model/year/engine? Your signature shows an XJS and X-Type, but I see some of your other posts mention an '03 S-Type V6.

Before condemning the alternator, charge that poor battery before any troubleshooting. I highly recommend an automatic charger with at least a 10 amp output. A trickle charger won't cut it, so don't waste your time.

13.2V is an odd number to see from a charging system. It shows the alternator is charging at least a little bit, but not fully. Normally it's either all or nothing. Two possibilities come to mind:

1) A tired or marginal battery. If there's any question about the battery's condition (not just state of charge), have it tested. When a battery starts to fail, it can draw down the charging system. It's like trying to blow up a balloon with a hole in it, or trying to fill a bathtub with the drain open. There may not be anything wrong with the alternator itself.

2) A slipping drive belt, which reduces the alternator's output. Here's an excerpt from a Jaguar training manual showing how to check the belt tension on an early V6. I believe all S-Type model years and engines are similar:




Once the battery is fully charged, and you've confirmed the belt tension is good, repeat the voltage measurement. The charging system on an '03 S-Type will initially show about 14.5V after engine start, slowly tapering off around 13.5V after several minutes.

If you have to replace the alternator, here's how I did it on my '02 V6:

https://www.jaguarforums.com/forum/s...rnator-241668/

There were some changes between the early ('99-'02) and late models ('03+), so not all details will apply, such as part numbers. But the cases and electrical connectors look similar, so that thread should help. Make sure any replacement has the proper Jaguar control board in it. Most aftermarket units have a generic control board that will not communicate properly with the rest of the car, and the battery light will not extinguish even though it is charging.

 

More symptoms of a slipping serpentine belt:

The power steering will feel sluggish, especially with the engine at low RPM.

The AC performance will be reduced. This may not be noticed with moderate ambient temperature. Toggle down the selected cabin temperature until LO is displayed. Measure the air temperature at the center dash outlets. You should see around 38F. If significantly higher, say around 50F, the drive belt may be slipping.

I mention the possibility of a slipping belt because it's pretty unusual to have an alternator only putting out a partial charge.

 

"The charging system on an '03 S-Type will initially show about 14.5V after engine start, slowly tapering off around 13.5V after several minutes."
about a month ago i tested the charging voltage and it was 15.1v
i thought that is going to cook my battery, i forgot all about it
i then found out that it is only charging 13.2v
after reading your posts, i had the battery checked and it wasn't to good
today i changed the battery with a battery that was testing excellent and the charging rate was again 15.1v
i took the car for a run and checked the charge rate again and it is still charging 15.1v

 

Making some progress. Glad you didn't change the alternator due to a wonky battery.

I found some more details in the service manual. Scroll down to section 414, or page 2742 in the PDF:

http://www.jagrepair.com/images/Auto...M-Workshop.pdf

"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 15.3 volts, measured with the generator at 25 degrees centigrade (77 degrees fahrenheit) and charging at a rate of 5 amps. These values decrease with a rise in temperature or current flow. The ECM determines the output voltage setting of the generator. 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 period of time is selected if the ECM determines that the vehicle has been 'soaking' for sufficient time to allow the engine coolant temperature (ECT) and the air intake temperature (IAT) to fall within 6 degrees centigrade (43 degrees Fahrenheit) of each other.

The intermediate time period is selected when the ECT and the IAT is below 15 degrees centigrade (59 degrees Fahrenheit).

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 over charged. The time periods are variable depending upon the temperature and 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."

End quote. To summarize Proust:

The 15.1V you're seeing is within specs after engine start. The voltage should taper down after a variable amount of time, based on certain conditions. How much time? The manual is sufficiently vague, so it's hard to say.

I will repeat myself to charge that poor battery, even though this replacement tested good. The test only determines the general health, not necessarily the state of charge. The charging system isn't really designed to fully replenish a run-down battery. As you've already seen, a marginal battery skewed the test results and could have led you to replace a perfectly good alternator. Could very well be if you hook up an automatic charger overnight, on the next drive the alternator voltage will quickly drop within specs.

 

well thanks, certainly complicated

 

Hey I'm also having the same problem!
Refer to this post I put on the forum for details: link