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Jaguar Repair/Early 4.2 power source


QUESTION: On a car wired without using The AL  TERMINAL on the alternator  what powers the 3AW relay. IIs it from b+  Terminal Circuit to the BATT or from the 6RA  relay in the ignition circuit  to the alternator terminal F +.  I still have an overcharging problem and am trying to eliminate any source that might cause a rogue ground in the field  return from the alternator terminal  F-     I appreciate your suggestion to check the wiring in your lanswer to my previous inquiry ......John

ANSWER: Hi John,

What year is your E-Type? I need to look at a wiring diagram of your car so I need to know what year it is.


---------- FOLLOW-UP ----------

QUESTION: The car is a 1967 American convertible

Hi John,

67 US version, Cool! We called that car the "Last Fast E-Type", 3 SU carburetors and Jaguar guaranteed it to run a true 150 MPH.

The alternator should have 4 terminals labeled AL, F+, F-, and B+.

My diagram shows the "AL" terminal on the alternator gets the brown w/yellow tracer wire which comes from the Ignition charge light unit (AL terminal) and it is powered by charge light (IGN)and the #6 fuse of the fuse box. This "AL" terminal must receive power from the charge light bulb to start charging.

It has been 40+ years since I worked on one of these but I seem to remember that Jaguar used a "Shunt Circuit" around the charge light. I seem to remember they powered the "AL" trminal on the alternator with the charge light (IGN light) and a small resistor circuit so if the bulb burned out then that "Exciter Circuit" could still trigger the alternator to start charging. The "AL" circuit is the "Exciter Curcuit". My diagram don't show that memory ha!

The "F+" terminal gets a brown w/purple wire from the charge relay (C-2 Terminal)(powered when the Key is in the "ON" position) and the "F-" goes to the voltage regulator terminal "F" with a brown w/green tracer wire.

The "B+" terminal on the alternator gets power all the time from the battery by a large brown wire.

The regulator, mounted low under the hood on the far left is a solid state unit with three terminals. "F" (which is the field circuit) gets the brown w/green wire from the "F-" terminal on the alternator.

The (-) on the regulator gets a black wire that is a ground wire

The (+) on the regulator gets a brown wire which is 12v power form the charge relay (terminal C-2) when the Ignition is in the "ON" position.

If all these wires do what they are supose to then it should work. Didn't you tell me this was over charging at 15+ volts at idle? This could mean that the alternator is "Full fielded" all the time meaning either the alternator is "Full Fielded" internally or the regulator or wiring is full fielding the alternator. When a alternator internal windings get shorted the alternator can produce very high voltages that the regulator can not control.

Here in the US most auto parts stores have a generator and alternator test stand and will test an alternator for free if it is off the car. If all the wiring is correct and you have a good volt meter that is correct (Test your volt meter by using it across the terminals of the battery) then you should have the altenator tested by itself. You can't test a regulator in the car without a known good alternator and wiring. So the regulator has to be the last item to suspect and test.

Alternators and generators control their voltage output by either grounding the field or powering the field circuit. This unit is a little different from others in that it uses a relay to power the field when the key is on and then controls the charge rate (voltage) like many others by grounding the field as per needed by the regulator.

Other items to test, Check the engine ground strap and the alternator to engine grounding. This is best done with a "Voltage Drop Test". (If you have not done this before, let me know and I will give you the procedure) Test all powers with a small load. (I often use a large bulb like a head light bulb or a 12v electric motor.) Just using a volt meter is not suficient because a volt meter only tests "Potential" not actual.

The car is 48 years old and there is no way to tell how many alternators and regulators have been put on this car and how many mechanics have been working on the wiring. So you must not leave any stone unturned. If you are lucky enough to still have the owners manual, look to see if it has the wiring diagram with it. Jags use to come with a wiring diagram. To see if my wiring matches yours and the car.
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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