Chromalox PQ402-6 service manual Contactor Fails To Energize

CONTACTOR FAILS TO ENERGIZE

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.

Figure 1

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 2 — Low Water Cut-Off Junction Box

Pressure
Control	Differential
Setting
	Control
	Setting

Bellows

Syphon

Tube

Figure 3 — Pressure Control