Whirlpool dryer-sensor dry

[mitch]

Hi all,

I purchased a used Whirlpool dryer (model # LET7848DQ1) a couple of weeks ago that I finally had time to fool with. I cleaned everything up on it, etc. and dried a load of towels in it. The issue I am having is with using the "Auto-Dry"--the timer never advanced, it stayed on the "Max" setting. Now, using the "Timed" dry setting---the timer worked just dandy.

Any ideas why the auto-dry is not working correctly? I suspect it may be the electronic control board, but thought it prudent to check with folks more knowledgeable than myself

As always--Thank you!!
Mitch

 

[whitetub]

Hi,

If it's Auto Dry, without a humidity sensor inside the drum (black part with two silver strips on it, at the back of the drum), then the timer should not move, until the air inside the drum reaches a certain temperature. Once the load is almost dry, then it will start moving to the cool down period, and shut off.

 

[coldspot66]

IF it is an electronic dry model with the black sensor block on the back wall at the 4 o'clock position, then it very well could be the small dryness board in the console. I beleive it controls the timer motor in the moisture sensing cycle, but it's not in the circuit in the timed dry cycle. Hope this helps.

 

[DADoES]

 
This model does have moisture sensing per parts diagrams.

Run it on auto-dry with no clothes, the timer should advance to off (at normal timing speed) being as there's no moisture to stall it.

The little board is Part Number 3390537 --> 8558178.  Kinda expensive, SearsParts lists $143.70.

RepairClinic.com $90.05

ApplianceZone.com $87.01

PartsDr.com $74.39

Numerous listings on eBay.

 

[mitch]

Thanks guys!

Sounds like you all suspect the board as well! I see an Ebay listing for a used one I may gamble on. I think I will take a very close look this afternoon and make sure everything (wires, etc.) looks good...again .

It does have the black sensor, with two metal strips attached to the rear of the drum.

 

[mitch]

Found one

I went ahead and ordered a board off of ebay--found a used one, warranted, for $4.95 with free shipping. I will gamble five bucks . This appears to be quite a common part number so if it turns out to be another issue I can keep this as a back-up.

Thank you all again!

 

[kb0nes]

Wow! ~$140 for a simple board with one transistor, a common quad comparator IC and a handful of discrete components. They probably have almost all these parts at Radio Shack even. About $6 worth of parts total...

Not many parts that seem failure prone, would be easy to fix.

 

[Chetlaham]

Radio shack parts.

You are correct indeed. Those parts are only worth 6 bucks. Funny you bring it up because I've found myself soldiering in new parts onto OEM control boards. Just one of those things to get you to buy a new machine.

 

[mitch]

That is funny!

I had the same thought about the board and the cheap parts I could probably get at Radio Shack. I have never changed out parts on a board, but understand the concept. My beloved, late father would have been all over this! He was a ham radio operator for many years and used to build (Heathkit) and work on all of his radios.

Once I get the old board out I may work on rebuilding it--or attempting to . Would it just be a matter of checking continuity with an Ohm meter?

Thank you!
Mitch

 

[kb0nes]

Mitch,

The board should be straightforward to repair if it had a failure. Its a pretty simple circuit and I am looking at photos of the board to reverse engineer it. I'll draw a schematic and post it when I get it done. Looking at the components on the board makes me even more curious as to how this beast functions.

I searched the Internet for a copy of the wiring diagram for your model but couldn't locate one. I need to confirm where the 4 wires on the connector go, my assumptions are:

O-W is AC power
G-Y and Y-R go to the drum sensor strips
W-B goes to the control
T-W is the Ground

If you could shoot a clear photo of the diagram on the back panel (or if anyone has the diagram) it would help.

Most likely parts to fail would be one of the diodes , the big electrolytic capacitor, the IC or the switching transistor. The resistors are unlikely failures unless there is visible evidence of failure. The card edge connectors can be a reliability issue too.

Finally any idea what your fathers Ham callsign was? I'm a Ham too

73, Phil

 

[Chetlaham]

Wires to connectors

If it helps I took a pic of the tech sheet that goes to one of my newer Maytag Dryers (MEDC300XW) It to uses I believe the same sensor card and from what Ive seen nearly all whirlpool made dryers in the last 10 years that use a sensor card use the same one.

 

[Chetlaham]

Gas Version:

...

 

[mitch]

You guys are awesome!

I will try to get a picture of this and post at some point this weekend...hopefully! It is a full weekend! Phil--I did send you an email via the board,

My 5.00 dollar board from ebay is set to arrive Tuesday, per the email notification. I do not see how the guy is making anything as it came with free shipping. I am scared this may dawn on him and he just "throws it" into an envelope with no packing and it will look like it went through a food processor when it arrives.

Thanks again!
Mitch

 

[jetcone]

PHIL

I have always wondered how these boards work, can't wait to see your diagram! 

 

[mitch]

Well, the plot thickens!

The new board arrived in the mail yesterday afternoon. Picture attached. I got the back of the control panel off and the old board was MIA! I did not notice that before--you can tell I do not make my living on appliance repair . The wiring diagram was also missing. I think what happened was the plastic end caps that hold down the console were broken to the point that the console was flopping around and not secured at all and I believe the board fell out along with the diagram. I actually had to tape down the console to keep it from blowing off during transport on the back of the truck.

The new board works just great! I did a load of towels using the sensor option and was qute tickled that everything worked correctly. This machine was a Craig's list find, btw. I did replace the broken end caps and the console is battened down correctly.

Thank you all again--too bad I do not have the old board to experiment with! Weird it was gone.

 

[mitch]

A picture of the dryer

Still have some cleaning to do.

 

[kb0nes]

A little update on my reverse engineering. I've got the circuit all drawn out but I was a bit unsure of the output device (the TO-92 case transistor looking thing). I was sure it was a TRIAC, so I bought a Ebay board myself to experiment with. Indeed its an MAC97A6 hah!

The circuit is a bit puzzling with its intermittent contact with the clothing during dry. I may have to hook this board into the sensor in my Cabrio dryer and watch how it senses the clothing on the oscilloscope.

In any case understanding is progressing...

 

[thefixer]

Here is a link to the wiring diagram for your dryer.

http://forum.appliancepartspros.com/attachments/dryer-repair/436d1221072415-dryer-timer-wiring-sheet-3401725.pdf

 

[mitch]

Cool!!

Phil--that is awesome! Cannot wait to get your knowledgeable update on the board and how the sensor dry actually works. BTW, the dryer I put the board in continues to work. I did a regular dry (high heat) and then a load on PP and it performed flawlessly!

@thefixer, thank you for the link!!

Mitch