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Three weeks ago the fridge alarmed and temperature start to climb up. Found the cover over the evaporator fully iced.

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Old 02-03-2017, 11:58 AM
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Default Defrost not working
Model Number: MFI2569VEM2   Brand: Maytag   Age: 1 - 5 years   

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Three weeks ago the fridge alarmed and temperature start to climb up. Found the cover over the evaporator fully iced. Removed everything and melted the grill ice with hairdryer. Everything worked for 3 weeks, last night alarm temperature again, cover iced. Repeated the manual deice. I saw the deicer resistance, use the opportunity to measure its resistance, removed the connector wires and measured around 25 Ohms. The element seems okay. Let to check other things later. Didn't know and ignored the thermostat over the grill (found out about it today). I can measure the thermostat on connectors wires 3 (J1) [RY2 STAT] and 5 (J2) [AC4 D-SENS], according to a circuit diagram I found online. Will remove the connectors from the main board first. Last month the fridge went crazy, nothing worked, pressing any key on panel lite all leds. Found two small caps 25uF altered, on the control board. Replaced them, fridge came back to life, so I have already friendship with the control board. Someone can confirm those pins and connectors for measuring the thermostat without disassembling everything again? I am old and fat, very difficult to go into freezer again just to check thermostat. Also, I don't know if the thermostat should be tripped (open) or closed in normal operation. I imagine that it will trip (open) if defrost (deice) go over temperature, and it must be closed all the time. Am I right? Please help.


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  #2 (permalink)  
Old 02-05-2017, 11:25 AM
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Default Updates - Defrosting not Working!

Measured pins on the control board cables, could measure the less than 30 ohms of the defrosting heater element, the continuity of the freezer grill thermostat (should be very low resistance to allow 120Vac to the defrosting heater) was okay. Since the heater wasn't working, and everything was okay out of the control board, the problem was isolated to be on the control board.

Measured voltage on the relays contacts. Compressor relay U1 was activated, so 120V was continuous present to the compressor. Defrost heater relay was not active - I know it turns on from time to time, do not know time periods of off and on, so inconclusive.

To make sure the defrost heater relay U2 was okay, I needed to remove the control board, feed it with 12Vdc, and exercise the relays. U1 and U2 have constant +12V on one pin of the coil. Is the pin that has a common PCB track among them. The other pin is controlled by the transistors inside the UL2003A1 chip (multi transistor array), finally controlled by the microcontroller on board.



Exercising the relays, I can hear the "click" noise from the relay armature attracking. Measuring the relay contacts when exercising did show good contacts from infinite to few thousands of a Ohm, what is correct according to the Google relays datasheet.

So, relays are okay, all the external wiring, thermostat, heater element are okay, the problem seems to be deeper into the control board.



Needed to make sure the driver UL2003A1 was okay. Its output at pins 10 to 16 are darlington transistors, they present very low resistance and voltage dropout (0.6V) when conducting current and high resistance when not conducting. To conduct, they need voltage around 2.6V between the input at pins 7 to 1, the base of the darlington transistores. Pin 7 controls output 10 and so on. When not conducting, the output of the darlington may present resistance around hundreds of kilo ohms.

Feed the 12V to the points presented in the first picture, and noticed U1 making contact noise, well, turning on the fridge should turn on the compressor, right? well. I don't know. Measured the voltage at pin 10 of the UL2003A1, it was 0.6V, indicating the darlingron was conducting. Just to make sure, measured voltage at pin 11, is what drives the heater relay coil, had 12V, meaning U2 was not active. Well, heater should not be active all the time, mostly when compressor is running, right? well... I don't know.

Measure pins 7 and 6, both inputs for darlingtons that control compressor and heater drives. Both had 0.023V, so not exciting the darlington drives.

Oops, if darlington 7>10 is not active, how its output is down, activating U1 relay????

Removed 12Vdc power from the board.

Now comes a delicate surgery. Very small soldering iron tip and a stainless steel needle. Melt the solder on pin 10 and lifted carefully the CI pin just a little bit, making sure it is isolated from the solder pad.

Now I can measure that darlington transistor inside the CI. Measured CI pin 10 to Ground (pin 8), positive multitester lead on pin 10... it showed 42 ohms... it should show hundreds of thousands of Ohms... That's the culprit.

UL2003A1 has that darlington transistor shorted out.

Hmmm, but it is from the compressor relay.
Yes, with the compressor running 100% of the time, the heater will turn on with compressor running, will never defrost, and worst, compressor is making ice all the time.

Need to replace the integrated circuit... but wait.
I see darlingtons transistors pins not used on the IC. Pin 5>12 are unused, there is a missing relay on that, empty solder position.

Just cut tracks (using a exacto blade) entering pin 7 and exiting pin 10, and the same for pins 5 and 12, reroute the tracks for compressor driver to use transistor on pins 5>12 using small thin wires and surgery soldering.

The red "X" on the picture are cuts with the exacto blade, and yellow lines are the new wires soldered.

Installed on fridge. Turn it on, nothing happens, waited 30 seconds, the evaporator fan turned on and the compressor also kicked in. Measured voltages around IC and relays, compressor is being controlled with the bypass darlington transistor. Now need to wait to see if no more ICE will be formed against the back white plate inside the freezer unit, meaning the defrost heater is working with compressor out.

I wish there were some LEDs on the control board to indicate status of what is happening in the fridge, or a communication port so we could plug a serial interface unit to read what is happening.

Lucky me that knows electronics and could debug the problem, avoiding to spend $180 with a new control board.

I think I will start to fix those control boards for $30 or so... make a living with it...
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Old 02-05-2017, 04:47 PM
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Quote:
Originally Posted by wagnerlip View Post
I think I will start to fix those control boards for $30 or so... make a living with it...
You should do that.

Did you manually defrost the evaporator coils, making sure to get rid of all the frost and ice?
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