1. No vacuum pressure or insufficient pressure 2. Excessive vacuum 3. Gyro horizon bar fails to respond 4. Turn-and-bank indicator fails to respond 5. Turn-and-bank pointer vibrates  Problem and Potential Causes  Isolation Procedure  Correction  Defective vacuum gauge  Check opposite engine system on the gauge  Replace faulty vacuum gauge Vacuum relief valve incorrectly adjusted Change valve adjustment  Make final adjustment to correct setting Vacuum relief valve installed backward  Visually inspect  Install lines properly Broken lines  Visually inspect  Replace line Lines crossed  Visually inspect  Install lines properly Obstruction in vacuum line  Check for collapsed line  Clean & test line; replace defective part(s) Vacuum pump failure  Remove and inspect  Replace faulty pump Vacuum regulator valve incorrectly adjusted  Make valve adjustment and note pressure  Adjust to proper pressure Vacuum relief valve dirty  Clean and adjust relief valve  Replace valve if adjustment fails Relief valve improperly adjusted Adjust relief valve to proper setting Inaccurate vacuum gauge  Check calibration of gauge  Replace faulty gauge Instrument caged  Visually inspect  Uncage instrument Instrument filter dirty  Check filter  Replace or clean as necessary Insufficient vacuum  Check vacuum setting  Adjust relief valve to proper setting Instrument assembly worn or dirty  Replace instrument No vacuum supplied to instrument  Check lines and vacuum system  Clean and replace lines and components Instrument filter clogged  Visually inspect  Replace filter Defective instrument  Test with properly functioning instrument  Replace faulty instrument Defective instrument  Test with properly functioning instrument Replace defective instrument Figure 10-141.  Vacuum system troubleshooting guide.  of an autopilot component, or whenever a malfunction in the  autopilot is suspected. After the aircraft’s main power switch has been turned on,  allow the gyros to come up to speed and the amplifier to  warm up before engaging the autopilot. Some systems are  designed with safeguards that prevent premature autopilot  engagement. While holding the control column in the normal  flight position, engage the autopilot system using the switch  on the autopilot controller. After the system is engaged, perform the operational checks  specified for the particular aircraft. In general, the checks  are as follows: Electronic and digital instrument systems utilizing LCD  technology may have special considerations for the care of  the display screens. Antireflective coatings are sometimes  used to reduce glare and make the displays more visible.  These treatments can be degraded by human skin oils and  When performing maintenance and operational checks on  a specific autopilot system, always follow the procedure  recommended by the aircraft or equipment manufacturer. Check to see if it is possible to manually override or  overpower the autopilot system in all control positions.  Center all the controls when the operational checks have  been completed. Disengage the autopilot system and check  for freedom of the control surfaces by moving the control  columns and rudder pedals. Then, reengage the system and  check the emergency disconnect release circuit. The autopilot  should disengage each time the release button on the control  yoke is actuated. If the aircraft has a pitch-trim system installed, it should  function to add down-trim as the control column moves  forward and add up-trim as the column moves aft. Many  pitch-trim systems have an automatic and a manual  mode of operation. The above action occurs only in the  automatic mode. 10-87 LCD Display Screens 4.  Rotate the pitch-trim knob aft; the control column  should move aft. 3.  Rotate the pitch-trim knob forward; the control column  should move forward. 2.  Rotate the turn knob to the right; the right rudder pedal  should move forward, and the control column wheel  should move to the right and slightly aft. Return the  turn knob to the center position; the flight controls  should return to the level-flight position. 1.  Rotate the turn knob to the left; the left rudder pedal  should move forward, and the control column wheel  should move to the left and slightly aft.