3 weeks ago my alternator bearing locked up and the lightning bolt and dash light came on, along with my voltmeter dropped into discharge. I had my mechanic replace it with a NAPA rebuilt. I also installed a new OEM battery. Although the new alternator charged, it was difficult to excite and even with the engine off and key removed, the lightning bolt and warning light came on? I installed a battery cutoff switch to prevent drain overnight. Progressively over 2 weeks the voltmeter showed erratic charging and difficulty in exciting ( had to rev it up) Today my voltage began dropping into red discharge, alternator will not excite. Belt is good. Bottom line, did I get a faulty alternator with bad diodes? I would appreciate input before I spend two hours uninstalling and returning the unit under warranty. Oh...I did check voltage at battery with a meter. 11.7 volts not running, same running.

 

You may have gotten a dud alternator. It happens. The warning lamp staying on even with the key 'off' is generally a clue that the internal diodes have failed.


Cheers
DD

 

15 years ago, I went through this with another XJS-C that I owned. If I recall I went through 3 bad rebuilt alternators in 5 weeks. It drove my mechanic crazy. 15 years later, the same bullshit....Junk alternators allegedly rebuilt and tested bought at NAPA. More crap from Mexico. Frustrating since it's gonna cost me for labor to swap out! I wish I could find a brand new one.

 

A fully charged battery will be 12.8 volts 50% charge will be 12.5 volts fully discharged 12 volts, these are with battery disconnected.

Charging voltage should be between 13.5 and 14.5 volts.

As Doug said sounds like a bad alternator. Most likely the regulator.

 

Fully discharged battery @12V? I guess I would like this kind of fully discharged battery...

My battery is not a spring chicken, but it gets to 11.7V-11.8V range after sitting in a garage for 2-3 days not driving. The engine starts from 11.7V. It can even start from 11.6V. If it drops below that, it requires connecting external charger

 

http://www.energymatters.com.au/components/battery-voltage-discharge/

The battery must be disconnected to measure it otherwise the reading will be I would suggest what you measuring the battery with is incorrect .

 

Not to take the thread off topic, thank you, warrjon, for the link. Good point about measuring the voltage with everything completely disconnected.

 

1. Curious me. Why would there be any difference in available volts at the battery posts, whether disconnected ort not? So, long as the probes are on the posts and not the clamps? Rules out poor connections, either way?


2. I made connector for my VOM. Wires to a device that inserts into the cigar lighter.
I can read volts in real time, under various rpm's and loads.


3. Rebuilds have suffered badly in quality. I do recall, decades back visiting a high volumn rebuild shop. Not reassuring at all. A bit like not destroying ones appetite for sausage by visiting a sausage factory. I suspect many are merely a reassembly of parts of failed units that still "'work"!!! The days of rewinding coils and armatures
probably long gone....


Carl

 

My luck with rebuilt alternators cannot be merely coincidental. Love my Jag, but this crap didn't occur with my BMW Z3 Roadster.

 

Which is why I began using a local rebuilder several years ago. 100% success rate versus 50% success rate with off-the-shelf rebuilt units.

Most cities and towns, it seems, have a local shop doing starter and alternator overhauls

Cheers
DD

 

Easy solution then Where were you getting your rebuilt alternators for the Bimmer? Use the same brand/source for the Jag !


Cheers
DD

 

Of course it's good practice, just on general principles, to check all relevant connections and wires. Doubly good practice on old Jags.

Some years ago I helped a pal with charging problems on his XJ6. Long story short, the output wire from the alternator was badly corroded under the insulation. Very high resistance.

Cheers
DD

 

So I can get a rebuilt for $39. Local rebuild shop says bring it in and we'll tell you, but between $85 & $145.

So, how hard is this removal / install. That is the question.

 

Hi Carl,

The reason is, any current draw on the battery will cause a voltage drop and the difference between a full and discharged battery is less than 1volt. Disconnecting the battery removes this unknown.

 

My other cars including Triumphs, Austin Healey, MG, Lotus, Sunbeam, Maserati, Bmw, and other numerous American cars and trucks...never had alternator issues. Only my Jaguar XJS cars....

 

Lucky you!

I've generally had good luck with alternators.....with the notable exception of the GM cars I've owned. But that's another story.

But, even then, when dealing with rebuilt anything, the onus shifts mostly to the rebuilder rather than the type of car.

You might wanna do some digging to see if there's a underlying issue with the car....some sort of wiring fault possibly. Also check that the alternator bracket is well grounded. The engine grounding wasn't very good on these cars. I ran a redundant ground cable to the engine on mine

Cheers
DD

 

Update: Swapped out the 3 week old alternator with a warranty replacement from NAPA today. It's now throwing a charge but now my lightning bolt fault light is illuminated! What the hells up now? If you recall these lights were coming on with the ignition off and key removed. My battery was down to 9 volts when swapping the alternator. Battery is brand new, and alternator is not Lucas,but upgraded type. Any ideas what's happened?

 

The lightning bolt will light in 1 of 2 scenarios.

1 if the Alternator voltage is higher than the battery

2 if the battery voltage is higher than the alternator

If your battery was down to 9volts this is the reason for the light. Once the battery is charged the light should go out as long as the battery is not damaged.

Lead acid batteries do not like being discharged to this low level and it damages the plates and/or dries out the acid. In a sealed battery low acid level will almost invariably mean the battery needs to be replaced.

 

So your opinion is that until my battery is fully charged by the new alternator ( Battery was very low because of dud alternator discharging) it's "Normal" for lighting bolt light to be on....fingers crossed!

 

With the battery being so low it might not charge. Also at 9volts any intelligent battery charger will not even see a battery connected so will not charge.

If you have this issue take the battery to a battery shop and have them load test it and charge it.