Tuga,

They must be 8d's since there is only two of them and since you have a series 60, I'm sure they're hooked in series. Like Jon said, you have to remove enough cable to isolate each battery. Also, is there a chance you got your meter hooked up bass ackwards, the bad battery usually drags down the good one. Check the voltage at the equalizer on the 12 and 24 lug.