car sits alot, been having dead battery issues after 3 or 4 weeks sitting,,battery about 1 yr old, has been tested twice and tests good,checked charging voltage, charges 14,3 or so under load fo 5 min then after 5 min its charging at 12,5 under load, bad alternator ? kinda dont know my hext step,,i do have a basic scanner,,any help from you guys ?

 

14.3 is good and suggests the alt is ok

 

true,,but 12,5 isnt really charging the battery, after 5 min it drops to 12,5, then i can shut it offt hen restart, its 14,5 for 55 min then back to 12,5 v.., with the huge battery in the car, id think it would charge longer,, wife only puts on 200 to 300 mi a year,,but i try to take it out for a drive weekly to charge it,, does a module control charging ?????

 

Could be a bad battery - those miles are far too few.

Also, read threads about quiescent current.

 

The optimal state of charge on an S-type is 12.8 - 12.9 volts. A charge rate of 12.5 volts is not going to get you there with all the other current draws taking place on an operating vehicle. The base charging rate should be over 13 volts.

 

If you plan to continue driving the car like that a very good idea is to put a battery maintainer on the car when it sits in the garage.
Many problems when cars are not driven regularly.
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12.5 is definitely low while driving. Where are you measuring the voltage? On the battery posts? Or is this measured at a point like the cigarette lighter socket, or via a scanner reading through the OBD port?

For a car seeing such little usage, a battery maintainer is highly recommended. Here's a good thread showing a simple method to connect one:

https://www.jaguarforums.com/forum/s...ng-port-87362/

Be sure to get a charger listed as a battery maintainer, NOT a trickle charger. The latter runs at a fixed output and will eventually cook the poor battery. A maintainer monitors the state of charge and adjusts output accordingly.

 

The car does have a current drain and sometimes these are NOT easy to find!
If you want to track it down here is a Jaguar document on their recommendations.
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im using the scanner to monitor voltage,, the 12,5 im getting is with lights on,etc in the driveway at 2000 to 300 rpm..what doesnt make sense is it will charge 14,2 and higher at idle, then after 5 nin it drops to 12,5. doesnt really increase in voltage w rpm, then i simply restart it, and we are back up to 14v and higher,,never saw a bad alt do that,,

 

Try a voltage reading directly at the battery posts and report back.

2003+ models have a "smart" charging system. After engine start, you'll normally see about 14.5V for several minutes, slowly tapering back to around 13.7V.

If you read voltage directly at the battery posts, I suspect you will see normal results. That's right in the downtown heart of the electrical system. The scanner reads via the OBD port, way out in the country so to speak, at the end of a fine branch feeding multiple other circuits, too.

If the numbers are good at the battery, and no quiescent drains are present (test as detailed in an earlier message), the pattern of infrequent usage is your main problem. A battery maintainer would be a good solution.

 

checked voltage at barrery,,high 14<s for a few min then to 13,2 ish,, taking a wild *** guess on a possible drain...turned headlights from auto to manual,,lol gotta try the current draw test,,

 

Hmm, 13.2V is still low. I was hoping we'd see a problem with where the voltage was measured. This is an odd one, as the initial charge voltage looks good. Even with plenty of electrical load, the alternator should be able to maintain more like 13.7V. That 0.5V difference may not seem like much, but it's important and this seemingly minor shortfall will keep the battery from getting a full charge.

Not quite sure what to suggest. The likely culprit isn't exactly clear cut. Might be a bad voltage regulator, which is integral to the alternator.

Is this a V6 or V8? On the V8, unfortunately you have to loosen the engine mounts and raise the engine a little bit for access. If a V6, a new alternator might be worth the gamble, but not on a V8. Might be worth taking it to an electrical specialist if a V8.

If you do have to replace the alternator, make sure it is guaranteed compatible for a Jaguar. Some aftermarket units will install and charge just fine, but the internal electronic wizardry does not communicate properly with the rest of the car. You end up with the battery warning light illuminated constantly.

 

So I gets to thinking. Don't worry, I don't charge for this service...

Why is the "smart" charging system behaving itself in the first few minutes, but then dropping too low when it scales itself back? Maybe it has to do with how the system controls itself.

Just a hunch, but maybe at startup, there's no feedback. The voltage regulator is like Scotty on Star Trek: "I'm giving it all she's got, Captain!" And so it goes for several minutes.

At this point, the voltage regulator is programmed to reduce output, and now looks at feedback from the battery to control the voltage. Only there's a problem with the feedback. It's slightly skewed, for reasons unknown. The regulator happily adjusts to this inaccurate feedback, totally unaware there's a problem.

I'm a genius, right? (It's important to nod and agree...) Only problem is I'm not exactly sure how inaccurate feedback would affect the charging system. I may be totally out in left field (We don't use the W word) so take this with a grain of salt.

There is one known trouble spot that could skew the feedback. Disconnect the ground cable from the battery. Follow this short cable to where it bolts to the body. This bolt extends into the wheelwell. Moisture and road salt can wick along the threads and corrode the connection. Inspect the joint between the cable and body. Make sure both sides are shinier than Kojak's noggin.

Reassemble the newly cleaned connection and see if the voltage reading is now within specs after the initial few minutes.

Is this a guaranteed fix? Not even close, but is cheap and easy (like me) so worth trying.

 

the body ground looks ok,,nut ill give it a go at cleaning it,, found a new oem alt at fcp euro for $370 ish... kinda expensive to be taking a wild guess at,,will be trying the drain test later,,, oh yea the car has 3.0 v6 , 127 k miles..of course wife says its the alt,, when its only driven 2-300 miles a year,,

 

Don't know if it's possible where you live but we have had numerous problems when people replace the alternator. As Karl posted above it can work just fine but the car is not happy with it.

Any chance you can take it to a re-builder and have your factory alternator rebuilt?
You know that is the correct alternator!
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Denso has a factory rebuilt exchange program. I got mine via Autohaus Arizona and it functions flawlessly. I also forfeited the core charge and had the original alternator rebuilt by a Denso authorized rebuilder, using all OE parts, for "the shelf".

 

Thanks for posting that!
A factory rebuild should be correct on all accounts.
Any idea of the cost?
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Here you go:

Denso XR837164, 2100877 Alternator - Jaguar (autohausaz.com)