Charge light staying on after replacing alternator

Tiny
DDIXONTULSA@AOL.COM
  • MEMBER
  • 1998 JAGUAR XJ8
  • 4.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 130,000 MILES
I replaced the alternator in my car but the charge light will not go off. The car is running fine. I took the new alternator and had it tested to make sure it was working correctly and it was putting off fourteen volts which was good. I can only think it is in the wiring system or in a sensor related to the alternator. I am trying to find a two wire charge system diagram for the car to find the sensors. Any other advice and or help would be helpful. Thanks.
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Saturday, April 1st, 2017 AT 7:45 AM

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Tiny
CARADIODOC
  • EXPERT
Testing off the engine is not a good idea. Most test benches have a half horsepower motor to run the generator. They actually take around five horsepower to run them at full output. Fourteen volts is just barely into the acceptable range of thirteen point seventy-five to fourteen point seventy-five volts. What I need to see is "full-load output current". Also, what is the battery voltage when the generator is running, in the car? If you find it stays near twelve point six, the charging system is not working. If you still find close to fourteen volts, a defective diode inside the generator becomes the best suspect, but the system must be tested professionally to identify that. You will not find a defective diode with a bench tester. The additional clue to a defective diode is "ripple voltage" will be very high.

Start by measuring the battery's voltage while the engine is running. If it is close to twelve point six or less, back-probe the white / gray wire in the generator's plug. That must have battery voltage on it. If it is missing, there's a blown five-amp fuse, number two in the under-hood fuse box.

If the charging voltage is within the acceptable range, a defective diode is still the first suspect. The voltage regulator may be responding to the dips in voltage that occur, by turning on the warning light.
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Saturday, April 1st, 2017 AT 3:12 PM
Tiny
DDIXONTULSA@AOL.COM
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The car is charging. It has fourteen point six voltage output at the battery when the car is running. So that is not the problem. The suspected problem is in the wiring. I purchased this car and it has on the title is a flood car and was totaled out by insurance company before I purchased it. I am looking for the wiring schematic for the two wire charge system, and ideas on where I can find this? Thanks.
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Monday, April 3rd, 2017 AT 3:48 PM
Tiny
CARADIODOC
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The fourteen point six volts is perfect, and it shows why I do not like test benches. You still did not have the charging system tested though with a professional load tester. The major concern is "full-load output current". You have a one hundred twenty-amp generator. If one of the six internal diodes has failed, the full-load test will show the most you can develop is forty amps, exactly one-third of the maximum rating.

The generator is controlled by the internal voltage regulator. We know all of that is working because of the fourteen point six volts. No need of a wiring diagram as there is nothing to fix. The voltage regulator is told to turn on by the current flowing through the "Battery" light on the dash. We know that is working because the regulator turned on.

Once the generator produces some output, the voltage regulator turns the "Battery" light off. That is the only thing that is not happening. There is no wiring problem since everything else is working. To add to the misery, a lot of late-model voltage regulators turn the "Battery" light on for a low-charge or an over-charge condition, as well as the no-charge condition. Two things can make your regulator turn the warning light on even though the system seems to be working. All AC generators put out three-phase output voltage and current. When one diode fails, (those are one-way valves for electrical current flow), you will get exactly one-third of the generator's rated current. One of those three phases is missing. At that time, output voltage drops a lot, possibly to as little as eight volts. You see what the battery has smoothed out, the fourteen point six volts, but the regulator sees the eight volts and turns the warning light on.

In response to the eight volts I suggested, the regulator may try to bump up the charging voltage a little to reach the desired thirteen point seventy-five to fourteen point seventy-five volts. Once the two working phases appear, it cannot cut back fast enough, so the output voltage goes too high for an instant. Again, you do not see that at the battery, but the regulator may turn on the warning light for what it thinks is an over-charge condition.

Either way, you need to have the charging system test done so we know for sure if my story pertains to your problem. Once the tester's cables are connected, the testing takes about ten seconds. We also want to see what is found for "ripple voltage". If a diode has failed, ripple voltage will be high.

On newer cars, the instrument cluster is a computer, and water damage could cause unusual problems, but if it is causing the warning light to stay on, it is likely you would have other problems too. In addition, there is a circuit in the generator that sends a sample of the output current to the voltage regulator so the regulator knows when to turn the warning light off. The circuit can fail to signal the regulator even though the regulator is running the generator perfectly. That is a case where testing shows you can get the full one hundred twenty amps, and ripple voltage is low, (acceptable), but the warning light is not being turned off.

I greatly prefer manufacturer's paper service manuals. Ebay is a good place to find them, Also, some new-car dealers will photocopy a few relevant pages for regular customers. You can also buy a one-year subscription for just your car from the same online sites the repair shops subscribe to. Those are Mitchel On-Demand, and AllData. Mitchel has better wiring diagrams.
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Monday, April 3rd, 2017 AT 6:50 PM

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