GE Microwave oven with burnt connectors on thermal cutoffs [USA] - Page 1
Forum section: electronics / repair

author: scootley
author_rank: Contributor
timestamp: February 21, 2021, 01:10:16 am
content: Hi,Note: I am generally aware of Microwave internal safety and the giant capacitor risk. The manual says it has an internal shunt resistor and is designed to discharge within 30 seconds. And I know there are shock issues if working on it with mains active, and the high voltage magnetron. And issues if I were to physically change the magnetron or case position/integrity where microwaves could be emitted to the exterior at harmful levels.I have a GE Microwave oven that shows "PF" on the display and if you try to activate it, it goes dark (light inside goes off, buttons/display do not operate) for a while until PF eventually reappears.So after taking it apart, I see two connectors that are brown/burnt a bit, one on each of two thermal cutoffs (TCOs) (labeled as "oven/cavity" and "magnetron").With the cord unplugged, the "oven/cavity" TCO reads open circuit and the "magnetron" TCO reads short. I'm assuming of course these TCOs are normally short and go open circuit when they exceed some temperature.Nothing else inside has any obvious visual signs of failure. The troubleshooting flow chart on the wiring diagram says to replace the TCOs that are reading open circuit.What could have caused this? Something causing a short in the circuit that flows through the TCOs? Fan motor failure? Turntable motor failure? High voltage transformer failure? Magnetron failure? I admit I have not yet fully internalized the wiring layout.Photo of oven/cavity TCO:Photo of magnetron TCO:Photo of internals overall:Wiring diagram:Schematic:Troubleshooting:Prior history of this unit is that:1) for years there has been some weird issue with the door interlock where either a) it thinks the door is open when it is shut, and won't activate at times without a wiggle, OR b) when the door is shut but the magnetron is off, the fan actually activate and be blowing out the back, also fixed by a wiggle. I just assume the interlock switches (there are 3) and/or their alignment with the door was bad.2) a few months ago, the main relay that activates the magnetron failed and was stuck on. I replaced it and it has worked fine ever since. I wonder if the fusing of that relay was caused by issue #1 where the flaky switches would make/break the circuit randomly causing arcing that welded it stuck on?Thanks

author: amyk
author_rank: Super Contributor
timestamp: February 21, 2021, 01:20:03 am
content: One of the interlocks shorts the power when the door is open. If one of the others hasn't opened yet, it is definitely possible to burn something.Another possibility is a poor connection, leading to heat, burning, and an even worse connection.(If you have pictures in your post, I can't see them.)

author: scootley
author_rank: Contributor
timestamp: February 21, 2021, 07:04:34 am
content: Quote from: amyk on February 21, 2021, 01:20:03 amYes there are 6 pictures in the post. Do you see the imgur.com links after each picture's title?Quote from: amyk on February 21, 2021, 01:20:03 amThanks. Yes I did a fairly crude simulator of the circuit here:https://tinyurl.com/yckgufadThis particular unit has 2 interlock switches that seem to be in direct control of power. Both of them are depressed when the door is shut, but one is wired "normally open", so it goes short with the door shut (called "primary interlock" in their diagram), and the other is wired "normally closed", so it goes open circuit when the door is shut (called "monitor interlock" in their diagram).So my deduction is that if the monitor interlock gets "stuck" as a short when the door is closed AND the main magnetron power relay is active, then there will be a zero resistance loop passing through the TCOs and they will burn out. Does that make sense? It could happen if someone wiggles the door while the unit is running, I guess. The magnetron would deactivate and the whole thing would shut off simultaneously with the burnout happening, since the monitor interlock's purpose seems to be to disable it by shorting the primary winding to it's transformer.So your theory is correct but only if the magnetron relay is also active, I think.Or an alternative scenario is where the door is open, but both the power relay AND the primarily interlock are stuck short, which is less likely because that involves 2 components being broken rather than 1 as above.(There is a third switch but it's more of an electronic interlock because it's wired to the control board and does not carry current through the circuit powering the main loads inside. It seems to be yet another backup "door is closed" sensor and shorts when door closed)So I guess I could get replacement TCOs. But each TCO seems to have it's push-on terminal connector welded to it, so maybe I would need new connectors too because I fear soldering to the TCO directly would activate it and kill it due to heat, right? How do I find terminal connectors that fit a given TCO and are rated for appropriate current?

author: CaptDon
author_rank: Super Contributor
timestamp: February 22, 2021, 03:46:00 pm
content: Those friction fit push on 1/4" spade terminals on the wires were always a piece of shit and GE knew it from day one. Now the insulation is also hard and brittle. 1/4" spades were never meant to carry 11+ amps for more than a minute or two at a time. Cut off the old lugs and both crimp and solder some new lugs on. Scrape the male spades for a shiny connection also. This is normal age and GE saving money. There is really no other problems except poor design.

