Trouble chart – Midco RS40 User Manual

Page 11

I. Motor Will Not Run

or Motor Runs

in Repeated Cycles.

II. Motor Runs Continuously,

But No Flame.

Trouble Chart

Make sure the thermostat and operating controls are calling for heat. Defective wiring or
loose connections can simulate the component defects outlined below. Check associated
wiring before replacing a component.
ELECTRICAL AND FLAME CHECKS MUST BE MADE IN ORDER LISTED.

Confirm 120V hot to neutral voltage and verify the circuit polarity and electrical ground,
between neutral and burner chassis metal.

Check 24V* operating control circuit between left T-T terminal and GND.

1. No voltage, transformer defective.
2. Very low voltage,* circuit overloaded or transformer defective.

Confirm 120V between hot wire and motor relay terminal 4:
1. No voltage, motor relay is defective.
2. Voltage present, motor is defective.

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Confirm that both Main Manual Shut-Off Valve and Manual Gas Cock Knob on
Combination Gas Valve are in the ON position.

Whenever the burner fails to light during the 6-second trial for ignition, or if the flame is
lost during the burner run and not re-established within 36 seconds (30 second
pre-purge) the Electronic Control will shut off the Combination Valve and LOCK OUT.
To RESET the Control for restart, set the operating control to OFF or thermostat below
room temperature for at least 30 seconds.

1. Check for 24V* between GND and Electronic Control 25V terminal. No voltage,

blower interlock circuit is defective.

2. If Electronic Control has a fuse, test for 24V from each end of fuse to strip terminal

GND.

For each of the following tests, reset the Electronic Control per step II.B. TESTS ARE
VALID ONLY DURING THE 6-SECOND TRIAL FOR IGNITION.

1. Turn Manual Gas Cock Knob to OFF. Check for 24V between the Electronic

Control 25V terminal and strip terminal GND. No voltage, defective Electronic
Control.

2. Check for 24V* between valve MV terminal and valve body:

a. With voltage, RESET Electronic Control and listen for audible CLICK as valve

operators open. No CLICK, replace valve.

3. Check for ignition spark (spark length approximately 5/32"). Since this is a capacitor

discharge system, observe closely when visually checking the spark as it is faint
and thread-like and may be overlooked in bright light.

a. Between Electronic Control high voltage terminal and GND. No spark,

defective Electronic Control.

b. Between Electronic Control high voltage terminal and Spark Electrode Wire .

No spark, broken wire, cracked insulator, or "spark gap" too wide, see Figure 7

c. Between spark electrode tip and distributor, see Figure 7. If spark is not visible

and/or cannot be heard, remove nozzle assembly (see Section IX Nozzle and
Electrodes) and ground it solidly to burner metal. No spark, replace ignition
electrode.

4.Turn Manual Gas Cock Knob to ON. Connect manometer to the manifold gas

pressure tap and during trial for ignition, check the gas pressure:

a. Pressure should be between 2.0" and 4.0" W.C. for NATURAL and PROPANE

and steady. Verify per Section VII Main Gas Spud that the spud orifice size
and manifold pressure are correct.

b. Zero, erratic low or high pressure: confirm that the inlet pressure to the

Combination Valve is between 5" and 14" W.C. for NATURAL and PROPANE
during standby and during trial for ignition.

c. If manifold pressure is zero, below 2.0" or above 4.0" W.C. for NATURAL and

PROPANE or erratic, the regulator section of the Combination Valve is
defective and the entire valve must be replaced.

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Trouble Chart