If the voltage climbs again to the acceptable range when the car is running on the road then don't worry about the drop when idling in drive at a traffic light. As others have said the engine RPM drops with the transmission in drive and the engine idling. It is natural, but if you are truly worried try a smaller diameter pulley on the alternator. The smaller pulley will spin the alternator at a higher speed so it should keep up during the in drive & stopped mode. The other possibility is that your battery is too small and does not have the reserve power available to maintain the voltage drain when the engine is running slow. The drop into the 9 volt range indicates something drawing a high load and the battery not keeping up with it for the short term.
Lots of amperage needed with the fans, stereo, etc. What amperage rating is your alternator. Not enough amps it won't be able to keep up with the draw.
When you had it at the shop, did voltage get tested at the battery when the car was in drive? Is the voltage test the same at the battery as it is on your car volt guage? I am wondering if there is a problem with the car volt gage. At 9 volts the car would be close to shutting down especially when the fans and other elects kick in. I would definatley check all grounds and connections thru the firewall. A heavier wire from the alt post would also help. When the car is put in gear the motor will move a bit. Is it possible a wire is touchong somewhere against an exhaust or a pinch point? If all else checks OK Jamie at 65specialconversion sells a nice cs130 alternator that works well with the extra electrical load of the fans, and the low rpm output is also better.
