last week I broke a alternator belt. I found a alternator with a bad bearing was the culprit. I bought a new alternator and new belts put them on what a job i might add, tricky to get alternator out. and yesterday my wife had a dead battery. she had a tow truck bring it home and when I charged the battery up and drove the car it seamed good. but when I turn off the car the lighting bolt light comes on in the dash my manual says that means a bad alternator but it seems like that would be when its running??? the alternator i had it checked today and its putting out 13.87 volts. My battery which is supposed to be a 750cca when tested was only showing 550. the guy said low but doable. he said i might get it load tested as they aren't able to do that on the jaguar with there equipment. any ideas to what this could be would be very helpful. If I unhook the battery cable when the car isn't running the battery keeps a charge but if i leave the cable hooked up then dead battery. Oh and i drove it to work today running the headlights and the heater the alternator gauge was running right above the red. on the way home i drove most of the way with no heater or lights and had the gauge sitting right where it should be then ran heater for the last ten miles and the gauge slowly worked its way down to just above the red

 

Sounds like a problem with the diode pack inside the alternator

Cheers
DD

 

I respecfully disagree, sounds like a bad cell on the battery. Change the battery first, how old is it?

 

I'm going to go with Doug on this one. If the battery were the culprit, it wouldn't keep a charge whether it was connected or not and certainly wouldn't make the light stay on when the key is off. It's backfeeding through the system causing the light to stay on and draining the battery.

 

Not that mine is the final word on this or any other subject but this ^ ^ is the classic symptom of a diode problem


Cheers
DD

 

This is a classic diode symptom, but before you pull the alternator again, check one thing. When you bought the new alt it already had a wire on it with a connector before you installed it. This is a noise filter. If the alt supplier put the noise filter connector on the wrong terminal (i.e. the terminal the noise filter should be on), you will have exactly the same symptoms as you are describing. Try switching the two terminal plugs on the back of the alt (the exciter wire from the car and the noise filter) and without turning anything on, see if your light is out now. If it is, someone at the alt rebuild place put the noise filter on the wrong terminal and all is well.

Don't ask me how I know this.

D

 

And by the way when the two wires are on the wrong terminal, the alt will still charge normally and behave fine except when you turn the key off.

 

ok i switched the wires the other day and the light went off and the battery was dead in the next morning so I took it to work an charged it up i just hooked it up and this evening and no light. gauge seems low when car running with all lights and heater running. but i will see if it charged up in the morning and go from there maybe my gauge is bad or my battery

 

Check to see what voltage you're getting out of the alt first. With all your lights and heater on, your gauge is most definitely going to read low. Try throwing the battery on a charger - trying to use the alternator to charge a dead battery takes forever. Even with everything off, my gauge never gets above the halfway hash-mark.

 

Also try blocking the throttle open to give about 1000-1200 rpm. You should be getting over 14v at the battery at that rpm. These alternators are definitely undersized but you still ought to be able to get over 14 volts from them.

Correct me if I'm wrong (it's been a couple months since I had this issue with my alt, fresh back from the rebuild shop) but I believe the noise filter wire goes on the terminal that sits down inside the case of the alt, and the exciter wire was the one sticking up out of the case? Or was it the other way around?

 

Alternator diodes can be checked easily with an old-fashioned analog ohm-meter, usually a multimeter. Unhook the heavy current charging wire from the alternator (be careful, it's 12volt live from the battery) and apply ohm-meter between this alternator connector screw and ground, such as alternator metal case. Have the meter switched to highest range. With meter leads one way round there will be good conductance, the other way round, no conductance at all. If there is any back leak, one or more diodes is faulty. These diodes can be replaced IF you can get them. Usually stud rectifiers pressed into a heat-sink.
Another way of doing the job would be to measure the back-leak current into the alternator with engine off. Even a small lamp may light if you wire it in series with the heavy current alternator terminal. Use your smallest test-lamp. Do it this way if you don't have an analog meter.
Sorry I don't post much now as I don't run a Jag anymore.

Leedsman.