Service manual

A DJM3.2 MANUAL
are met. (Example: heat switch turned on.)
NOTE: During “Service Mode” operation or safety operation, this step is ignored.
2. When a heating call is received, and before the combustion air blower is allowed to start, the
combustion blower tachometer checks for a false signal. If no signal is present, the combustion
air blower starts.
3. After the combustion blower starts, the DJM waits until the blower reaches pre-purge speed.
4. When pre-purge speed is obtained, the pre-purge light comes on and the pre-purge timer starts.
During pre-purge the heat exchanger is purged of any unwanted gases that may be present.
5. After the pre-purge cycle is complete, the combustion fan speed reduces to the internally set
ignition or light off speed.
6. The set of relay contacts between DJM terminals 4 and 2 close, thus energizing the flame relay
or ignition control device. The flame relay or ignition control device lights the pilot, proves the
pilot flame and then energizes the main gas valve. At the same time, and from the same ignition
control terminal as used for the main gas valve, a "feedback signal" is also sent to the DJM3.2
on terminal 3.
NOTE: At all times the combustion blower speed is continuously monitored. If any
problem is detected, the burner is shut off and the control logic goes to step 3.
7. After the main flame has been established and the DJM3.2 has received the feedback signal,
the combustion blower speed usually slows down. After about 10 seconds, the pilot contacts
between DJM terminals P and 1 open. If the pilot valve is wired through these contacts, the pilot
is turned off.
8. Combustion blower speed and gas valve modulates to maintain the discharge temperature and
proper combustion.
9. When the heat call is satisfied, the burner cycles off. The combustion blower returns to the
intermediate light off speed.
NOTE: Opening the fan or heat switch cycles the flame off. The combustion blower runs 1 to 4
minutes to cool burner.
10. If there is a requirement for heat within 4 minutes the control logic goes to step 6.
11. If there is no heat call within 4 minutes, the combustion blower is shut off and the control logic
goes to step 1, (unless the DJM3.2 dipswitches are set at constant purge for door heater applications,
in which case the combustion fan runs continuously).
Page 4 Revised: 3/11/99