Hi I have a 2001 4.o s type. I have a new battery 2 weeks old or so.I was driving home a few days ago and the battery/charging light came on.I tried to get home but before I made it the lights went out and the engine puttered out. I believed this to be an alternator, so I changed that, got it all buttoned up started it and the volt meter read 11.9 volts. so i charged it, removed it from the charger overnight and these are my test numbers. 12.9 volts to start, then after starting the car and turning it off it read 12.5 volts, starting the car and leaving it running it read 12.05 volts and after shutting everything down reads 12.5 again. now this looks like an alternator to me. before I tear someone up for shipping me a defective part what else should I test, o and I plan on bench testing the alternator I removed tommorrow. any Ideas???

 

I think you are doing the correct thing. I have also had rebuilt alternator problems. I would also make sure my battery connections were rock solid. No corrosion, clean and tight.

Please post back if you get it fixed.
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Minimum voltage to charge is 13.6V I would, as you say expect Alternator not working correctly, I found this page very useful for info
Charging System Basics

 

has anyone heard of an alternator suppression module? someone showed me that the xj8 of that year had one, and said that s type did too. If it does does anyone know if it is a lincoln Ls compatible part and if there is an easy test if it is functioning properly?

 

with the new battery we cleaned the hell outta the terminals, the old battery case had a crack around the positive terminal, got all gunked up.but cleaned it up real nice and all the copper feels solid.

 

faulty alternator confirmed.

 

Thanks for coming back and letting us know.

 

ok I am back, and it turns out the bench tester that i checked my old alternator on was faulty, on another bench tester it tested good, after the replacement failed. so 100%now a good alternator in the car, and good battery double checked under load.I am still not charging.alternator suppressor maybe? whats up here?