I recently ran across a problem with a 2000 cougar that took me back to school. I got my ass kicked on it, but it made me a bit wiser, so i'd tought i would share the knowledge.
Customer came in complaining that the battery went dead on him. I did a battery test, it failed. I put it on a charger, and threw in a test battery. Started the car, and the battery wasn't being charged. Took my DVOM (Digital Volt/Ohm Meter) and tested the POS wire at the Alt. Only 11.2v. Ok, so it's a bad Alt. Sold the customer a Alt install, as his was under warranty from the parts store. The install was a pain in the ass, as we all know, but i got it done, and re-checked my work. Battery was still not charging, alt was still puitting out 11.2v.
Time to go to out technician's web site (direct hit.com) and look up similar problems.
As it turns out, there is a 175 Amp fuse that runs from the POS Red wire from the battery, to the Starter, then behind/above the rear exhaust manifold. It's hidden back there, and the only way you'll know it's there is if someone TOLD you it was there. It's covered, so you can't see it. I took my DVOM, set it to CONTINUNITY, and put one lead on the battery POS side. Tested at the starter wire, and it was good. Tested to the fuse, and it was good on one side, but i lost continunity on the other. The fuse had blown for some reason.
I removed the complete wire, from the battery, starter, and the Alt. What had happed was whoever did the previous install of the alt did a shotty job of putting the POS wire back together. It was routed so that it was right up against the EGR tube. Over time, the EGR tube had gotten so hot, it had melted the protective sleve of the POS wire, causing it to arc on the EGR tube, causing the 175 amp fuse to blow. It had been doing this for some time, and had arced a hole in the EGR tube.
I replaced the complete wiring harness, and it included the 175 amp fuse and new holder/cover. Installed it, re-tested, and all is well.
I've seen a lot of problems here concerning this, and haven't ran across someone actually posting a fix, or a way to accuratly diag that it is indeed the ALT that is bad.
Here is a pic of the fuse. Only way to tell if it's bad is to actually take it out, or test with a test light, continunity tester.
I hope this helps some of you out.
Got any questions, just let me know.
Mark