Simplified Tests For Troubleshooting Autopilot Servos

Questioning the functionality of the servos is a normal part of troubleshooting autopilot squawks. This often requires removing the servo from the aircraft and substituting another or exchange unit to determine functionality. While substitution or bench-checking is the best solution, in many cases, there are 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. 

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 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.

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

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 the same performance but with different resistance readings.