author: scootley
author_rank: Contributor
timestamp: February 22, 2021, 10:33:54 pm
content: Quote from: CaptDon on February 22, 2021, 03:46:00 pmThanks! These are actually only 3/16" too.Wait so do you mean that you think the lugs overheated from carrying too much current during normal magnetron operation, and that heat is what tripped the TCO, rather than a short circuit somewhere? Interesting.I definitely need at least one new TCO (the oven cavity one) because that one reads open circuit now and so it doesn't seem to have been the auto-reset type or it was just pushed way beyond its capability and broke. Thing is that I don't know what the current rating for it was. The wiring diagram says it's "110", so I assume that's the trip point in degrees Celsius (not Fahrenheit), but I can't figure out who makes the part and it pains me (on principle more than budget) to buy it from a "microwave parts distributor" because it will cost like 5x in price. The wiring sheet also says 14.3 amps (1650W) "line current" (it has a 20A fuse). Unit is branded as "1150W". The only parts I see on digikey that seem similar are rated at 15A max. Maybe that will be ok.

author: CaptDon
author_rank: Super Contributor
timestamp: February 23, 2021, 02:47:51 pm
content: Throw a cup of water into the oven and jump out the bad thermal cutout, crank it up and see if it works. If there is a real serious problem it will blow the fuse. Note that most microwaves use a 3AG style fuse but made with a ceramic body, some have a 15 amp fuse some have a 20. If the unit fails to operate properly with the TCO jumped just junk the thing. You may have a problem with an appliance store selling you the TCO. They act like you will blow up the earth if you replace it. They feel 'only' factory service technicians can properly and safely replace such an item. I had to replace one on my Mom's built in over-the-range microwave. It was such bloody hell trying to convince the brain feeble morons to sell me the TCO that I ended up disecting the old one and fixing it. It could be internally reset!! Mom fat fingered the buttons and cooked a potato for 40 minutes!!! Nice fire until the TCO tripped and shut everything down as it was supposed to!! The fire in the oven cavity starves for oxygen by design and the fire goes out.

author: scootley
author_rank: Contributor
timestamp: February 26, 2021, 06:31:57 am
content: Haven't gotten a chance to do more testing or fixing yet, but here is some info on the parts.The thermal cutoffs are all this component typically identified as the KSD201 or KSD301. In my case the former.I've found maybe 6 manufacturers in China but they are hard to find from typical US distributors like Digikey.Mine are 160C and 110C cutoff temperatures. (I assume they are C and not F, but honestly the manual and the parts do not indicate this. 110F would be way too low as far as I see).The highest current rating on any of these components that I've seen at 120VAC is 16A. So I assume mine are no higher than that, although my parts don't say any current limit.Some employees from the manufacturers in China have stated that the KSD201 (which does not seem to be actively manufactured) uses a "Wheatstone bridge", and they seem to require either manual reset or act like a fuse (where they have to be replaced after tripping)And the KSD301 (which is still made) uses "bimetallic strips" that allow it to auto-reset after it cools off and be used again.

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2021, 06:54:38 am
content: CaptDon nailed it. Those connectors are problematic under the best of circumstances. Add in moisture from cooked food, salt vapors, grease and years of use and it's not unusual at all to see failures like that. I've mostly encountered it in clothes dryers where the high current to the heating element is really more than a connector like that ought to be used with but microwaves are also fairly heavy loads and I've seen it in those too. Clean up the lugs and replace the connectors on the wires with good quality ones and hope for the best. In some cases you might have to replace the thing the wire is plugged onto if the terminal has oxidized badly.

author: scootley
author_rank: Contributor
timestamp: February 26, 2021, 07:15:11 am
content: Thanks. Two of them are actually welded on from the heat, it seems, so not sure if I can remove them without damage to the lugs (I assume the "lugs" are the terminals on the thermal cutoff). But I will try to get new thermal cutoffs anyways since one of them is dead.How can I find good quality connectors for the wires that are meant to carry the right load. I guess maybe a digikey search.

author: scootley
author_rank: Contributor
timestamp: February 28, 2021, 08:52:56 am
content: Quote from: james_s on February 26, 2021, 06:54:38 amWho makes good quality ones? Molex?

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2021, 04:36:48 am
content: Quote from: scootley on February 28, 2021, 08:52:56 amI don't remember what brand is carried by the local (real) hardware store near me but they're quite good. Molex ought to be good in general too. See if you can find some meant for high current applications.

