The following is a theory of operation of the pitch/pitch-trim system. The attached drawing will help, and it is advised to opening the drawings while reading through the paragraphs below. The lines highlighted in blue are grounds, and the lines highlighted in yellow are bus voltage (28V).
Pitch Down Trim Relay De-Energized
The relay is drawn in the de-energized state. The ground coming in on pin 87a passes through the closed contacts of the relay, and through pin 30. This ground is then sensed at DFC90 P2 pin 15 which the DFC90 uses as a sort of discrete input to determine the trim down is inactive. Pitch Down Trim Relay Pin 30 also passes this ground to the Pitch Autopilot Relay Pin 1 which travels through the closed contact and out pin 5, which travels through the normally closed contacts of the Pitch Trim Motor Down Limit Switch and to the Trim Motor.
Bus voltage from the Pitch Trim Circuit Breaker is fed to the Pitch Down Trim Relay at pins 87, and 86. Pin 87 is the normally open contact of the relay. Pin 86 feeds the cathode side of the diode and coil.
Pitch Up Trim Relay De-Energized
The relay is also drawn in the de-energized state. The ground coming in on pin 87a passes through the closed contacts of the relay, and through pin 30. This ground is then sensed at DFC90 P2 pin 16 which the DFC90 uses as a sort of discrete input to determine the trim up is inactive. Pitch Up Trim Relay Pin 30 also passes this ground to the Pitch Autopilot Relay Pin 2 which travels through the closed contact and out pin 6, which travels through the normally closed contacts of the Pitch Trim Motor Up Limit Switch and to the trim Motor.
Bus voltage from the Pitch Trim Circuit Breaker is fed to the Pitch Down Trim Relay at pins 87, and 86. Pin 87 is the normally open contact of the relay. Pin 86 feeds the cathode side of the diode and coil.
Pitch Down Trim Relay Energized
When the yoke mounted pitch trim down switch is closed, the switch provides a ground to the anode side of the diode and coil. This allows current to flow through the windings, which generates a magnetic field which switches open the contact between pin 87a and pin 30, and closes the contact between pin 87 and pin 30.
Pin 30 then passes bus voltage through to DFC90 P2 pin 15 which tells the DFC90 that the manual electric trim is being commanded down. If the DFC90 is engaged, this action will cause it to disengage. Pin 30 also feeds bus voltage to the Pitch Autopilot Relay Pin 1 which travels through the closed contact and out pin 5 which travels through the normally closed contacts of the Pitch Trim Motor Down Limit Switch and to the Trim Motor.
Since the Pitch Up Trim Relay is still de-energized, it is still providing a ground through the Pitch Trim Motor Up Limit Switch to the other side of the Trim Motor which drives the trim in the down direction.
Pitch Up Trim Relay Energized
When the yoke mounted pitch trim Up switch is closed, the switch provides a ground to the anode side of the diode and coil. This allows current to flow through the windings, which generates a magnetic field which switches open the contact between pin 87a and pin 30, and closes the contact between pin 87 and pin 30.
Pin 30 then passes bus voltage through to DFC90 P2 pin 16 which tells the DFC90 that the manual electric trim is being commanded up. If the DFC90 is engaged, this action will cause it to disengage. Pin 30 also feeds bus voltage to the Pitch Autopilot Relay Pin 2 which travels through the closed contact and out pin 5 which travels through the normally closed contacts of the Pitch Trim Motor Up Limit Switch and to the Trim Motor.
Since the Pitch Down Trim Relay is still de-energized, it is still providing a ground through the Pitch Trim Motor Down Limit Switch to the other side of the Trim Motor which drives the trim in the up direction.
Pitch Autopilot Relay De-Energized
When De-Energized, the Pitch Autopilot Relay passes the commanded trim (ground, or bus voltage) from the Pitch Down Trim Relay coming in on pin 1, through the normally closed contact to pin 5, through the normally closed contacts on the Pitch Trim Motor Down Limit Switch, to the Trim Down side of the Trim Motor.
The Pitch Autopilot Relay also passes the commanded trim (ground, or bus voltage) from the Pitch Up Trim Relay coming in on pin 2, through the normally closed contact to pin 6, through the normally closed contacts on the Pitch Trim Motor Up Limit Switch, to the Trim Up side of the Trim Motor.
Pitch Autopilot Relay Energized
When any mode is engaged on the DFC90, the DFC90 sends out bus voltage via P1 pin 20 to the Pitch Servo Solenoid Pin 1, engaging the Pitch Servo. This output is also paralleled to the Pitch Autopilot Relay pin 7, which feeds the cathode side of the diode and coil. This allows current to flow through the windings of the coil, generating a magnetic field which opens the normally closed connections between pins 1,5 and 2,6 respectively. And closes the normally open contacts of pins 3,5 and 4,6 respectively.
At this point, the DFC90 is commanding the trim motor up/down via P2 pins 32 and 17. The commands are fed through the now closed contacts of the Pitch Autopilot Relay pins 3,5 out through the normally closed contacts of the Pitch Trim Motor Down Limit Switch, to the Down side of the Pitch Trim Motor; and pins 4,6 out through the normally closed contacts of the Pitch Trim Motor Up Limit Switch to the Up side of the Pitch Trim Motor.
Pitch Servo Engaged
When the Pitch Servo is engaged via the Pitch Solenoid output from DFC90 P1 pin 20, all Pitch commands from the DFC90 are driven directly to the Pitch Servo via DFC90 P1 pins 21 and 23, to the Pitch Servo pins 4 and 3 respectively.
At this point, the DFC90 is NOT feeding any trim commands to the Pitch Trim Servo.
The Pitch Servo has internal switches that you can think of as torque switches. Once a large enough force has been applied to the Pitch Servo to cause one of these "torque switches" to close, the switch sends a discrete signal out of the servo at pin 6 or 8, and back to the DFC90 P2 pins 4 or 22.
When the DFC90 receives either of these discrete inputs, it then commands the Trim Motor in accordance with the description above for "Pitch Autopilot Relay Engaged," Driving the trim motor in the direction determined by the discrete input (Trim up, or trim down).
Essentially, this retrims the aircraft to a point where the excessive force is removed from the Pitch Servo, at which point the "torque" switch opens, removing the discrete input to the DFC90, causing the DFC90 to cease commanding the Pitch Trim Motor in either direction.
Closing either Manual Electric Trim switch on the yoke will result in the DFC90 disconnecting, which will de-energize the Pitch Autopilot Relay, allowing the Pitch Trim Motor to be driven from via the Pitch Down, and Pitch Up Trim Relays once again.