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Jaguar Repair/Overcharging Etype 4.2


Charging system produces 15.5V or more.I have changed the regulator, battery and the alt. is new. Could it be a problem with the Lucus amp gauge?It shows a charge of too much which is correct. Have any suggestions?  Thanking you in advance,,,,,,,,,,,John

Hi John,

My standard procedure is to first charge the battery and then put a volt meter directly on the posts of the battery (not the cable ends) and read battery voltage. It should read 12+ volts.

Next start the engine and set the RPM on about 1500 RPM and check the voltage again at the battery posts. It should read from 13.8 to 14.5 volts. If it is higher or lower then that Something is wrong with the Alternator or Regulator or you have a wiring problem between the alternator and the regulator.

Just because a alternator and regulator are new does not mean they work correctly. However, your first step is to look at your wiring diagram of your car and check each wire at the regulator to see that it goes where it is supose to go and no where else. don't leave this step unturned. A wiring problem between the alternator and regulator can "Full Field" the alternator causing it to over charge.

The gauges in the car are not reliable for testing.

Most shops do not have any way to test a regulator but many auto parts stores and garages do have a way to test an alternator that is off a car. So if you test the wiring and it is correct then you should have the alternator tested.

If you take a copy of your wiring diagram and the Alternator and the regulator to an Auto Electric shop that specializes in auto electrics. They can usually test both. If either tests bad you need to get a statement form them of the test results and you should be able to exchange it for a good one at the parts supplier that you bought them form. The reason you need something in writing from the testing shop is that most companies that sell alternators and rgulators will not accept any electronic part once it was installed on a car.

In the US even a mechanic in a car dealership can not return an electrical part to his own parts department.

Let me know,


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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