Service manual

A DJM3.2 MANUAL
If the combustion blower IS RUNNING and the tachometer signal is absent,
weak, or grounded. With a digital AC voltmeter, measure the AC volts
present on terminals “YS to G”. When the combustion blower is running
there should be 4 to 6 VAC present. If the AC voltage IS NOT PRESENT,
check the tachometer sensor to magnet gap. It should be 1/16 to 1/8 inch. If
the gap is OK, replace the speed sensor.
NOTE: No part of the speed sensor’s sensing element should be located over the end of
blower motor shaft.
If AC voltage is present, note the DJM3.2 is sensitive to the shape of the
waveform produced by the speed sensor. A sensor with leakage to ground
will provide a distorted waveform, which will cause the DJM3.2 to reject the
signal. Also a bad magnet or speed sensor will produce an uneven
waveform. Try flipping the magnet over, then flipping the speed sensor over,
before replacing the speed sensor to correct this problem.
If the supply fan were running when a speed sensor problem occurred which
activated the speed sensor light, the heat would go off. If the discharge
temperature then falls below 40ºF the L/L failure light would come on, the
supply fan turn off, and the speed sensor failure status light will be lost.
ON The pre-purge timer is active.
LED 3
OFF There is no heat call and/or the fan switch is open.
SLOW FLASH There is a heat call and the fan switch and/or night contact is made, but the
heat switch is open. If the discharge temperature is below 40ºF, the S/A fan
can't run.
IRREGULAR FLASH Dipswitch 5 is in the service position and the fan switch “FS” is on.
ON Valid heat call present, the system should operate normally.
LED 4
OFF The flame relay or ignition control device is not energized.
FAST FLASH Flame failure. The DJM3.2 energized the flame relay, after 90 seconds there
wasn’t a 24 V feedback voltage (to terminal 3) proving pilot flame was
established and the main valve energized. If the flame failure occurred
before the S/A fan started, then this failure light would hold its status. If the
S/A fan is on and the discharge temperature falls below 40ºF, the "L/L" light
will come on, the L/L alarm contacts close, and F/F light goes off.
NOTE: On some units, if the externally wired HIGH LIMIT control is wired to interrupt the
power to the Flame Ignition device without interrupting the power to terminal HS,
the fast flash code may indicate that the high limit contacts opened.
The DJM3.2 flame failure code/function is disabled if the service switch is on.
ON The flame relay or ignition control device is energized. The ignition control
device now has the responsibility to light the pilot flame, prove the pilot
flame, and then energize the main valve.
X. AUTO BYPASS LOW LIMIT
NOTE: Low limit can be disabled (dipswitch 8). Refer to Section XXIII re service
information.
The low limit set point is fixed at 40ºF. There are two low limit bypass timers, start-up and anti-noise.
The 5 minute start-up auto bypass low limit timer is started every time the night terminal energized
Page 7 Revised: 3/11/99