I've owned my car about 3 years now, and for the last year I'vehad a problem with the battery losing charge. I've replaced the battery twice, but with an aftermarket battery - not a "Jaguar" direct replacement.

Sure seems to me like the alternator is charging because the dashboard voltmeter goes to 13.8V when the car is running.

Yesterday, I went to go to work. I had the battery on charge all night the night before. I checked the battery voltage with a good voltmeter - said 12.63V. Drove 2 miles to work, and 2 miles home that evening - checked voltage again - 10.3V.

I've had a small squeak in the area of the belt and alternator, which I've tried to adjust out. Got it better, but still there a little bit. Afraid to overadjust it and break it.

I know Jags have a funny Lucas electrical system, so I don't know if it's possible there's a drain on the battery when it is shut off or not. Anyone know what off-key battery amperage should be? I could measure it with a clamp on style DC ammeter, if I knew what the current should be.

Last question, is it possible for the alternator to be bad, even though it shows 13.8 volts on the dash voltmeter when the engine is running?

 

Yeah, most fascia gauges are mere indicators and slow to react. I made a patch cord to connect my meter via the cigar lighter outlet. I was able to read voltage in real time under various rpm and load conditions.

The Jaguar and all other cars care not as to the pedigree of the battery, just want volts and amps, the more the merrier.

Yes, most if not all modern or near modern cars have a residual drain to power any memory units. Radio station presets for example.

So, a meter in series on a battery cable. All loads off. About 045 MA's or so. A mere trickle. If more, some load remains on. Boot lights glove lights, after market alarms, car phones?

If not found that way, tis a bit more complex, start pulling fuses til the reading goes
down. When it does, track down why the circuit that should be open is closed. A sticky switch or relay?

If the drain is substantial, suspect is a diode in the alternator.

If the alternator charge rate is low, first go to clean and tighten connections. if a belt slip, they howl something awful!! And once they slip,they glaze and are dun for. Don't even dream belt dressing!!

Carl

 

Thanks for the tip on the cigar lighter - I'll do it.

I measured 20mA after the dome light went out. The diode thing might be the ticket. I think it would be best to go ahead and replace the alternator at this point. Direct replacement is a good, valid troubleshooting technique for the mere $150.

 

Whew, Easy come easy go, huh??? Just kidding. Might well be that is where you end up anyway.

But, as to the drain via bad diode, just disconnect the alternator and run the drain test.
If it persists it is probably innocent and a rebuilt one will change nothing.

And, there are rebuilds an there are rebuilds. You run the risk of getting one that is worse shape than yours. I aw the interior of a shot that "rebuilt' alternators and the like. Kinda like sausage, don't ever visit one, it will spoil your appetite for wurst!!

Carl