Troubleshooting Autopilot Servos

During the normal course of troubleshooting an autopilot issue, the process often comes around to questioning the functionality of the autopilot servos. Often this can require removal from the aircraft and substituting another servo or exchange unit to prove the functionality of the unit. While substitution or bench checking the suspect servo is the best solution, there are, in many cases, some simple in-aircraft measurements that may aid in the troubleshooting process.

These are not manufacturer mandated or approved tests, but procedures we have utilized over the years for troubleshooting.

Most servos are simple mechanical structures consisting of a motor, a tachometer and an electronic clutch or solenoid. While not proof of serviceability, an ohmmeter can be utilized to perform passive measurements of these mechanisms as a go/no-go test to determine issues with them. 

Because the different model servos usually have differing components and different readings, I have provided a chart.

Tachometer resistance measurements:

MFG                            M/N                             Approximate Resistance Reading 
Collins                         334C-6()                      40 ohms
Collins                         SVO-80()                      50 ohms *
Collins                         SVO-85()                      75 ohms
Honeywell                   SM-200                        20 ohms or 120 ohms ^         

Motor resistance measurements:

MFG                            M/N                             Approximate Resistance Reading
Collins                         334C-6()                      10 ohms
Collins                         SVO-80()                      10 ohms
Collins                         SVO-85()                      8 ohms
Honeywell                   SM-200                        25 ohms or 150 ohms ^         

Synchro resistance measurements:

MFG                            M/N                             Approximate Resistance reading
Honeywell                   SM-200                        4.5 ohms each leg to leg X-Y-Z

Clutch solenoid measurements:

MFG                            M/N                             Approximate Resistance reading
Collins                         334C-6()                      40 ohms
Collins                         SVO-80()                      40 ohms
Collins                         SVO-85()                      32 ohms
Honeywell                   SM-200                        45 ohms or 55 ohms ^

*    Across pins D and E
^    MFG supplied two different parts with same performance, but with different resistance readings.

Measurements are made by accessing the servo in the aircraft. Unplug the aircraft wiring from the servo and measure directly across the servo pins for the motor, tachometer or clutch of interest. Use the ships wiring diagrams to determine the correct pins. Values within approximately 10% should be considered good. If tapping on the servo while taking the reading results in a large resistance change, it should be considered defective.