Service manual

A DJM3.2 MANUAL
DJM3.2 Locks Up on Trial for Ignition
When all regular troubleshooting items have been addressed about possible lockup problems, look
at this possible problem.
Some chips supplied to us in controls manufactured about mid 1995 that will lock up very shortly
after the spark starts. This is often an intermittent problem and usually will have the DJM3.2 lock up
without any lights on, however, some lights may be on in certain cases. The DJM3 will reset if power
is turned off for 30 seconds and then restored. If you witness this problem you will note that just after
the combustion fan goes to light off speed the lockup will occur within a fraction of a second of the
spark ignition beginning. Usually a pilot does not get time to light. The problem is radio frequency
noise created by the spark interferes with the DJM3.2 electronics.
Installation of a radio frequency filter will resolve the problem. Disconnect the wires from T1 and T2
on the DJM3. Connect them to the RFI filter. Connect the outlet of the filter to the T1 and T2
terminals with wires no longer then six inches in length if possible. Ensure the RFI filter is well
grounded.
A filter that will resolve this problem is available at most electronics wholesalers. One we have used
successfully is the CORCOM model 1VK1 EMI filter.
L-2X1.86 MH
C-2X.1UHF (x2)SH
2X.0055UF (Y)
R-1.5 megohms
Internal changes have been made to DJM3.2 controls to eliminate this RFI problem on controls
manufactured about April 1996.
Rumble During Light Off
The low fire combustion air quantity being too great usually creates this condition, or the low fire gas-
firing rate is set too low. Always check the main gas inlet pressure first before making any adjustments.
At high fire it should be a minimum of 6½ inches on units designed for 7-inch natural gas inlet
pressure. (Minimum 12 inches on 14-inch natural gas design and 11½ inches on propane units.)
Rumble is usually not a problem with the rectangular shaped high turndown burner. On the standard
burner it is possible to get small amount of rumble whenever a burner is lit after being cold for an
extended period of time. After the first light it settles down. This is because the warm burner has a
greater back pressure than the cold one; thus a lower air velocity occurs when the burner plate is
warm.
One of the more common causes on the regular burners (round burners) is if the combustion blower
light off speed is set too high. Check if the light off speed is too high with an accurate tachometer or by
using a meter that measures hertz connected to the “Y and G” terminals of the speed sensor. Light off
speed is 2700 RPM (45 Hz).
Another item that can create rumble on the standard (round) burner is if the turndown is adjusted too
low. At low fire the standard (round) burner can only modulate to 40% of the high fire rate. This leaves
too much air at low fire for the quantity of fuel. To correct increase the low fire manifold pressure to the
proper firing rate. A rough rule of thumb for low fire manifold pressure is:
DJ40 0.6"
DJ60 0.6"
DJ100 0.8"
Ensure that these pressures are set with the modulating gas valve at low fire (1.8 VDC on its terminals)
and the combustion fan running at 1800-1850 RPM.
Combustion Fan Motor Surging in Speed
There are two possible causes for this. First, both the DJM3 and the combustion fan motor
transformers need to be powered from the same two phases of the three phase power supply. Second
Page 40 Revised: 3/11/99