Service manual

A DJM3.2 MANUAL
“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 2 for
more info.
If the test is done only for voltage and it proves to be correct, there could still be a problem as the
DJM3 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.
Scale @ Megohms Scale on Diode
Positive Lead
(Red)
Neutral Lead
(Black)
Value (Megohms)
Neutral Lead
(Black)
Value
Red
Yellow
Green
Infinite
25.2
Yellow
Green
Infinite
1.6
Yellow
Red
Green
Infinite
Infinite
Red
Green
Infinite
Infinite
Green
Red
Yellow
Infinite
4.6
Red
Yellow
Infinite
0.652
Pre-purge Light Coming On (when not supposed to)
There can be two reasons for this:
1. A power spike is being picked up through the “O, YS” or, “G” terminals. Make sure the DC wiring
is not run common with the AC power wiring. If separating the wires doesn’t cure the problem,
shielded wire may have to be used.
2. A problem with the DJM3.2.
Constant Purge Light, No Ignition Attempt
This can only apply to units that have a relay connected to the main gas valve connection of the
ignition control. One set of the relay contacts control power to the gas valve, the other set controls
power to the DJM3.2. If the heat call light and the pre-purge light are both staying on and there is no
attempt to slow the combustion fan down to ignition speed, check if DJM3.2 terminal 3 has constant
power on it. If the previous heat call was completed and terminal 3 did not have the signal removed
because the relay contacts are stuck closed, it can result in the lockup as described.
Failed Operation or Solid State Control Contacts by Others
There have been a number of new controls introduced into the marketplace recently that allow a
building to be controlled by a central computer (BMS or Building Management System). Many of these
devices use relays to switch external power devices on and off and their operation is fine. However,
some of the new devices use “opto-couplers” or “solid state contacts”. These devices will not
interface properly with the DJM3.2 contacts “FS, HS, K, or terminal 3”. The solid state contacts allow
enough “voltage leak” through them when they are in an off (open) position to cause the DJM3.2 to
malfunction. Voltage leaks of 4 to 5 VAC are common. Only dry contacts should be tied to the
DJM3.2 terminals unless instructed to do otherwise.
S/A Fan Off Before Room Satisfied in Night Heat Mode
Problem is on DJM3.2 only. This problem occurs when the room thermostat calls and the discharge
air sensor gets too warm too fast (above 120ºF), due to overheating warm building air in the discharge
Page 37 Revised: 3/11/99