Cambridge Engineering, Inc.
74
M-Series Technical Manual
TROUBLESHOOTING GUIDE
Problem
Possible Cause
Corrective Action
II. BLOWER RUNS;
NO HEAT;
FLAME SAFETY
RELAY DOESN’T
LOCK OUT
1. Mode Selector Switch
a) Switch in VENT position
a) Place switch in HEAT position.
2. Burner Service Switch
a) Switch in OFF position
a) Place switch in REMOTE position.
3. Multi-Functional PC Board
a) No input voltage on Terminal W (RCS)
b) Thermostat satisfied
c) Improper wiring
d) Defective board
a) Check voltage in RCS.
b) Adjust thermostat.
c) Check wiring to multi-functional PC
board.
d) Replace multi-functional PC board.
4. Airflow Switch
a) Blower running backwards
b) Belts slipping
c) Blocked intake or discharge
d) Clogged airflow tubing or pickup ports
e) Defective switch
a) Reverse motor direction.
b) Tighten and/or replace belts.
c) Find and remove obstruction.
d) Clean or replace tubing or pickup ports.
e) Replace switch.
5. Flame Safeguard Relay (FSR)
a) No input voltage
b) Defective FSR
a) Check wiring.
b) Replace FSR
6. Entering Air Thermostat (EAT)
a) EAT set too low
a) Increase EAT setting
III. BLOWER RUNS;
NO HEAT;
FLAME SAFETY
RELAY LOCKS OUT
1. Igniter
a) No current (open igniter)
b) No voltage
During trial for ignition:
a) Check igniter current
b) Check FSR output to igniter.
2. High Limit
a) High limit tripped
a) See Problem Number VI
3. High or Low Gas Pressure Switches
a) Low gas pressure switch tripped
b) High gas pressure switch tripped
c) Defective gas pressure switch
a) Check gas supply for low gas pressure or
no gas
b1) Check manifold gas pressure for high
pressure reading and reset pressure regu-
lator
b2) Check gas supply pressure against name
plate
b3) Verify the high pressure regulator is a
lock-up type
c) Replace gas pressure switch
4. Gas Valve
a) No input voltage
b) Gas valve does not open
c) Defective solenoid
a1) Check FSR output to R1 relay
during ignition trial
a2) Check gas valve circuit and wiring
b1) Compare supply voltage to nameplate
voltage
b2) Clean and/or replace gas valve parts.
c) Replace solenoid or valve assembly.
5. Modulating Valve
a) Minimum fire set too low
a) Adjust minimum fire on modulating valve
6. Multi-Functional PC Board
a) Low fire start set too low
b) Thermistor open or not connected
a) Adjust modulating valve voltage between
10 and 13 Volts DC
b) Properly install or replace thermistor
