1995 XJS V-12 voltage problems

There ya go!
Cranking should be no less than 9.5 or so.
I don't like less than 10.

The BIG problem was getting new owners used to voltmeters when they were used to ammeters or simply idiot lights!

I've got this big, whacking handheld battery tester.
Simulates a starter load to check condition of battery.
Hold the load for 10 seconds, see if it drops below 10 volts or so.
You'll spot a dud battery (that may still work for a while) right away.
It will drop down to 6 or 7.

Dave

Hey Dave you are the journeyman... it seems.
Just checked my 95 with a tester @ the battery.........
Sitting but not running, battery reads ....................................12.80 Volts
Running with full load................................................................14.18 Volts
So running the battery should be way above ........ ...............12 volts........
However the battery need not be way above this figure to start the engine easilly....
Could the terminals somewhere be corroded on David's car, or even loose on the alternator, I guess if the alternator belt was loose he would hear it slipping........
I guess I do this now for a LOVING more than a living ..........
Good luck...Art

Either of you guys ever do this for a living?
About 40 years Journeyman auto mechanic and electric.
14.0 volts to just under 14.5 volts is what the battery should be maintained at, without full loads.
(remember the 12.6V actual)
http://www.egauges.com/ATM_Tips.asp?TipPage=voltmeterammeter.htm
read the last paragraph.

http://www.egauges.com/ATM_Tips.asp?TipPage=voltmeterammeter.htm

http://autorepair.about.com/library/faqs/bl685b.htm

And here is something I learned: The newer computer controlled vehicles actually turn the alternator off to save gas when the battery is fully charged (gee, all these decades I thought that's what the voltage regulator did):
http://forums.trailvoy.com/showthread.php?p=635919

Bottom line:
If I was out in the middle of nowhere and my voltmeter went to 12, and not above while driving, I would be finding a place to stay before it got dark.

I agree....

Voltmeters, unlike ammeters, react very slowly and generally show what was happening 10 minutes ago until the alternator is running constantly. The initial voltage usually is the cold rate of the battery, starting will reduce it a little, it will then climb upto the output of the alternator, as the battery charges the alt output will decrease etc. Bottom line a12 V battery has a maximum output of 2.2 Volts per cell, 13.2V MAXIMUM! Todays cars (since the electronic age) need to have a minimum of 12V to operate. As a consequence, the charging rate was raised up to 15V on some cars to ensure the minimum is met at all times. In other words they are OVERCHARGING the battery most of the time. Newer cars with heavy loads now use two and even three batteries to seperate the circuits, some have water cooled alternators that have very high output.. Battery manufacturers are having a field day. AsIsaid in my earlier post this car is behaving normally!

"is this at the battery voltage or on the gauge....... gauges can be wrong..... "

Absolutely!
But, generally, when they are wrong, they are wrong.
Voltmeters generally don't slowly drop off like that with running.
I would have called that as something to look into if it started at 12, and stayed at 12.

But, it starts normally at 14.5V, then drops to 12.

is this at the battery voltage or on the gauge....... gauges can be wrong.....

Which is what I'm saying.
At 12V, you have a problem.

Generally speaking, electronic voltage regulators don't act like that.
Old mechanical ones could when hot.
Seen it.

Did you check the power feed to the alternator and the belt for tightness?

Hey Dave thanks for your info. My charging system starts at above 14.5 in the mornings but the more I drive the car the lower it gets. It has gotten past the 12.5 in the past with lights and AC on. I"m afraid somewhere along the line it is going to strand me . I would like to keep it at 14 volts. I have a XJ6 that runs 14.5 always.

"It sounds like everything is working OK to me! It is a 12V battery.

Jaguar affectionado and etc. "

I really hate doing this, and don't want you to think I am being contrary.......but......
I did this for too many decades.
Nominal cell voltage is about 2.1V.
A 12V battery at rest, no surface charge, needs to be at least 12.6V.
If it's 12V, it is discharged.
An alternator puts out higher voltages to keep it charged (newer cars sometimes have a voltmeter instead of an ammeter, and the 12v mark is at the red line).
14.5V is what I like to see out of a fully functional charging system.

It sounds like everything is working OK to me! It is a 12V battery.

14V is about the minimum for a functioning charge system.
The symptoms you describe almost sound like a slipping alternator belt.
Have you checked belt deflection with your thumb?
Engine off, of course.

Also, the terminal on the back of the alternator...
is yours all plugs or is there a battery feed that is bolted on?
Loosen (do not short your wrench), rotate the wire back and forth to eliminate any corrosion, and re-tighten. Since you've cleaned your battery cables, you aren't concerned about resetting clock and radio, so safest bet is to disconnect one leg of the battery for this.

Those are the simple things.

I am guessing internally regulated alternator (by 95, most were).
Removal of the alternator for testing is next.
Make sure they load it if possible.

First get a digital voltmeter and hook it up directly to the battery,start the engine, then I would simply turn everything on while stationary, the power should stay around 12 @ idle, and when revved to about 2,000 rpm it should maintain over 12 volts for sure......... the proof of the pudding to me is right at the battery where you can see what is truly happening. Art