Service manual

A DJM3.2 MANUAL
likely find the supply fan off and the combustion fan off when you arrive at site. Note the condition of
all of the lights. If you now turn the DJM3.2 power off for 30 seconds and then turn it back on, watch
the lights as the ignition process retries. If an ignition or sensor problem exists it should be evident
within a few minutes.
If the combustion fan is seized, or the Hall Effect Speed Sensor is faulty, etc., the DJM3 will display
pre-purge lockout until the discharge sensor cools to below 40ºF. When the low limit bypass timer -
that has now started to time - completes its timing, the low limit light alarm will be displayed and the
pre-purge problem alarm light will turn off.
Two Speed VAV Applications or High Limit Tripping
The DJM3.2 likes to see stable operation. If the set-point is changed rapidly (lowered), the DJM3.2
may take up to 3 minutes to fully respond. The same conditions apply to a load that is suddenly
reduced. (Vanes closing or low speed operation.) Under these conditions the discharge temperature
may rise. If a problem is encountered with the high limit tripping, consult the factory.
Checking Speed Sensor with the Fluke 80 Series Meter
The DJM3.2 has a speed
sensor to allow the control to
closely monitor the combustion
fan speed. There is a fixed 10
VDC output from the DJM3.2
terminals “G” and “O” to the
speed sensor chip. The chip
allows a return signal to be
measured on the “YS” and “G”
terminals. If the fan is rotating
above 500 RPM, the signal can
be read as either an AC or DC
voltage and it will be about 5
volts. This is because the fan is
turning at a high rate of speed
and your meter is averaging the
reading. If the DJM3 has 24 volt
power to it, and the combustion
fan motor has been
disconnected from its power
supply, turning the combustion
fan slowly by hand will result in
a reading of 0, then 10 VDC.
Repeat as the north and south
poles of the magnet pass by the
Hall Effect chip. If the pre-purge
problem light is flashing the
above steps should be checked.
If the DJM3.2 indicates a pre-purge problem the problem can be analysed more completely using
one of the Fluke 80 series meters. The DJM must see a near symmetrical square wave coming from
the speed sensor or it will reject the sensor signal. This wave is formed as the magnet spins on the
shaft with the north and south poles passing a Hall Effect speed sensor. This speed sensor is a very
small “chip” located under the shrink wrap at the end of the copper tube. A simple method for
estimating the waveform when the unit is running is by setting the Fluke multimeter to AC volts.
Attach the meter to the DJM3.2 terminals “YS” and “G”. Press the Hz button once. The number
should read between 15 and 59 Hz depending on the speed of the fan. If the reading is over 59 Hz
the sensor may be grounding out as the motor’s maximum RPM should be 3450 RPM. Press the Hz
button a second time and the meter is reading % duty cycle. This number indicates how symmetrical
the waveform is, and should read close to 50% ±2.5%. If the number is outside those parameters the
Hall Effect
Sensor
Magnet Retainer
Ring
Magnet
1/8” Gap
Bl
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Page 36 Revised: 3/11/99