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Tuesday, November 30, 2004

Appliance Repair Revelation: Burner Operation in a Gas Dryer

appliance tip of the day archiveGas valve solenoid coil kit, 2 and 3 wire, new style. Dryers, gas or electric, are the simplest appliances you'll ever work on. But judging by the amount and type of questions I get about gas dryers, specifically about the gas valve and burner, has prompted me to bring down another pearl of appliance wisdom from on high. This Appliance Repair Revelation explains how the burner in a gas dryer is controlled by these cute, button-looking coils on the valve, shown here to the right. Ok, hang on for some major geek-speak.

Many gas burners use a split coil design. Valve No. 1 (see diagram below) has two coils to actuate its plunger, thus giving the split-coil burner its name. Neither the hold coil nor the assist coil alone is strong enough to open the valve. The combined magnetic action of both coils is needed. Once the valve is open, however, the hold coil can hold it open by itself. The circuits of both coils will serve as a quick shut-off function.

When voltage is applied to 1V and 3V, a circuit is completed from 1V to the ignitor, and through the sensor to 3V as shown below.

Simultaneously, the hold coil is energized, and the assist coil is energized through the radiant sensor. This action means that the ignitor is heating and No. 1 valve is open. No gas flows, however, until No. 2 valve also opens. Note the resistor in series with the assist coil. This is a balancing resistor which is used only on one style of valve. It will not be present on all split-coil valves.

The ignitor has reached a temperature sufficient to open the radiant sensor contacts as shown below. This action causes valve No. 2 to be energized through the ignitor. Gas flows through the valve and is ignited instantly by the still hot ignitor. Current through the assist coil on valve No. 1 is very low at this point.

Magnetism created by the hold coil is sufficient to hold the No. 1 valve open.

Another important thing to be aware of is that the radiant sensor is closed when it does not sense light and it opens when it does sense light. This means that in order to ohm out the sensor to verify its proper operation, you first need to make sure the sensing port is covered. In this condition, you should have a very low resistance reading between the terminals. Then, uncover the sensing port and shine a flashlight in it. In this condition, you should have a very high resistance reading (ideally, open) between the terminals. A bad sensor will also cause the meter reading to jump all around during either test.

If you suspect the valve coils are burned-out, you can use your meter to make resistance measurements. The diagram below shows you what to measure:

gas valve coil resistance readings

Recommended reading:

For more information on your dryer or to order parts, click here.

grasshoppers visualizing current flow and magnetism in a gas burner valve control circuit

Samurai Appliance Repair Man cast these pearls at 01:52 ET.  [permalink]
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I have ordered a new valve for my Maytag dryer. It should be at my local parts store on Thursday. The old valve that is being replaced is a White-Rodgers model 25M01A-101.

I removed the old gas valve from my dryer. Since I'm an engineer, I was curious about the valve so I removed the top to see what was inside. [Don't worry, I'm not going to reuse the valve.] I noticed that there were 2 valve post plungers that get lifted by the 2 separate solenoid coils. When I removed the plungers from the valve body I didn't pay attention to the location of the plungers in the valve cavities. Does the longer valve plunger get lifted by the holding/boost coil or does it get lifted by the secondary coil?

Also, what will make a valve go bad over time?

By Blogger jawalter, at January 30, 2007 2:10 PM  

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