On my 95 V-12 on start up I am running 13.92 volts, on warm up it drops to 13.32, with air on it drops to 12.92 and with air and lights on it drops to 12.68. I have been on another forum but cannot get the problem resolved. It seems to drop voltsage when engine warms up and lately it goes to 13 volts without anything on. I am checking it with a volt meter at the battery. This may seem crazy but on a trip to the mountains there is about a five mile stretech that is either down hill or up hill all the way. When going back down hill the engine cooled off and the charging rate went up to normal, but of cource when engine temp went back up it decreased..I have check all the connections at the fire wall and have went so far as to change the aloternator even as my old alternator seemed to be okay on the check at the alternator shop. It is a nipensso alternator. I have checked the pully along with the connections but have no change. I did change the battery with one that has more amps so the car starts great. It has never let me down but ocasionally it has a real pull on the battery when trying to start when hot. Any help would be apppreciated.

 

Sounds as if your alternator is working as it should. The slow cranking when cold may not be a low battery problem. Electrical experts should be able to help you as soon as they read this post.
RagJag

 

If your electrical setup is the same as the pre-facelift V12s as far as the alternator/starter motor/firewall post is concerned, it might be worth running a thick cable DIRECT from the alternator to the firewall post (copyright G Francis). As standard, this goes from the alt to the starter and then to the firewall. It just could be that something in the starter or connecters is getting hot and providing a resistance. Certainly cleaning the starter connections and checking them would be a good idea.

Greg

 

From what you said it seems to me your battery and alternator seem ok. The charging system is designed to provide a variable voltage depending on the charge state of the battery. Temperature will have an effect on voltage, resistance will change with temperature and this will effect the volts.

The voltage will also drop as you draw more current, ie turning on AC lights etc. If you draw enough current the voltage will drop to 0.

Does your car crank slowly?
Do you ever have a flat battery?

If the answer to these questions is no your battery and charging system would appear to be working correctly.

 

Dave,

I am reading this over and over, and I think you are doing the readings at idle??. That reading at idle is "about right". Most charging tests are done at 1500rpm.

Then you are travelling thru the mountains, so maybe at revs is also at play here, dunno??.

Most "internally regulated" alternators do have a "temp compensator" as part of the circuitry, and that can/will affect charge as the temps rise to protect the battery apparently.

I am thinking a bad earth, either at the engine, or alongside the battery, as in the earlier cars I think??.

On my '85 V12 I ran that extra +ve wire to the firewall post, as Greg mentioned, AND another -ve wire from the alternator casing bolt direct to the chassis of the car, bypassing the subframe altogether. That sorted the charge issues with that car.

I am also thinking that the "rubber" vulcanised section of the drive pulley on the crankshaft is slipping as it gets hot. I do read occasionally that this does happen, usually associated with a "squeal", BUT, not always. Might be worth a look, since the 2 alternators are testing OK on the bench?.

 

Thanks for the quick response. The car has never failed to start but a couple of times has been relucant. I may be worring about nothing but I see that the alternator reading on my 96 XJ6 stays at 14 volts without going down when something is turned on. It seem from your guys everything is as it should be. I have had the car for 3 years now it has always been like that. Its the only vehicle that I have ever had that the currant to the battery fluctuates like it does.