GM9C96 Service Manual

SERVICING
30
12. Remove the manometer hose from the hose barb tting
or outlet pressure tap.
13. Replace outlet pressure tap:
White-Rodgers 36J54 valve: Turn outlet pressure test
screw in to seal pressure port (clockwise, 7 in-lb mini-
mum).
14. Turn on electrical power and gas supply to the system.
15. Close thermostat contacts “R” and “W1/W2” to ener-
gize the valve.
 
Natural Low Stage 1.6 - 2.2" w .c. 1.9" w .c.
High Stage 3.2 - 3.8" w .c. 3.5" w .c.
Propane Low Stage 5.7 - 6.3" w .c. 6.0" w .c.
High Stage
9.7 - 10.3" w .c.
10.0" w .c.


Using a leak detection solution or soap suds, check for leaks at
screw (White-Rodgers valve). Bubbles forming indicate a leak.
SHUT OFF GAS AND REPAIR ALL LEAKS IMMEDIATELY!
 For gas to gas conversion, consult your dealer for
appropriate conversion.
T  RT RB RT R QT G T
T G  RR T B    T T
RTG T T  TR  G R 
RTG





High Stage
3.2 to 3.8" w.c.
3.5" w.c.
Low Stage
1.6 to 2.2" w.c.
1.9" w.c.



G TG


T

TR R  T  G
 BR RG R TG T T

T R R  B RT

R T
    RRT G R JR R T

G TG
T  TR R  T  G 
BR R
G R TG




High Stage
9.7 to 10.3" w.c.
10.0" w.c.
Low Stage
5.7 to 6.3" w.c.
6.0" w.c.



  use a 120V silicon
nitride igniter for ignition. The normal operating temperature
is approximately 2156°F - 2678°F. At room temperature the
ignitor ohm reading should be from 37-68 ohms.
7. Place unit in heating cycle, measure current draw of ignitor
during preheat cycle.
The steady state current at 120V is 0.37 to 0.68 amps.
8. After checking and/or replacing of hot surface ignitor,
reinstall burner compartment door and verify proper unit
operation.

Flashback will also cause burning in the burner venturi, but is
caused by the burning speed being greater than the gas-air
ow velocity coming from a burner port.
Flashback may occur at the moment of ignition, after a burner
heats up or when the burner turns o󰀨. The latter is known as
extinction pop.
Since the end results of ashback and delayed ignition can
be the same (burning in the burner venturi) a denite attempt
should be made to determine which has occurred.
If ashback should occur, check for the following:
1. Improper gas pressure - adjust to proper pressure (See
CHECKING GAS PRESSURE).
2. Check burner for proper alignment and/or replace burner.
3. Improper orice size - check orice for obstruction.

The pressure control is a safety device to prevent the
combustion cycle from occurring with inadequate venting
caused by a restricted or blocked vent pipe. In addition to
the inducer pressure switch, this furnace has a “front cover
pressure switch” wired in series with the gas valve. This
pressure switch keeps the gas valve from opening in the event
of condensate backing up in the secondary heat exchanger.
This could occur from improperly connected drains or a
plugged drain tube.
1. Remove burner compartment door to gain access to
pressure switch(es).
2. Remove wires from the pressure switch(es) electrical
terminals.
3. Remove the pressure control hose from the control and
interconnect with an manometer as shown in the following
gures.
4. With an ohm meter connected across the pressure switch
terminals; with the inducer running the switch should close