Let's take a look at a problem that can stump a lot of techs, especially ones who aren't as confident troubleshooting circuits. The scenario: a no heat complaint in an electric dryer. Let's pull up the schematic and take a look at the heater:

Pretty standard stuff -- that's a dual heater element, but that doesn't affect our troubleshooting too much.

As always for an electric dryer, the heater is a 240 VAC load. So it needs to get both L1 and L2 to function. L1 and L2 each have 120 VAC with respect to Neutral, but they have 240 VAC with respect to each other.

Here's the power supply for this circuit sketched out. The meter that's drawn in there should read 240 VAC, since you're reading the voltage drop across one of the heater coils.

Now, to our scenario: since you have no heat, you decide to check the voltage supply to the heater. You decide to check each leg one at a time, measuring it with respect to Neutral. Here's our first test, where we're intending to check L1 to Neutral...

...and you read 120 volts. Good! That's what you would expect. We're using the loading function on our meter, as you should almost always do when testing AC circuits.

Let's check the other side to Neutral!

This measurement also reads 120 VAC. That is also what you would expect! Again, in a 240 VAC circuit, you should have 120 VAC with respect to neutral on each side of the load.

So is this just an open heater? Well, let's check the voltage across the heater. If it's open, we would expect to see 240 VAC across it.

With this test, we get a reading of 0 VAC.

Huh? But I thought that if we're reading from L1 to L2, we should have 240 VAC? How could we have both legs of the power supply on either side of the heater, but no voltage difference between them?

Well, our question leads us to what must actually be the case: one of the legs of our 240 VAC power supply is missing. This makes sense of our previous findings -- when we measured from each side of the heater to Neutral, we were just measuring the same leg of the power supply to Neutral both times.

If you're having trouble visualizing that, let's imagine the problem is an open safety thermostat. In that case, you would simply have L1 on both sides of the heater, like so:

Okay, we're zeroing in on the problem! We just need to figure out how to identify which leg of the power supply is missing -- L1 or L2. 

The easy way to do this is by half-splitting the problem. That just means dividing the circuit in half somewhere. Let's do that by unplugging the bottom terminal of the heater, like so:

And now we just repeat our voltage measurements to Neutral.

Our measurement on the L1 side of the circuit (above the disconnection) reads 120 VAC.

Our measurement on the L2 side, however, reads 0 VAC.

That's different from last time! We have now identified that this no heat problem is caused by an open somewhere in the L2 side of the heater circuit. All that remains is to figure out which component has failed. Our prime suspects are the safety thermostat, hi-limit thermostat, and the centrifugal switch.

Normally at this point in your troubleshooting, there's nothing for it but to disassemble right down to each component and check across it with your meter to see if it's open. But to round out this troubleshooting odyssey, let me leave you with one more trick you could use to check components from the control board instead.

Because of the sensing line on YL3, we can actually read through the heater and the safety thermostat right from the control board, like so:

And there you have it -- enjoy never getting tricked by this 240 circuit head fake again!

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