Alternator and Charging System problem

[p1michaud]

Hi folks,
This evening my girlfriend informed me that the battery light came on after she started the truck on 2 occasions and it remained on for approximately 5 minutes or so. When I got home I dressed up since it's 5F here, put the head light on, grabbed the multi meter and headed out the door to the apartment parking lot. I can just imagine what the neighbors were thinking.

Anyhow here is some background information. I have a dual battery system and am using a Sure Power (model 9523A). The main battery is an Optima Red Top 34/78 and the Aux battery is an Exide Orbital Marine Deep Cycle. Click HERE for more details on this system. The isolator has 4 terminals, #1 for the main battery, #2 for the aux battery, #A for the Alternator and #E for the exitation terminal.

I did a few tests as follows...
Test #1
With the truck off I measured the battery voltage. Main = 12.82 V and the Aux. = 12.58 V.

Test #2
With the truck started and runing (battery light on). The truck was warm and I did not rev it up (RPMs approx 750). I took the readings again, Main = 12.14 V and the Aux. = 12.58 V. The Alternator output was 0.61 V (reading taken at the #A terminal on the Isolator)

Test #3
With the truck still runing (battery light still on). I reached over and reved the engine while watching the multi meter. As soon as the engin reved up the readings were, Main = 14.25 V and the Aux. = 14.38 V. The Alternator output was 15.27 V (reading taken at the #A terminal on the Isolator again). This seems normal as per SurePower's documented 0.8 to 1 V dropp.

This is leading me to believe that my alternator is on it's last legs. Do you agree, or should I check something else before rebuilding or replacing the alternator?
Cheers and sorry for the long post
P

I've also posted this on TTORA HERE