I have a 1982 xj6 4.2L

I am not really sure where to start, so I am just going to give a list of events in the order they happened.

About a month ago, I was driving home from work in the dark and all of the sudden my engine seemed to lack power.
The stereo turned off and my headlights were dimming. I was begining to lose speed. My amatuer mechanic instict told me to turn the head lights off. After turning the head lights off the rpms and engine power returned to normal.
I drove to the nearest parts store.
I arrived at the parts store and the engine was still running fine, but I still had all accessories off.
Just to see what would happen, I switched my head lights to on and it immediately killed the engine.
I had the battery tested and it was bad.
I replaced the battery.
The car ran fine for about 2 days, then I had another dead battery. I attempted to jump start, but every time I got the engine running and I disconnected the jumper cables, the engine would slowly start to die and no amount of gas could keep the rpms up. I concluded I had a bad alternator.
I ordered a new alternator and trickle charged my new battery to full charge.
The new alternator arrived and I replaced it myself.
I had no problems for about 2 weeks.
Last night my roommate and I went out of a little night cruise around town. In the 2 weeks that it has had the new alternator, this was probably the longest amount time I had had the headlights on in one drive.
We stopped at a store to grab some food, and when we came back out to the car and tried to start it the battery was dead. I did not leave the lights on and we were only in the store for about 10 minutes.
We got a jump start from someone.
The engine turned over without having to wait very long at all for the battery to charge.
We headed for home and about half way there the vehicle starting doing the whole loss of power/headlights dimming/stereo turning off thing.
I flipped all my accessories and headlights off and the engine started to act normal. We got home okay but had to leave the lights off. Stressed and mad about the situation(and because it was too dark to do anything about it), I went to bed and decided to deal with it in the morning.

I got up early this morning to go see to it because I had work around noon. I went out to the car and tried to start it, guessing that the battery would be completely dead. I was wrong. There was just enough charge to turn the engine over.
I wasn't really sure what to think of that.
I got in and decided to test drive it.
With all the accessories off, I made it about a mile before the engine started to die. I pulled over and tried to fight it by putting it in nuertal and raising the rpms, but again, no amount of throttle could keep it alive.
The battery was totally dead now. Nothing worked and the starter motor didn't even try to crank the engine when I turned the key. I called my mother and asked if she could come give me a jump.
She arrived and I connected the jumper cables, waited about 10 seconds, and turned the key. The engine started and I disconnected the cables, not allowing the battery to charge off the other vehicle. I waited and waited, expecting to watch the engine slowly die from lack of battery power. Nothing happened. The engine continued to run at a normal sounding idle.
I asked my mom to follow me and that I was going to try to get to the nearest parts store. The car ran fine all the way there, and my stereo even came back on. I parked at the parts store and just for kicks and giggles I flipped my lights on. The engine continued to run with no difference in the idling rpms. I turned the engine off, turned to my mother and said "watch this thing turn itself on". I turned the key and the engine fired up. Somehow, the battery had recieved some charge on the way to the parts store. I grabbed a parts guy and had him test the battery. It read 12.12 volts. He also tested for energy coming from the alternator which was 12.04 volts. Not good enough. How could my brand new alternator already be going bad?
My mother suggested driving the vehicle to her house and then I could take my dads truck to work. I agreed and we left the parts store to her part of town, about 10 miles away.
The car ran great the whole way there.
We were almost there when I decided to stop at another parts store to see if the battery had any difference in charge from the 10 mile drive it just went on.
The battery tested higher voltage than before, 12.6, but still lower than it should be. I also had power coming from the alternator tested again, only this time, there was none. The tester read that no voltage was coming from the alternator. The parts guy said "your battery is dying as we speak.".
We drove the small remainder of distance to my parents house with no problems.
I don't know why, but right before I got went to leave for work in my dads truck, I decided to check my cars alternator belt. When I replaced it, I made sure I adjusted the tensioner accordingly until the belt was tight. Now the belt was a little loose. Not way loose. But looser than it was. I never thought to check it earlier because the belt was never whining/squealing (I know they don't always do that when they are loose). I didn't have enough time to fiddle with it before I had to leave to work, so when I get home tonight I tighten it and see if that helps at all. But I have a feeling that isn't the problem, so I typing up this novel.
I am really really hoping that one of the alternator's cells hasn't already gone bad.

Any suggestions/feedback is greatly appreciated.
Thank you

 

A marathon indeed.

The alternator SHOULD be reading 13.5V or better with the engine at about 1500RPM, and NO load as in lights etc.

