First generation smart valves act as ignition controls and gas valves. There are internal and external safeties that can prevent the valve from working properly. As the valve operates on a 24V signal from a blower control or thermostat, diagnosing these valves is relatively simple.
The smart valve 1 has a 4 pin Molex plug that attaches to the top or side of the control. This 4 pin plug houses 4 wires: a 24V hot, 24V common, electronic fan timer output, and a 24V input from the pressure switch (see diagram below). You should find 24V present between the 24V hot and common at all times when power is applied to the furnace. With a call for heat, the blower control will energize the induced draft blower, which will close the pressure switch. When the normally open pressure switch closes, 24V will be introduced to the pressure switch wire at the valve. This allows the furnace to attempt to light the pilot. After ignition has occurred, the valve will send a 24V signal back out on the EFT terminal to energize the blower motor.
If the pilot light will not light, verify voltage to the pressure switch terminal and the 24V hot terminal. Check for voltage to the igniter with the igniter plug still attached to the valve. If the pilot will light but not the main burners, check flame sense to the valve. Verify that the pilot tubing is securely attached to the valve.
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