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Old 29-08-2020
werd  werd is offline
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Default CHECK GUAGES LIGHT and BATTERY GUAGE BOTTOMED OUT

The check gauges light has come on and the battery gauge bottoms out at 9V.
Thinking it was the alternator, I swapped it for another. Everything looked good for about 10 minutes and then the same problem. Turning the engine off and then back on resets the problem but then comes back after a couple of minutes.
Below is my observations:
The battery is good. Six months old. Load tested OK. Voltage starts at 13.8V after overnight charge.
When the car is started, the voltage drops to low 12V
Alternator output is very similar to battery reading. Increasing motor speed doesn't change voltage.
The battery temperature sensor resistance seems OK. (I put it in the fridge and watched it warm up)
I have cleaned the battery connections, alternator connections, power powertrain control module (PCM) plugs.
No error codes using scantool.
My guess is the PCM is getting a reading from a sensor and turning the voltage down.
But I have no idea..... I'm way over my depth on this one.
It's been a week now and it's doing my head in.
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Old 29-08-2020
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have you cleaned all your earth connections? I don't know what year you are running but on a 96 the main pcm earths are on the oil dipstick tube bracket to the block and on the 99 they are on the coil bolts maybe check and clean them ?
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Old 29-08-2020
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Default

Hey Werd, what year is yours.
Some years have the voltage regulator in the computer.

As Mark suggested, definitely check your main earths.
Also check to see if your year had the voltage regulator in the pcm.

You may have to swap pcm,s to see if the problem goes away.

Cheers,
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Old 09-09-2020
werd  werd is offline
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Mine is a 97.
I did clean up the ground points 1. Beside the battery on the side wall and 2. Firewall to intake manifold.
The other earths are on the block below the coil. I will clean those up.

I think the regulator is in the PCM. On top at the back of the alternator is a field coil to control the output voltage.
So, can the output from the PCM be tested?
Can the field coil be disconnected/removed from the alternator to test the alternator output voltage? I understand that if the alternator is working it would put out 18V? Would it be safe for the battery if this was done for about 30-60 seconds?
Another thought I had was to remove every fuse that was not essential for the running of the motor to eliminate other circuits causing problems.
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Old 09-09-2020
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https://youtu.be/ntLuEZp049c

Have you tried this to see if it's the instrument cluster? Not sure if it will work on an XJ but it's worth a crack.

Sent from my SM-T805Y using Tapatalk
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Old 09-09-2020
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"To test voltage regulator circuitry, refer to the appropriate Powertrain Diagnostic Procedures service manual."

Does anyone have this pdf?
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Old 09-09-2020
werd  werd is offline
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Quote:
Originally Posted by spoondog View Post
https://youtu.be/ntLuEZp049c

Have you tried this to see if it's the instrument cluster? Not sure if it will work on an XJ but it's worth a crack.

Sent from my SM-T805Y using Tapatalk
Interesting test! It does work on the XJ. All guages and lights are A OK
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