Cessna 1980 172RG Cutlass Pilot Operating Handbook page 166

CAUTION: Any failure of the above procedures indicates that a
failure exists in the system and the system shall not be
operated until the failure has been located and corrected.
4-2 TRIM PROMPTER
The autopilot has the feature of sensing and annunciating the
need, amount, and direction of pitch elevator trim when the
autopilot is engaged.
required will appear on the autopilot annunciator in either the
upper or lower right hand corner positions for "TRIM DN" (trim
down) or "TRIM UP" (trim up) respectively. The amount of trim
required at any given moment is indicated by the frequency of
the flashing annunciation.
greater out of elevator trim condition. Both trim annunciators
continuously extinguished is a properly trimmed condition.
1. When the autopilot is engaged and a flashing trim
annunciators is noted, the pilot should manually move the
aircraft's
indicated until the annunciator is extinguished.
2. An aural alert will sound approximately every 4 minutes if a
continuous out of trim condition in the same direction exists
for 4 minutes or longer. If this condition occurs, the pilot
should immediately follow the procedure of step 1.
CAUTION: Prior to autopilot disengagement, the pilot should
verify both trim prompting annunciators are extinguished and
the aircraft control wheel is in hand. In the event that there is
excess nose UP or nose DN Trim Prompting the "G" forces of
this airplane may be exceeded when disengaging the Autopilot.
CENTURY FLIGHT SYSTEMS INC.
FAA/DAS APPROVED DATE: 1-22-93
P/N 68S1063
The need and direction of elevator trim
elevator pitch
A higher frequency indicates a
tri m system in the direction