Cambridge Engineering, Inc.
70
M-Series Technical Manual
DAMPER MOTOR REPLACEMENT & ADJUSTMENT
1. Before re-installing the damper motor into the heater
assembly, remove the black plastic weather cover
from the damper motor by removing the screw that
secures the cover to the damper assembly using either
a flat tip screw driver or a 1/4" nut driver. It may be
necessary to twist the cover slightly to disengage it
from its mounting to the end of the motor after the
screw is removed.
2. Verify the damper motor is in the closed position
(Drive tube is retracted with a minimum of 1/4" gap
between drive tube and body of the damper assem-
bly).
3. Verify the inner cam has fully engaged the lower limit
switch plunger and the lower limit switch (next to the
body of the damper motor) shows continuity between
the common (C) terminal and the normally open (NO)
terminal.
CAUTION:
If the lower limit switch does not indicate continuity,
the damper motor must be driven to the fully closed
position before connecting the damper linkage to the
drive tube of the damper assembly.
CAUTION:
If the drive tube bottoms out against the damper
motor housing, damage to the PC board or trip-
ping of the onboard fuse may result. If the fuse
trips, place the blower service switch in the “OFF”
position for 15 seconds to allow the fuse to reset.
Proceed with steps 4 through 10 BEFORE connect-
ing the damper motor to the linkage arm.
4. Verify the cam location on the upper cam is set just
short of the four o’clock position with respect to the
switch plunger being the twelve o’clock reference
position. If it is beyond the four o’clock position, use
a deep well 1/2" hex socket or nut driver to manu-
ally rotate the cam counterclockwise a few clicks, as
required, to obtain the correct starting position (see
Figure 1).
Figure 1
5. Verify the heater disconnect switch is turned off and
the Service Switches are in the “OFF” position.
6. Connect wires to terminals as shown in Figure 2 and
re-install the weather cover.
Figure 2
7. Remove the push-in plug (located in the weather
cover) to have access to the upper cam adjusting nut.
8. Connect the motor pivot to the damper motor support
bracket using the 3/8" diameter clevis pin. Do not
connect the damper linkage at this time.