author: scootley
author_rank: Contributor
timestamp: March 01, 2021, 05:08:10 am
content: Thanks. Most quick connect "data sheets" or spec documents don't seem to specify a current rating. Not sure why. Maybe there are more generic (e.g. UL) standards that they are tested to that cover many brands/parts and I didn't bother digging into that.I will try some of these, which are at least rated for fairly high heat (150C) and voltage (600V)https://www.te.com/usa-en/product-521081-1.html

author: scootley
author_rank: Contributor
timestamp: March 01, 2021, 05:19:48 am
content: This document:https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=82004_FASTON_TERMINALS_-_FULL_CATALOG&DocType=CS&DocLang=ENSeems to imply that 250 series (1/4 inch) TE quick connects are tested to 20-24 amps with 12-10 AWG (respectively), but doesn't say the voltage.(page 8 )The standard is UL 310.In my microwave they used 16 AWG, which seems crazy to me for how much current it supposedly needs at 120VAC.At least I will try 1/4" rather than the existing 3/16" spades that failed.Ah yes the UL standard test for 16 AWG seems to be only 10 amps for these connectors at 1/4".See here:https://en.wikipedia.org/wiki/FASTON_terminalAnd you can sign up for a free UL account to view the standard too.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 01, 2021, 05:31:12 am
content: Those faston lugs need to be steel and then nickel plated. Anything less will fail at those temperatures. If they are the usual tinned brass lugs, it’s no wonder they failed. Tinned brass or bronze has two problems. The tin plating burns off and the surface oxidizes, causing an increase in contact resistance and the brass expands due to the temperature, causing a loss in contact pressure. Again, more high contact resistance which becomes a thermal run-away. It’s a double whammy failure mode.The problem may be the size. You can usually source crimp on nickel plated 1/4” females at an appliance parts store, they are used on clothes dryers, for the heating element connections as james_s pointed out. I’ve never seen 3/16” but then I haven’t asked. Good luck.

author: scootley
author_rank: Contributor
timestamp: March 01, 2021, 05:49:31 am
content: Quote from: WattsThat on March 01, 2021, 05:31:12 amThanks. Yes I noticed the factory 3/16 ones did seem to oxidize. I am going with new TCOs that have 1/4 spades and so will of course use 1/4 connectors on them.I didn't see any insulated steel ones that can do higher temps than the insulated brass ones; clearly the issue seems to be the insulation temperature rating and not the metal.If I do the insulation myself, I wasn't sure what the easiest and least expensive way to insulate them would be in a way that can handle 160C. Maybe capton tape?Side point: I have no idea what causes heat in these wires/TCOs during healthy operation vs failure scenarios. Clearly the oven is generating heat. But maybe there is some non-insignificant heat from the current too given the small gauge wires. I would imagine they will be nowhere near 160C (or even 110C) during normal operation, so maybe trying to insulate for 160C is going overboard.

author: scootley
author_rank: Contributor
timestamp: March 07, 2021, 08:19:12 am
content: I fixed it with these parts:Faston connectors:https://www.te.com/usa-en/product-521081-1.htmlhttps://www.digikey.com/en/products/detail/te-connectivity-amp-connectors/521081-1/288134TCOs: (which are also physically compatible with the existing mount points)https://www.cantherm.com/product_post_type/r53/https://www.digikey.com/en/products/detail/cantherm/CS715025Z/2195567https://www.digikey.com/en/products/detail/cantherm/CS711525Y/2195559Also should note that wiggling the door (in particular upwards) will cause the lowermost interlock switch (referred to as "door sensing" or "secondary interlock") to toggle to an incorrect state (open circuit). This switch is a logical interlock, in the sense that it does not make or break current through the main components, but feeds into the control board as a logic input of sorts. When this happens, the unit activates the fan, turntable, and light, but not the magnetron. This to me is poor control design. You can see why this happens in my simulator in my earlier post. The only way this unit can turn on the light when the door is open is by activating what is labeled as the "motor relay". But if the aforementioned door sense interlock gives a false reading, the motor relay still activates, which then not only turns on the light but also activates the fan and turntable, since that one relay is wired to (and the only way to turn on) all 3 components. In other words, there is no relay/switch that toggles only the light.(the fan/turntable are normally not on when the door is open because the upper interlock cuts power to them even though the motor relay is active)However I don't see how this door sense interlock could have caused the TCO failure, so my earlier theory remains the best explanation.

author: Zakilorado25
author_rank: Newbie
timestamp: January 17, 2024, 11:02:55 pm
content: I have the same problem, but I cannot remove the white connectors from the terminals...How do I remove the connectors from the terminals?

author: amyk
author_rank: Super Contributor
timestamp: January 18, 2024, 02:33:27 am
content: Quote from: Zakilorado25 on January 17, 2024, 11:02:55 pmPliers or a screwdriver, depending on how stuck they are.Of course make sure you're pulling on the right part.

author: Zakilorado25
author_rank: Newbie
timestamp: January 18, 2024, 07:38:09 pm
content: Yes I'm pulling on the right part....but I just don't know how to wiggle my pliers to get it out...is there a video or something that I can watch on how to remove it...or can you explain a little more detailly please..

author: ggggizli
author_rank: Newbie
timestamp: October 10, 2024, 05:21:21 pm
content: I ran into exactly the same problem with my Thermador microwave. Like many pointed out in this thread, the problem seems to be bad quality connectors on the TCO.I have a dumb question before I jump in and start ordering parts. How do you crimp hard-shell plastic insulated connectors like these (https://www.digikey.com/en/products/detail/te-connectivity-amp-connectors/521011-1/288143) ? The insulation does not seem to be removable but may be it is?