Help with failed speedo??

[Mikrodeere]

Hi guys. First time poster here needing help. Just bought a 91 CTD for my step son and the speedo quit coming home. The odometer still works but the speedo needle stays at zero. The drive unit on the side of the 5 speed seems OK. Took it out and the gears are seem fine. Assuming since the odometer still rolls the miles up that the signal is getting to the speedo unit. What is inside the unit that makes the needle move? Thanks in advance!!