charging system

[JBEnt2@aol.com]

I have a 79 MGB that is at the local garage. The mechanic (after replacing
the regulator and a bearing) has done some testing on the alternator. He
reads 12.8 volts @ 2500 RPM (he says that it should be 13.4) and 24 amps
(should be 34). The problem is the ignition light on the dash stays on mostly
dim. He checked diodes and says that they are all right. He said that the
alternator  is charging the battery enough to keep it from discharging.

Question:  What is causing the low read outs?