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Charging System Check

Charging System Check

Barrie Dawson


For my first contribution to the XJ list I would like to offer some support to Joe and his electrical nightmare.

I have been reading Joes' accounts about charging circuit etc., the problem is looking very much battery related. I think the original alternator has cause serious damage to the battery and it is now shorting the new alternator. I would suggest getting the battery checked out before going further, also avoid jump starting you may damage the other car as well. When you are certain there is no battery problem I suggest the following course:

    PHASE 1
  1. Remove all fuses NOT required for starting and running the engine.
  2. Check tightness of ALL battery and alternator wires, especially earth straps.
  3. Check tightness of engine earth strap.
  4. Connect 60 AMP + ammeter in main feed from battery and measure current draw with everything off - should be zero.
  5. Turn ignition key through each step measuring current draw then start engine.
  6. When running the current draw should not exceed 40 AMPS and should drop after 3 - 4 minutes to about 10 -20 AMPS. at 750 - 1000 RPM.
  7. Assuming everything okay till now, switch off and start next phase, you have proved the problem is neither alternator/battery/engine system.
    PHASE 2
  1. With ignition key turned to off replace a fuse and measure current draw with ignition off and related circuit off = zero.
  2. Carry out same measurement with circuit on, ignition may need to be on for this test depends on circuit.
  3. Make a note of the results and remove the fuse.
  4. Repeat steps 1 & 2 until either a serious discrepancy occurs or every system shows up okay.

I hear remarks like "How do I know what is the correct measurement?" Instruments, radios, computers, cigar lighters, will draw very small amounts so ensure you check these first (probably no more than 1 AMP). ALL lights on at once will cause a reading of 10 - 20 AMPS so when testing here you would expect some lower readings when having marker light only.

  • Direction indicators, reverse lights, brake lights, rear fog lights will draw 1-2 AMPS.
  • Interior lamps will probably draw 3 AMPS total.
  • Headlamps will draw 5 AMPS each.
  • Any motors running, eg. window lift/sunroof would draw in the same area, about 10 AMPS each. Treat door servos the same as any other motor they should only draw when energised 3 -7 AMPS.


In either test when a failure occurs replace or at least disconnect offending part until repairs are effected.

The art of the auto electrician is complicated and doesn't appear to follow any conventional rules regarding electrical circuits. I can only recommend conventional rules though and so far they have worked for me. I have used the above PHASE 1/PHASE 2 process many times and with success even if it does take a long time. You may be lucky and get a result in the early stages.

All the best

Lew Plummer

Place a multi meter in series with the neg. battery cable. Meter and leads set for the 10 amp scale. Approach this cautiously, the current may be out of meter range -- usually not. Set the meter up so it can be read from inside the car. Remove one fuse at a time from the right and left fuse blocks, and the lighting fuses at the left edge of the engine bay, etc. If the fuse you pull drops the current to a expectable level then you have isolated the primary circuit causing the problem.

The real hitch in this process is the Jags unusual electrical system. Most circuits branching from these fuses (some don't) are further protected by inline fuses scattered throughout out the car. But, if your vehicle has 90 to 100 thousand accumulated miles look first at the alternator if nothing shows up in the primary and lighting circuits.

If a diode is shorted in the full wave rectifier of the alternator it will pull current from the battery. To prove this unplug the small battery wire attached to the alternator -- the battery meter current will drop if the rectifier is to blame.

A healthy electrical system should pull approximately 5 to 10 ma with the key off. Vehicle electronic loads, clocks, trip computers, etc., creates this little trickle from the battery. Conductive material on open connections, broken down insulation and the like can cause the quiescent current to be higher than it should be.

Jaguar chose to route the engine harnesses in some really warm places. In time these harness will brake down. Check the harnesses routed over the fuel injection and around the front of the engine for deterioration.

I am in the process of rebuilding these harnesses in my Jag. I am replacing the old stuff with a higher temperature wire for the fuel injection, ignition and pollution control harnesses. The new loom is housed in high temperature loose fitting convoluted tubing. The original connectors are being maintained. I am solder/shrink splicing the new cable into the original connectors. These connectors are located in a relatively cool area of the engine bay. The wire insulation near these connectors is still soft and pliable.

I keep a current draw record of my system and check its health occasionally. One of my routine checks found the alternator problem in my shop before the "prince of darkness" manifested its self along the roadway.

I replaced the alternator with Johns Cars Delco conversion kit. The kits works well, except for some belt adjustment limitations. Replacing the alternator is a royal pain. Why didn't they install the air pump on the bottom and the alternator on top? Next time I have alternator problems I will seriously look at the possibility.

Before your trouble shooting gets to far along you should purchase electrical diagrams from Jaguar Services (800-842-0912) they are "worth their weight..." and not expensive considering the time they will save.

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