LG LST5651SW stove: oven not heating properly

[stj]

before you do, make sure the relay that does not appear to be getting power is not simply shorted - or the diode shorted either.

[redwire]

The two big oven relay coils are about 144 ohms each (12V). Across a diode D36 (bake), or D35 (broil) I would expect to see that reading, same for both. With power off of course.
Oven relay datasheet: T9AS1D22-12

[R_J]

I had to read the whole thread again to see if the diodes across the coils were checked and I don't see that.
So using your meter in diode test, check across each of the three glass diodes, hopefully D36 checks shorted

[Curious808]

The main board looks fine to me, both sides. Nothing loose, nothing burned.

[R_J]

Originally Posted by Curious808

The main board looks fine to me, both sides. Nothing loose, nothing burned.

Did you check if the diode across the relay coil on the relay board is shorted?

Also can you trace the wire from the relay board, CN8 pin 4 to the main controller board CN7, I will guess CN8 pin4 connects to CN7 pin3, if it does then there are two transistors that must be turned on to enable the relay

Q206 must be turned ON AND Q36 must be turned ON to ground the RY6 relay to energize it. CN7 pin1 is the +12v source

[CapLeaker]

Originally Posted by R_J

Most ovens will turn on both the main element and the broil element to get the oven up to temperature faster (preheat), In this case it seems that only the broil element is being tuned on to get the temp to 365, If the lower element is not operating it is either open or not receiving any voltage.
Locate the relay board and check it.

I side with R_J on this. My broil element has no problem heating up the store way past the 365. Matter effect, that’s also how the oven clean function works too. The bottom element isn’t always on. I don’t think there is a short either, rather an open on either the element, a bad relay or whatever turns the relay on. I am not sure maybe a temperature control of some sort sounds viable too, but that should throw some code, I would imagine.

[redwire]

Not to derail the thread - but this range has a Jewish "Sabbath Mode" which I don't fully understand but it does limit cooking. Do any of these stay activated despite a cold power up, as if remembered in EEPROM? I dunno.
To Cancel SABBATH mode
1. Press and hold “BROIL” pad for 3 seconds
2. Press START pad.
-----------------------------------
Oven Lockout
1. Press and hold START pad for 3 seconds (toggles LOCKOUT on/off).
-----------------------------------

Just to confirm the control board isn't doing this (no bake heat) on purpose.

[petehall347]

Originally Posted by redwire

Not to derail the thread - but this range has a Jewish "Sabbath Mode" which I don't fully understand but it does limit cooking. Do any of these stay activated despite a cold power up, as if remembered in EEPROM? I dunno.
To Cancel SABBATH mode
1. Press and hold “BROIL” pad for 3 seconds
2. Press START pad.
-----------------------------------
Oven Lockout
1. Press and hold START pad for 3 seconds (toggles LOCKOUT on/off).
-----------------------------------

Just to confirm the control board isn't doing this (no bake heat) on purpose.

wow thats the first i heard about that . the appliances really are taking over .

[Curious808]

Here's all the relay board diode tests:

1) Already posted, with the oven set:

R6 0.0 VDC
R5 13.5 VDC
R8 13.5 VDC

Unplugged:

2) diode test

R6 0.213 V
R5 0.092 V
R8 0.092 V

Not sure if I did diode test properly, got same readings when I switched red and black probe position. Attached photo shows multimeter setting.

3) resistance

R6 349 ohms
R5 139
R8 140

The diode photo shows where I placed the probes.

[Curious808]

Originally Posted by R_J

Also can you trace the wire from the relay board, CN8 pin 4 to the main controller board CN7, I will guess CN8 pin4 connects to CN7 pin3, if it does then there are two transistors that must be turned on to enable the relay

Q206 must be turned ON AND Q36 must be turned ON to ground the RY6 relay to energize it. CN7 pin1 is the +12v source

Not sure what to do here. All of the wires seem to be securely connected.

[redwire]

If was just a thought to check if the oven controls are set strange, before going after the control board.

Off topic - had Samsung fridge that would not turn on the compressor, repair tech swapped out the control board to fix it.
Happened again, looks fine but no cooling. It turns out if changing the fridge temperature button pressing wonky puts it in "showroom" or "demo" mode.
It will appear fine but not turn on the fridge compressor. If appliances have WiFi who knows it it would make repair easier.

[redwire]

Those relay board test readings look fine to me. Around 144ohms coil resistance is seen.
That leaves the little control cable having a break in it, or an oven control board fault.
I will post the cable's pinout, it's not in the LG service manual at all.

[R_J]

Originally Posted by Curious808

Not sure what to do here. All of the wires seem to be securely connected.

Is this how the wires are connected between the two boards (ignore the colors they are just for clarity)
I suspect this is how they are connected, so for the RY6 to activate, first Q206 needs to be turned on, then Q36 gets turned on. So one other condition must be met for the bake element to turn on.

The RY6, RTH34012, coil resistance spec is 360Ω +-10% so 349Ω is ok. and the diode is not shorted. Next check is make sure the wire between the two plugs that controls this relay is not open, If the wire is not open, then we can move on to checking the control board.

