Major Appliances/Continued from "To keep thermal fuse from blowing..."
QUESTION: [AllExperts didn't let me post enough follow up questions, so I'm asking a "new" question to try to continue.]
Hi again Eric, you wrote:
"Dryer diagnosis is relatively simple as long as you can get to all the various components. In your case, you cannot take the unit out of the closet. The thermal blows because the dryer rear panel is reaching temperatures above 220 degrees. To me, this means air is not being pulled across the heating element coils fast enough. As you can see, the heating element is on the rear of the dryer mounted behind the dryer drum. Your blower fan pulls the air from the rear vents across the heating element and then into the drum, through the clothing and then the lint screen and finally the blower housing and out the dryer exhaust vent. Running it without clothing in it should make the unit cycle faster since the temps getting to the cycling thermostat get there quicker and without any obstructional items such as wet clothing. If the cycling thermostat opens at the correct temperature and the heating element still stays on, there has to be a short causing the heating element to stay on. For now, you can jump out the thermal fuse for testing purpose. Do you or did you ever have the dryer drum out? If not, I suggest you remove it and inspect the heating element. Over the years they do get weak and can possibly sag enough to touch one of the brackets securing it inside a non-conductive ceramic protector. If the heating element is touching metal, the ground is enough to keep the element on continuously." -snip-
The heating coil looks fine--not sagging and not touching the enclosing metal pan. The heating coil impedance measures 11.6 ohms, which is about right, and neither end is shorted to the enclosing pan. I don't know if the heating coil is staying on all the time; if it is then the fault must be some other component failure, such as the cycling thermostat not opening at the right temperature, or lack of air flow to the cycling thermostat, or some other component providing a circuit path to the heating coil.
The cycling thermostat (L135-15F) opens when hot but I don't know if it opens at the right temperature. It contains a bias heater controlled by a second pair of terminals, but I don't know whether it properly biases the opening temperature. When I shop online for a replacement cycling thermostat, the parts websites always lead to a L140-10F and I don't know if that's compatible since Frigidaire made at least two versions of the FSE747GES1 dryer. (Mine is the version that has a pin R on the timer. The fabric temperature control switch is different on the two versions, and is in series with the cycling thermostat. How can I determine if the L140-10F is for the other version of the dryer, or for my version?)
Your suggestion about insufficient air flow from the heating coil is interesting. I assume you're talking about exhaust vent blockage or a leaky seal. How do I check for a leaky seal?
ANSWER: A leaking seal would not cause an overheating issue as the leaking seal is after the blower wheel. Is there anyway you can remove the rear vent and run the unit? IF NOT....Pull the drum and run the unit without the drum in it. While running and heating, pull one wire off the operating thermostat. Did the heat go off?
Waiting to hear from you,
Eric & Ann Campion
---------- FOLLOW-UP ----------
QUESTION: Hi again,
When you say the only seal that could leak is after the blower wheel, I think you've overlooked a couple: (1) the felt strips attached to the inside of the front panel, which surround the opening of the drum, and (2) the foam rubber ring seal where the blower opening meets the duct below the lint trap. If one of those has a significant leak, the blower would pull less air than normal away from the heating coil through the drum, and the leaked air would cool the exhaust. I assume either of these conditions could cause overheating inside the back where the thermal fuse lives (and dies).
The foam rubber ring seal looks fine. The felt strips (and nylon upper glides, upon which the top front of the drum rests) are only a couple years old. How would I determine whether they leak?
I tried the test you suggested. When I pulled the wire from the cycling thermostat, the heating coil shut off as it should. I think that test rules out some possible causes. (I assume the absence of the drum during that test is unlikely to affect the result. If the drum had been causing a short that kept the heating coil stuck on and blew the fuse, I think I would have heard a scratchy sound as the drum rotated.)
Still in the running for possible causes: leaky felt seals, cycling thermostat that won't open at a low enough temperature, partially blocked exhaust vent. Are there other possibilities?
Can you also answer my other question, about whether I would need to purchase a L135-15F or L140-10F if I need to replace the cycling thermostat? The part in my dryer is the L13-15F, but the part sold at the parts websites is the L140-10F, which might be for a different version of the FSE747GES1 dryer. I have wiring diagrams for two different versions of the FSE747GES1, which show that several impedances differ. In particular, the impedance of the bias heating element inside the cycling thermostat, and the impedances of the fabric temperature control that's in series with the cycling thermostat. Where could I purchase a new L135-15F if that's the part I need?
I've attached two image files: the wiring diagram of my dryer (photographed from the underside of the dryer's top panel) and the wiring diagram of the other version of the dryer (extracted from a pdf file downloaded from Frigidaire).
Thanks again for your assistance!
Your unit calls for part number 131298300 for the operating thermostat which is indeed a L-140. I do have this in new. I also have the L-135 under the same part number in good quality used if interested. Over the years, the manufactures change things without reason or rhyme. I do not know why but installing either one of the thermostats with only a five degree difference would not cause the thermal to blow. The inside smaller terminals on the thermostat are not a heater as you suggested but rather another set of contacts that close when the operating thermostat main terminals reaches temp and opens. Opening will close the smaller terminals and send power to the timer motor for the auto dry cycle only. Have you ever replaced the thermal and simply tried it running the unit? For testing purpose jump the wires and monitor temp coming out of the exhaust. That is the only way I know to answer your question. Thanks for the wiring diagrams but they were not very readable...If I enlarge or use a magnifier, they are too blurry.
Good Luck, Eric