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I'm having an issue with my 2001 Jaguar XJ8, where when reading the voltage from an OBDII reader shows the voltage is bouncing around between 13.3 and 12.8 volts at a very rapid rate. If I were to post a picture of it it would almost look like someone who is running on an EKG monitor, my question is what would I look at in order to try to stabilize this voltage, as well as find out what the actual charging voltage should be coming out of my alternator/generator? I suspect that the suppression module has gone bad... I just put a brand new alternator in, which didn't solve the issue. Other cars that I have viewed the voltage on the OBD have showed 14.7 volts charging while the engine is running, I have never seen that high of a voltage on my OBD, as well as have never seen a stable smooth line going across my display. Any help would be greatly appreciated!
Sorry it took awhile for the response...I didn't see your post.
I would possibly suspect either you have a bad/dead cell in your battery or the voltage regulator stopped regulating in the alternator. Take it Autozone or O'Reilly's and have them test it.
Also, if you have any corrosion on your terminals (another sign of a bad battery) replace the terminal, you won't get the corrosion off it. Also, check your grounding points, Jags have a few.
As far as alternator goes, I just replaced it, one would assume it would be good. As for battery, and bad cell, that could be a possibility, although there is zero corrosion on the terminals. What I thought it actually might be is the noise suppression module has possibly gone bad, considering that the spikes that I am seeing could be considered noise. And that module would straighten that out and flatten them out. Please see attached the OBD voltage that I recorded while driving one day.
It should be pretty steady around 14.1-14.4V when running. Dead or dying batteries can cause all kind of weird issues with these cars...
First thing I'd do would be to test the voltage with a multimeter, to have a solid point of reference vs the onboard sensors. Easy probe points, aside from the battery, are using the engine block for ground, and the right hand false bulkhead junction for positive. While the car's running of course. If the alt was recently replaced, it's possible the new alt could be bad, but somewhat unlikely.
One test that you can do to isolate the battery/alt would be: start the car, and with the engine running, disconnect the ground/negative terminal from the battery. Measure voltage between chassis ground and positive and see what it shows with the battery disconnected, to measure the alternator output voltage with no battery load. Make sure to reconnect the battery before turning the car off...
You can also try measuring battery voltage without the engine running, after the car's sat for a few hours or so since last driving, to make sure the "overcharge" from the alternator has dissipated. Should be 12.4V or near there.
One more bit of food for thought, "typical" acceptable alternator voltage fluctuation, given steady load, no rapidly switching accessories, etc, is ~.2V (I believe RMS? Maybe peak-peak?)
I had similar voltage fluctuation on my car. I first realised there was something wrong when I noticed slight "flickering" of the dash and interior lights. Cleaning all contacts on the large fuse box in the trunk plus replacing the positive battery cable solved the issue. I found the positive battery cable warm/hot while the car was idling. Even though it looked very good, its crimps had probably developed resistance inside (a fairly common issue, as I have read).
04-15-2020, 09:03 AM
Voltage instability
