After letting my 1987 LB-15 sit idle for a week or so, we needed to start it for a run. It boots up like normal and the y axis moves fine...but when you try to jog the z axis an alarm pops up.

100-4 Alarm-A DIF
over z-axis 38B98

We looked at the servo and screw for the Z. Powered down, you can move the screw by hand easily both ways. When you try to jog, it does not move the servo at all. I looked at the drive card. Before you put the machine into fault (alarm) mode, the x axis has a green light on but the z does not. When it goes into fault, there is a yellow light on y axis (card) and again no lights on the z axis.

Any thoughts?