If you got this reading, keep the revs at 1500 and start to load it up, lights, note the reading, a/c, note the reading, heated rear window, note the reading. With all that on is should NOT drop below 13.2V. If it does the alternator is simply not keeping up.

Bear in mind that at idle the alternator WILL NOT charge, so you are then running on battery only, until you drive off and the alternator kicks in.

12.6V is exactly what a fully charged battery should read, maybe 12.7v, thats all there is. 2.1V per cell, and you got 6 celss.

I would also do some SERIOUS checking of ALL the earth leads, mainly the one from the engine to the chassis, and the body end of the battery earth cable. I bet they are cruddy, and that will screw with any cars electrical system.

The belt must be tight, and ensure NO oil is on it.

Charge that battery with a battery charger, NOT the alternator. The alternator is designed to maintain a charge plus a "bit more", but will NOT properly charge a flat battery.

They may be more as I think about it, but that will keep you busy for a few hours.

 

Josh:

Grant, as usual, has you on the right path.

I might suggest that you invest in a simple Volt, Ohm meter. VOM!! Cheap at Harbor freight or the like. I have both analog and digital versions. And, I made up a couple of test lamps. Ought be standard equipment for stewards of old Jaguars. In addition, I made up a patch cord to enable me to connect my meters to the car in the cabin via the cigar lighter. I've not smoked for decades, so lt is good for something!! The patch cord enable one to read available volts in real time under various loads and RPMs. Excellent diagnostic stuff.

Invest in a Smart Charger. Mine is a Schumacher from O'Reilly's at about $100.00!
Their techs are very helpful. A lesser one will do, but needs some interpretation and not OK for the AGM batteries.

And, your alternator has two wires off the back. The large brown one runs to a post under the battery box on LHD cars. It must be intact, have good connectors and be clean and tight at both ends. if not, the alternator can do what it wishes and the car get little or no benefit. The other is a smaller wire and serves as an excitor to kick the alternator into producing juice and to warn you via idiot light if it isn't. Same caveats. Intact, clean and tight connectors + good bulb in the fascia!!

Fun, ain't it!!

Carl

 

Time for a drink, then off to work, AGAIN.

Lots of connectors in lots of places that need attention. Simple old age is the demon here now.

I believe it was Doug that always suggests to spend a Saturday and go over ALL the connectors in the WHOLE car, and clean and refit. That sorts/prevents 98% of the gremlins that appear on here regularly.

Another V12 engine fire being flatbedded here today, some people simply do not listen, NO sympathy from me.

 

Well I think this is your problem, frankly. When generating, an alternator puts quite a load onto the belt, and if the belt is not sufficiently tight, slippage will occur, Do you have the air con compressor running as well on that belt ?

One night many moons ago in the early 70s, when I had an Austin 1100, (a small family car made in Birmingham in the 60s), I was going home for Christmas to my parents and the lights just got dimmer and dimmer. I was on unlit country roads, so eventually I stopped and had a quick look and the dynamo drive belt was loose, but not all that much !

So I levered the dynamo to tension the belt with whatever I had in the car but the battery was now so flat the starter motor wouldn't turn. Fortunately I was at the top of a little slope, so I pushed the car, jumped in, put it into gear and bump-started it and away it went, just sufficient to get home.

 

Hi there,
Does the alternator light act as normal? ie it comes on nice and bright when you turn the ignition on then goes out once the car has started? Make sure it has the right wattage globe for the alternator and as other have suggested check the connections to the lamp socket and the connections beyond it. The alternator relies on this lamp to 'excite' the field winding in the alternator and without it the alternator will not produce a charge.

To me, from skimming over all the posts its sounds like there is a burnt / snapped / corroded wire or connection that makes connection some times but not others. If you cant find the issue by inspecting the wires and connections ide go and buy some new 100 amp wire or 50 amp or what ever you alternator is rated at +20 amps and run a new main wire from the alternator to the battery, then buy a charge light globe (the wattage is important, refer to the alternator manufacturer for the right rating) and connect that in line with the alternator and battery (positive terminal i think) then if the alternator is a 3 wire run the sensor wire TO THE BATTREY not the output terminal of the alternator as it uses this wire to read the status of the battery. If there is a corroded connection ect then the alternator could have 15 volts at the terminal of the alternator and 8 volts at the battery and it would think it was fully charged.

Lastly, the alternator relies on its mount bolt and bracket for earth and if these arnt clean it wont charge.

Hope this helps somewhere along the line,

Regards Jay