Technician's Bench Troubleshooting Autopilot Engagement Problems by Scott Howard, Tech Rep Upon occasion, S-TEC Autopilot Systems may experience some type of engagement difficulty or problem. These problems, while a brief nuisance, may be attributed to the RPMs of the rate gyro motor or an absence of voltage to the AP disconnect switch, but whatever the reason, engagement problems can be relatively easy to troubleshoot and isolate. Gyro Tach Mode engagement of all S-TEC autopilots in current production requires an illuminated "RDY" indication on the controller and/or annunciator. This "ready to engage" condition is enabled by a valid tach signal from the turn coordinator and becomes present when the rate gyro motor in the coordinator achieves 80 percent of its full operational (RPM). Typical tach values are Valid = 7 to 10 VDC and Not Valid = 1 to 2 VDC. If the rate gyro fails to achieve the necessary 80 percent of its full operational RPM, the autopilot will not engage in any modes. Some systems (Thirty, 50, and 55) will also require a successful self-test completion in conjunction with valid tach to produce the "RDY" light. The tach output of model 6405-() turn coordinators can be easily tested on the bench as follows: Apply rated power (14/28 VDC) to pin A. Apply ground to pin B. Allow unit to run at least 90 seconds. Read tach voltage on pin E. Valid = 7 to 10 VDC After mode engagement, any momentary interruption of the tach signal will cause the autopilot to disconnect, and a continuous interruption will prevent the re-illumination of the "RDY" light. Autopilot Disconnect Switch S-TEC Autopilots all have provisions for an AP disconnect switch. In most systems, this is a normally closed switch supplying required voltage to enable mode engagement. See Figure 1 for pin contacts on different systems. An absence of this voltage will cause the system to remain in a "RDY" condition and prevent autopilot engagement. Momentary interruption of the voltage will cause the autopilot to disconnect. Other Engagement Problems All S-TEC systems require that the "RDY" light be illuminated for the autopilot to engage in a mode. If a system fails to produce a "RDY" light: for all systems, verify power and ground to autopilot, verify presence of valid gyro tach at appropriate prog/annun pin, for System 60/65 verify presence of regulated 10VDC on pin 11 of annunciator, 50-pin connector, and on System Thirty, 50, and 55, insure that unit goes thru self-test sequence. Some Systems (Thirty, 50, and 55) also require a successful self-test completion in conjunction with a valid tach to produce a "RDY" light. Here is a check list for Systems that do not pass the self-test: System Thirty - Disconnect remote pitch computer and try again. This will isolate the problem to roll or pitch unit. System 50 - Pitch servo must be connected for normal self-test. If trim lights do not illuminate at all during test, this indicates open 10v-regulator fuse. System 55 - Controller will indicate "FAIL" if does not pass self-test. Check for regulated 10 VDC at transducer pin 3. If the "RDY" is present but the autopilot does not engage modes, check for presence of disc A+ at appropriate pin (see Figure 1). If the autopilot is disconnecting intermittently, check all power, ground and signal connections in Figure 1 for secure, positive contact. Momentary open contact at any point can cause the autopilot to disconnect. Finally, if an autopilot will not engage, these few tips can help you pin down where the problem is located.

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