DJM2.4 MANUAL
25 Oct 00 R2
at any place in the programmed burner control process is by setting the Fluke multi-meter to AC
volts. Attach the meter to the DJM2 terminals “YS” and “GS”. Press the Hz button once. The
number should read between 15 and 59 Hz depending on the speed of the fan (which would be
3450 RPM or 59Hz if the unit were in pre-purge mode). If the reading is over 59 Hz the sensor may
be grounding out as the motors 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
“Pre-purge Problem” light may be flashing. Check the sensor location and magnet quality.
Occasionally poorly manufactured magnets produce an irregular magnetic field. Sometimes
reversing the magnet on the shaft will correct this problem (turning it around). Also refer to LED 1
for more information.
If the test is done only for voltage and it proves to be correct, there could still be a problem as
the DJM.42 has a refined sensing circuit. This circuit is looking for the magnet to be split fairly
evenly. The duty cycle test gives a reasonable view of the magnets split between north and south
poles.
Speed Sensor Values
With the speed sensor disconnected from the DJM, the following are approximate values read
with a fluke meter at 50ºF.
Pre-Purge Light Coming On (When Not Supposed To)
There can be three reasons for this:
1. A power spike is being picked up through the B, W, and G terminals. Make sure the DC wiring
is not run common with the AC power wiring. If separating the wires does not cure the
problem, shielded wire may have to be used.
2. Problem with the Hall Effect speed sensor.
3. Faulty DJM2.4.
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