[redwire]

The little cable:

Oven Relay board 6871W1N012B
Connector CN8
Pin 1 org RY8 Bake {D38, jumper R38}
Pin 2 blk RY9 (not populated, 6871W1N012A models convection bake rear heating element)
Pin 3 brn RY7 (not populated, 6871W1N012A models W/D)
Pin 4 white RY6 DLB safety relay {D36, jumper R36}
Pin 5 blue? RY5 Broil {D35, jumper R35}
Pin 6 red +12VDC power
---------------------------------------------
Oven Control board 6870W1N002C
Connector CN7
Pin 1 red +12VDC power
Pin 2 blue? RY5 Broil {Q35, Q205}
Pin 3 white RY6 DLB safety relay {Q36, Q206}
Pin 4 brn (not connected)
Pin 5 blk (not connected)
Pin 6 org RY8 Bake {Q38, Q47}


I am not 100% sure of the cable's colour codes. Not all wires are used.
But to me, BAKE relay control from the control board is CN7 pin 6. So I would test transistor Q38 and Q47. If you could give the numbers written on them, that helps too.

[R_J]

Originally Posted by redwire

The little cable:

Oven Relay board 6871W1N012B
Connector CN8
Pin 1 org RY8 Bake {D38, jumper R38}
Pin 2 blk RY9 (not populated, 6871W1N012A models convection bake rear heating element)
Pin 3 brn RY7 (not populated, 6871W1N012A models W/D)
Pin 4 white RY6 DLB safety relay {D36, jumper R36}
Pin 5 blue? RY5 Broil {D35, jumper R35}
Pin 6 red +12VDC power
---------------------------------------------
Oven Control board 6870W1N002C
Connector CN7
Pin 1 red +12VDC power
Pin 2 blue? RY5 Broil {Q35, Q205}
Pin 3 white RY6 DLB safety relay {Q36, Q206}
Pin 4 brn (not connected)
Pin 5 blk (not connected)
Pin 6 org RY8 Bake {Q38, Q47}


I am not 100% sure of the cable's colour codes. Not all wires are used.
But to me, BAKE relay control from the control board is CN8 pin 6. So I would test transistor Q38 and Q47. If you could give the numbers written on them, that helps too.

The relay that is not getting 12v is RY6. CN8 pin 6 is +12v common to all the relays
The safety relay is RY8

[redwire]

Hmmm, not sure what is correct regarding the relay board. 6871W1N012 has A and B rev's but look the same.
I think the smaller orange relay RY6 is the DLB safety relay, which is CN8 pin 4. It switches off the return for both bake and broil.
But the service manual calls it RY8. It could be a typo or silkscreen error top vs bottom?
The two large relays are for bake and broil, as I figure. Please check so we are on the same page.

[R_J]

Originally Posted by redwire

Hmmm, not sure what is correct regarding the relay board. 6871W1N012 has A and B rev's but look the same.
I think the smaller orange relay RY6 is the DLB safety relay, which is CN8 pin 4. It switches off the return for both bake and broil.
But the service manual calls it RY8. It could be a typo or silkscreen error top vs bottom?
The two large relays are for bake and broil, as I figure. Please check so we are on the same page.

RY6 only has one wire going to it that is connected to the bake element, that is the only relay that does not have 12v across the coil, and it is the bake element that is not working.
RY8 is the safety relay that has two connections, one to the broil element (connected to the relay) and one to the bake element (connected to the board terminal)
RY5 is the broil relay, The broil function works, also RLY 5 and RLY8 both have 12v across there coils.

[redwire]

You are right. I was (wrongly) assuming the two big relays were for each heating element. Broil is 4,000W (16.7A) and Bake is 3,400W (14.2A) or almost 31A both on. But the small relay is only for Bake.

Anyway, I think we are at the oven control board (and that small cable) and the relevant transistors to test are little Q36 and Q206. Both transistors must be on for the bake relay command signal to appear.
Interesting that LG uses a very small transistor for Q36. Q206 looks like a digital transistor (no base resistor). I believe the connector CN7 pins #'s get flipped between the two boards.
I can post a pic of the multimeter diode-test readings to take on Q36, Q206 to confirm. I don't have their marking codes.

With both boards connected (small cable), at the oven control board connector CN7 pin 1-2 around 144Ω (Broil), pin 1-3 around 360Ω (Bake), pin 1-6 around 144Ω (DLB). (note you got 140Ω and 349Ω which is fine). This confirms the relay board + cable connections, leaving the problem as the control board I think.

[sam_sam_sam]

Do the checks that are mentioned on pages 4.2 and 4.3 and follow the directions carefully for best results

Especially the ones that cover the temperature sensors there are two different types

https://www.badcaps.net/forum/attach...4&d=1673369253

[Curious808]

Originally Posted by redwire

Not to derail the thread - but this range has a Jewish "Sabbath Mode" which I don't fully understand but it does limit cooking. Do any of these stay activated despite a cold power up, as if remembered in EEPROM? I dunno.
To Cancel SABBATH mode
1. Press and hold “BROIL” pad for 3 seconds
2. Press START pad.
-----------------------------------
Oven Lockout
1. Press and hold START pad for 3 seconds (toggles LOCKOUT on/off).
-----------------------------------

Just to confirm the control board isn't doing this (no bake heat) on purpose.

No Sabbath mode: long press BROIL does nothing.

START is oven lockout, it's marked on the pad.

Long-pressed other main controls. Ones that did something:

BAKE -> Adj
COOK & WARM -> Opt