DJM2.4 MANUAL
9 Oct 00 R2
IRREGULAR (2 short ON flashes followed by a long off)
LED 1 (Pre-Purge)
ON The 50 second pre-purge timer is active.
OFF The heat exchanger ignition pre-purge is completed or there is no call for
heat.
SLOW FLASH There is trouble with the combustion air blower tachometer. The DJM2.4
is receiving a tachometer signal that is not consistent with the
combustion blower status. This can occur with a blown combustion fan
fuse or failed combustion fan motor as well as a faulty Hall Effect speed
sensor or a faulty magnet.
If the combustion blower is not running, first check the Hall Effect speed
sensor ensuring it has power and can respond to the magnet as described
under “Checking Speed Sensor” on page 24. Next remove all three wires
to terminals "OS, YS, and GS". If the motor starts, replace the speed
sensor. If the combustion blower fails to start, check the combustion
motor. Reconnect the speed sensor, fuse, and power.
If the combustion blower is running the tachometer signal is missing,
weak, or grounded. With a digital AC voltmeter, measure the AC volts
present on terminals "YS" to "GS". 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 okay, replace the speed sensor. Further
information found in section “Checking Speed Sensor” located on page
24. If the voltage is constantly 10 VDC and the sensor is located near the
magnet then the sensor if faulty (always “on”).
NOTE: No part of the speed sensor’s sensing element should be located over the
metal blower motor shaft. (See diagram section XV.)
If about 5 VAC voltage is present on YS and GS, note that the DJM2.4 is
sensitive to the shape of the waveform produced by the Hall Effect speed
sensor. A sensor with leakage to ground will provide a distorted
waveform, which will cause the DJM2.4 to reject the signal.
Sometimes a bad magnet or Hall Effect speed sensor will produce an
uneven waveform. Turning the magnet around and/or replacing the
speed sensor will correct this problem.
Refer to "Checking the Speed Sensor" on page 24 for further information.
LED 2 (Heat Call)
OFF There is no heat call.
ON There is a valid heat call and the system is allowed to function normally if
all systems downstream are operating correctly.
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