Testing an autopilot drive unit

[Chuck - Raymarine - Moderator]

Testing an autopilot drive unit

When an autopilot fails to move the rudder, the fault may be rooted in the autopilot course computer, drive unit, or rudder reference transducer.

Move the rudder to the central position and then turn the wheel to Starboard- the rudder angle display on the Autopilot control unit should show the rudder moving to Starboard (green) repeat the test turning the wheel to Port and the display should indicate rudder moving to port (red). if correct, then the rudder reference is operating correctly.
if not, follow these instructions click here to view the FAQ addressing how to test a rudder reference transducer. When using a rudder reference transducer, it is also very important to ensure that rudder limits have been correctly configured to establish angular limits to the amount of rudder travel and to ensure that the rudder angle indicator correctly indicates the position of the rudder with respect to the centered position.

Drive units feature reversible DC motors, inside the motors are carbon brushes, an electromagnetic clutch, epicyclic gearbox and belt drive.

The clutch of a drive unit is tested by placing the autopilot into AUTO and checking the steering wheel becomes 'locked', then selecting STANDBY will release the clutch and the wheel should become free to turn by hand.

A properly functioning drive until will engage the clutch (causes the drive to appear locked) when 12 VDC is present across the Clutch leads. Similarly, a properly functioning drive until will release the clutch (causes the drive to disengage from the steering system) when 12 VDC is not present across the Clutch leads.

Accordingly, a clutch related problem may be fault isolated by measuring voltage across the Clutch terminals of the ACU or course computer. When the autopilot is in Standby mode, the measured voltage across the clutch terminals should be 0 VDC. When the autopilot is in Auto or Track modes, the measured voltage across the clutch terminals should be 12 VDC. Note that even 24v Raymarine drives operate on a 12v clutch, some 3rd party drives use a 24v clutch and so be sure to check the correct voltage required by the drive.

If there is correct voltage present at the clutch terminals on the computer or ACU, but the clutch still fails to operate, check the wiring to the drive unit, if all OK, then connect 12v direct to the clutch wires at the drive unit, if the clutch fails to operate, then the drive is faulty and needs to be serviced.

If there is not the correct voltage present at the clutch terminals on the computer or ACU, then the computer or ACU is faulty and needs to be serviced.

Should a drive unit's clutch not be releasing and the measured voltages across the Clutch terminals of the ACU or course computer be as specified above, then the autopilot drive unit should be replaced or sent to Raymarine’s Product Repair Center to be bench checked / serviced. However, a drive unit's clutch not be releasing and the measured voltages across the Clutch terminals of the ACU or course computer not be as specified above, then the ACU or course computer should be replaced (if not longer serviceable) or sent to Raymarine’s Product Repair Center to be bench checked / serviced.

Should rudder reference transducer (if so equipped) be operating properly, then failures related to rudder movement may be fault isolated to the drive or autopilot course computer.
This can be tested by the drive unit's ability to move the rudder in each direction when driven by a source of power other than the autopilot course computer. To do so, center the rudder, select AUTO to energise the clutch and then disconnect the drive unit's motor leads from the autopilot course computer's / ACU's MOTOR terminals and instead momentarily connect them to the course computer's / ACU's power terminals.
This connection should be momentary in nature and only long enough to verify that the drive is able to move the rudder. After verifying movement of the rudder in one direction, center the rudder again, reverse the motor wires polarity on the computer's/ ACU's power terminals and verify the drive moves the rudder in the opposite direction.

Should the drive unit move the rudder in both directions, then the fault has been isolated to the autopilot's course computer, necessitating that it be serviced or replaced. Should the drive unit move the not rudder in both directions, then the fault has been isolated to the drive unit, necessitating that it be serviced or replaced.

Should the operation of the motor be erratic, sometimes moving the rudder and sometimes not, this could be sticking or worn brushes inside the motor, which will need to be replaced through servicing.

RAYFAQ#