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Furnace: suburban + atwood/dometic (12v dsi)

How the safety string is wired

It is ONE series circuit, not a bunch of switches each wired to the board. 12V comes from the fused coach feed, through the thermostat and the furnace ON/OFF switch (or breaker), and the thermostat output is the board's call-for-heat input. The board turns the blower on. The circuit to the gas valve then runs through the SAIL switch and the LIMIT switch in series before it reaches the valve side of the board. Any one switch open = no gas.

Suburban: on some models the sail switch is before the limit switch. Atwood 79/80 series also has the limit and sail reversed. It does not change the test: walk the string and find where 12V stops.

Sequence (Suburban fan control module board)

1. Thermostat calls. The board checks the sail switch is OPEN, then energizes the blower. 2. As the blower comes up to speed the sail must CLOSE. If it stays open 30 seconds after the blower starts, the board locks out and shuts the blower off. 3. About 15 second pre-purge. 4. Board opens the gas valve and sparks the electrode for 7 seconds. 5. Flame is sensed through the same spark wire and electrode, so the electrode must sit in the flame. 6. Heat satisfied: valve closes, blower runs a 90 second post-purge.

No flame after 7 seconds: another 15 second purge and another trial. After 3 trials the board closes the valve, runs the blower (3 minutes on some boards, 90 seconds on others) and locks out. Limit open for 5 minutes during the heat cycle also locks out. Reset by cycling the thermostat.

Sequence (Atwood/Dometic)

Older relay units: the thermostat energizes a time-delay relay that starts the blower in 1-25 seconds (1-2 seconds when the relay is on the ignition board). 15-17 second purge, valve opens, spark. No flame in about 6 seconds = retry; 3 tries then lockout for one hour unless power is removed or the thermostat is cycled. Blower post-purges about 90 seconds.

Digital/controller-module units: the board checks sail open, voltage, limit and its own processor first. 15 second purge, 7 seconds of spark, 24 second purge between tries, 3 tries then a one-hour 'soft' lockout with an LED code.

Atwood/Dometic LED codes

1 FLASH (3 s pause)|Air flow / limit fault|Check blower speed, return air, ducts, sail switch and limit switch. Measure 12V both sides of each.

2 FLASHES (3 s pause)|Flame sense fault|Electrode position, gap, porcelain, ground.

3 FLASHES (3 s pause)|Ignition lockout|Gas supply/pressure, spark, valve. Reset by cycling the thermostat or power.

STEADY ON|Internal control failure|Verify voltage and inputs, then replace the board.

Digital-thermostat controller-module boards use a different table: 1 = low input voltage, 2 = ignition failure, 3 = open high limit, 4 = stuck APS (sail switch), 5 = module fault. Read the label on the board you are looking at.

Specs

DC VOLTAGE AT FURNACE|10.5 to 13.5 VDC, checked while running, interior lights on and off

GAS PRESSURE|11 in. W.C. minimum with all LP appliances running

ELECTRODE GAP|1/8 in. between spark probe and ground

ATWOOD AMP DRAW|4.3 to 11.1 A depending on model

Walk the string (Suburban flowchart)

Thermostat closes? 12V on the blue wire at call. Check stat amp draw and anticipator (shorted anticipator = replace stat).

12V at the furnace? If not: furnace ON/OFF switch, then fuse/breaker.

Blower running? 12V at the board power terminal and on the red motor wire at call. Check the ground wire on the terminal block before condemning the motor.

Blower runs but no spark after 15 s? Measure 12V on BOTH sides of the limit switch, then BOTH sides of the sail switch. 12V in, none out = that switch.

12V between red and yellow at the module but no spark: check the electrode plug, the HV wire and the 1/8 in. gap.

Spark for ~7 s but no flame: 12V at the gas valve? Yes and valve won't open = valve. Check manifold pressure, orifice, and the combustion air intake (wasps).

Lights then drops out: flame must hit the electrode and be hard blue.

Never jumper a sail or limit switch to 'test' with gas on. Find why it is open: slow blower from low voltage, blocked return air, crushed ducts, lint on the sail.

Most furnace calls are low battery voltage. The blower slows, the sail never closes, the board locks out. Measure at the board WHILE the blower runs.

SUBURBAN|Suburban Furnace Service Manual: sequence of operation, 12 VDC troubleshooting flowcharts (via myrvworks.com)

ATWOOD|Atwood Furnace Training Manual: sequence of operation, LED diagnostic tables, troubleshooting chart (via myrvworks.com)

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