LG LST5651SW stove: oven not heating properly

[Curious808]

Here's the sheet of diagrams.

[redwire]

From the pictures: Everything looks OK, board soldering has a few rough spots but no big fractures. There is no thermal safety switch in this model.

I do see a blackened diode with splat of solder, on the relay on the right. I'd look to see if the relay spit some molten metal or what. Both diodes should look identical.

RY6 is Bake, RY5 is Broil, RY8 DLB is the safety relay for both.

[R_J]

Since the top element (broil) works, I would suspect that RY6 is bad.

It may or may not be getting told to turn on but to check that it would require working on a live oven.

You could give the smaller relay a couple taps with a screwdriver handle and see if the bake element comes on, then you know the relay itself is bad.

[Curious808]

Here's the diagram sheet.

[Curious808]

Originally Posted by R_J

Is this wire (red arrows) all connected to the same point (red ac line in)?

Yes.

[R_J]

Originally Posted by Curious808

Yes.

I figured that after seeing the diagram but wanted to be sure.

[Curious808]

Originally Posted by redwire

I do see a blackened diode with splat of solder, on the relay on the right. I'd look to see if the relay spit some molten metal or what. Both diodes should look identical.

One doesn't look like the others. Is this the problem?

[R_J]

D38 does look a bit different but I doubt that is the problem, The only one that would cause the problem you are having (no bake element) would be if D36 was a dead short across the relay coil, causing the relay to not operate.

[redwire]

The diodes look fine, one was black in the picture but it must be lighting.

I would put it back together and run the oven and make a (live) voltage measurement. Are you OK to do this? As long as the dog/kid etc. don't come running in, and your multimeter+probes are reasonably good quality.

I would measure the output of the relay board going to the bake heating element and confirm there is 240VAC across the wires to power the bake element. Like this:

"-->Set the oven to bake and check for 220 Volts AC across BAKE and L2 or DLB-OUT. The terminals are marked on the board. L1 is voltage into the board. then, depending on what's selected, bake or broil, it comes out through the relay and to one side of the element. The L2 voltage goes into the board and comes out the Double Line Break relay OUT and is common for both elements.
Touch one meter lead to BAKE and touch the other to L2 then BAKE. Your meter should indicate 220 Volts AC for both measurements."

pic taken from Jake at ApplianceBlog.com thread but no solution was posted.

[Curious808]

Originally Posted by redwire

The diodes look fine, one was black in the picture but it must be lighting.

R8 looks broken up inside, compared to the others. That's fine?

Originally Posted by redwire

I would put it back together and run the oven and make a (live) voltage measurement. Are you OK to do this? As long as the dog/kid etc. don't come running in, and your multimeter+probes are reasonably good quality.

Sounds ominous. I get the risk. Does this multimeter look up to the job?

Is it possible to continuity test the relay board first (the tech sheet says to do that)?

[R_J]

The meter should be fine, but PLEASE be careful, you are checking 240vac, so find a way to attach one lead, and probe with the other keeping one hand only to probe the high voltage.

to check the dc voltage across the diodes you can use both hands to hold each probe but again be careful.

If you are comfortable doing the above test, you can also check the dc voltage across each of those diodes, the voltage across the diodes would be around 12vdc, this test will confirm there is drive to each relay and if the voltage IS there, confirm a bad relay

[petehall347]

i also suggest if very low or no volts to the element try disconnect the element and see if something like full voltage is there .if so there is a bad contact . possibly inside the relay .

[sam_sam_sam]

I would check the bake relay because these relays are notorious for going bad

The easiest way to check them is when the bake relay is being called to be on check the terminals on the relay terminals for voltage if you have voltage a crossed the terminals then your relay has issues with it and you can do the same exact thing with the broil relay as well and compare the results

[Curious808]

Originally Posted by redwire

"-->Set the oven to bake and check for 220 Volts AC across BAKE and L2 or DLB-OUT. The terminals are marked on the board. L1 is voltage into the board. then, depending on what's selected, bake or broil, it comes out through the relay and to one side of the element. The L2 voltage goes into the board and comes out the Double Line Break relay OUT and is common for both elements.
Touch one meter lead to BAKE and touch the other to L2 then BAKE. Your meter should indicate 220 Volts AC for both measurements."

Oven turned on, preheating to 350. There were several clicks over a couple of minutes. Tested several times.

BAKE YELLOW / BAKE BLACK 0.01 VAC
BROIL YELLOW / BAKE BLACK 240 VAC

No connector post at L2. The holes are bridged with solder.

L2 (holes) / BAKE BLACK 0.05 VAC

The photo shows what was tested.

[R_J]

Replace the orange relay RY6, as long as it is being told to close (12vdc across coil) the contacts are open.

Looks like Digikey pt# PB2478-ND, P & B # 1-14155369 (RTH34012WG) SPST-NO (1Form A) 16A

[Curious808]

Tested the diodes. Not sure if that's how, but touched probes to lead wires on either side (see photo).

Set oven to 350, preheat started.

R6 0.0 VDC
R5 13.5 VDC
R8 13.5 VDC

Tested 3-4 times each.

(Thanks for the help, and sorry if it's taking so long. First time doing most of this, so taking it slow.)

[R_J]

Now this is why I asked about the coil voltage, R6 (RY6) it has no voltage, so the coil is not energized, so this relay will not close, so it looks like the problem is somewhere else, It looks like the wire for that relay is pin4 of CN8.
So check the wires and that plugs connections, if they are ok, trace that 6 wire cable, it likely goes to the main controller.

Looks like that cable connects to CN7 on the main controller

[Curious808]

Originally Posted by R_J

Now this is why I asked about the coil voltage, R6 (RY6) it has no voltage, so the coil is not energized, so this relay will not close, so it looks like the problem is somewhere else, It looks like the wire for that relay is pin4 of CN8.
So check the wires and that plugs connections, if they are ok, trace that 6 wire cable, it likely goes to the main controller.

Looks like that cable connects to CN7 on the main controller

Sorry, not clear. Does that mean that the whole relay board is fine, and the problem is elsewhere?

[redwire]

I'd say yes, the oven relay board looks OK and the problem is upstream. That means a little scream and then pulling the control board out. Or first taking clear pictures of the control board in the area that goes to the little cable that goes down to the relay board.

There is power to the oven relay board, Broil and DLB relay have coil power, but zilch for the Bake relay coil. That means troubleshoot the command signal to turn on the bake heater relay. This comes from the control board I believe.

It's worth the patience to troubleshoot, at least down to the board level because it's gets expensive otherwise. Hang in there. This is fun. So much fun.

[redwire]

Trying to find the LG service manual for the LST5651 and I couldn't find anything. So, looking for its EBR74632606 Oven Control Board PCB Assembly, I find the same board is also used in American model number LSB5682.
I did find that service manual, give it a looksee and I think it can help, it has a few extra options that might not be the same as your stove.

So it seems to have no test modes, the service manual advice is replace the temp sensor, then the relay board, then the control board. OUCH that's crazy expensive.

Next would be to remove the control board and post pictures here. LG does have low quality soldering so hopefully that's all we have to look for.