Alt vs ECU/pcm

[blackwaterfrog]

OK,sorry, i guess I posted this in the wrong section earlier. Here we go again.
My 97 has a charging issue. The 6 year old optima went bad (i think) so I replace it and also replaced the battery tray so I could go with a 34 group battery. Drove it around, everything seems great. Next day…..totally dead.Voltmeter says 13.3 v engine running….hmmm. So I charge the battery and drive on over to the local O'reilly where I had them test the battery and alternator. Test says…battery good/ low charge, alt good. So i figure maybe it's the battery temp sensor, so I remove the battery and tray and low and behold the BTS wire had gotten under the tray and smashed a bit. I buy a new one and install and re-install the fully charged battery,put the VM on and read 13.9v at the battery engine off, 13.9 engine running…turn on all accessories…13.2v….damn. I also checked for AC current, .02 or so. It should be putting out more than 13.2 with accessories on? This model is voltage regulated at the computer. There are no other electrical symptoms other than seemingly low alt output. Belt tight,no squeals,good connections,no corrosion,no broken wire that I can see. Anyone know how to determine if the problem lies in the computer?
Thanks in advance.

[4.3LXJ]

When they checked the alternator, did they check just voltage, or did they check amperage also? It is possible for an alternator to have voltage and little or no amperage. Had it happen several times

[blackwaterfrog]

I assume so. They used a device that puts a load on the alt. He did say that it had a "big ripple" meaning variation in voltage.

[4.3LXJ]

OK, I am going to assume then that your alternator is in one of those marginal states. I think your problems would be solved by replacing it

[autotech98]

I have to go with Steve on this one I had roughly the same problem and it ended up being the alternator

[blackwaterfrog]

OK. Here is the final prognosis. The battery was bad, replaced it with a new battery from Oreilly. The new battery turned out to be bad as well. The alternator was on its way out as the battery was failing. I removed the alternator and took to an alt shop who replaced the brushes for $40. Ended up taking the XJ to a master mechanic to do a parasitic drain test. Result=no drain. He also tested the new new battery and the alternator. The mechanic discovered the bad (new) battery with a load test.
What have I learned?
Do not rely on the testers that auto parts stores use. They will give false positives and false negatives.
Alternators cannot be tested properly on the car.
When buying a battery inspect it carefully for swelling. A swollen (bubbles or bows in the case) battery is on the way out.
Do not assume that a "new" battery is any good.