Electrical fault identification

  • 1999 JAGUAR XJ8
  • 4.0L
  • V8
  • 2WD
  • 125,000 MILES

- Car completely "blacks out" intermittently through the week whether it be after a drive or getting in the car to start.
- Multimeter reads consistently around 13.8 volts at idle.
- In order to start car, both positive and negative cables have to be disconnected, then reconnected. Car starts up fine without any problems.

(Before disconnecting, positive cables is usually warm; even at times when car has been sitting overnight)

According to O'reilly and AutoZone, with their on car battery/alternator testers, they concluded that the built in voltage regulator on the alternator is the issue.

- Purchased and installed a brand new AGM Interstate battery.
- Purchased and installed a brand new alternator.

- Experiencing same symptoms of the complete "black out" of the car.
- Multimeter with New Alternator/Battery reads consistently around 14.2 volts at idle.

Need help identifying fault in order to resolve issue.
Do you
have the same problem?
Tuesday, July 6th, 2021 AT 2:46 PM

1 Reply


When you say it blacks out, I suspect nothing works that is electrical including lights, horn, radio, and so on.

If that is the case, first there are two main high-power 250 amp fuses in the luggage compartment next to the battery. Power is run directly to them. From that point, it is distributed throughout the fuse boxes and vehicle components.

If you lose everything, the problem has to start before these fuses or both fuses are losing power at the same time. The interesting thing is when you disconnect and reconnect the battery, it works again. Therefore, I have a feeling the wire from the battery to the fuses is weak or damaged in some way. Or you have a bad ground.

If you look below, I attached the wiring schematics for the entire power distribution circuit. In the first pic, I highlighted the two fuses that distribute power throughout the vehicle. if everything goes out, move that brown wire around to see if that changes things.

Also, check that wire for evidence of corrosion or damage. If it is good, then disconnect the battery and inspect the fuses. Make sure they are tight, the connections in them are in good condition too. Also, check the condition of the ground wire between the battery terminal and where it grounds to the vehicle.

The last pic below shows the location of the high-power protection module. That is where the 250 amp fuses are located.

You mentioned that the battery cable gets warm. If there is a weak connection that causes power to arc or excessive resistance, that can happen, so check for corrosion and damage.

Here is a link you may find helpful:


Take care and let me know what you find.


See pics below.

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Tuesday, July 6th, 2021 AT 8:45 PM

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