Chromalox PQ402-6 - Manual (Page 3)

If the contactor fails to energize, check for blown fuses in
the control circuit. If the fuses are not blown, check the contac-
tor holding coil. The holding coil can be checked by jumpering
Terminal L1 of the line and Terminal No. 4 (coil lead) on the
contactor. See Figure 1.
A. If the contactor fails to energize, then the coil is defective.
B. If the contactor energizes, it is an indication of failure in
either the Pressure Control or in the McDonnell-Miller three
wire mercury switch.
Pressure
Control
Setting Differential
Control
Setting
Bellows
Syphon
Tube
Since the Pressure Control and the three wire mercury switch
are wired in series, they can be checked one at a time by placing
a jumper across each control. If the mercury switch or Pressure
Control fail to energize, check the McDonnell-Miller controls by
jumpering Terminals 5 and 6. See Figure 2.
At this point, the contactor should energize unless there is an
open wire in the control circuit.
PRESSURE CONTROL — Malfunctions in the pressure control
can be mechanical or electrical.
The bellows should be removed and the small orifice at the
bottom cleaned out. The bellows must be replaced if it is punc-
tured. The syphon tubing should also be cleaned by steam while
the control is removed; it will be necessary to join the two loose
leads in order to activate the boiler. See Figure 3.
The electrical controls should be checked by referring to the
Contactors, Elements and Voltages section.
NOTE: If control is purchased locally, remove #8-32 orifice
screw.
Figure 3 — Pressure Control
Figure 2 — Low Water Cut-Off Junction Box
Figure 1
CONTACTOR FAILS TO ENERGIZE