[ICOM] Pro III ALC in SSB-D mode

[NZ8J]

I have a digital interface connected to ACC (1) and it seems to be working
fine, however on page 54 of the ProIII manual it says:

 "When operating in the SSB data mode adjust the PC output so that the ALC
meter doesn't go outside the ALC zone"

The ALC meter does not seem to function in the SSB-D mode, am I doing
something wrong, or is there a problem with the radio?  I can adjust the
output of the sound card to vary the power output of the radio, but the ALC
meter does not move...

Thanks in advance for any info
73
Tim
NZ8J