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Appliance Repair Tech Tips

A blog by Son of Samurai in General

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Using Schematics and Wiring Diagrams to Figure out Why the Lights Won't Turn Off on this GE Wall Oven

Son of Samurai

Entry posted by Son of Samurai in Tech Talk

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At one of our recent Live Dojo workshops, a tech shared an interesting problem he had encountered.

He was called out on a GE wall oven where the upper oven lights stayed on all the time, and the door lock wouldn't activate. He investigated further, discovering that when he opened and closed the lower oven door, he heard a relay click as the light turned on and off — but there was no such relay sound for the upper oven.

Presuming this meant a stuck relay on the control board, he went ahead and replaced the board. However, the problem persisted. Only then did he find something else: a pinched wire harness going to the door lock assembly. After replacing this harness, the symptoms disappeared. How is this possible? Is there some combination of broken wires in the pinched harness that would cause the relay for the oven light to stay closed and the door lock motor not to work?

Let's take a look at the tech sheet to figure out how this could happen.

Schematic of Upper Oven

Here's the schematic of the upper oven with the door lock motor and the oven lights circled. These are our Loads of Interest.

Screenshot_2024-09-30_122153.png

If we analyze their power supplies to see how they get Line and Neutral, it looks like this:

Screenshot_2024-09-30_122153-2.png

That's about as straightforward as it gets. Each of our LOIs has an unswitched, direct connection to Neutral. L1 is provided on the other side by a relay, one relay for the door lock motor and the other for the lights. These relays are directly controlled by the computer board.

These two circuits are completely parallel to each other. Maybe the door lock motor’s lack of operation could be explained by a broken wire in its circuit, but that wouldn’t explain the light staying on at all.

Wiring Diagram 

Since we’re dealing with a wiring problem here, we have to go to another part of the tech sheet: the wiring diagram. Unlike the schematic, the wiring diagram shows us the physical layout of the wires and harnesses in the machine.

Screenshot_2024-09-30_122359.png

Kinda gnarly looking, right? To help you make sense of this, I've circled what we're interested in. At the top is the connector where our pinched harness meets the control board. The loads numbered as 18 are the lights, and the assembly marked as 20 is the door lock assembly.

So how do we read this diagram? What we're interested in finding out is what circuits could possibly be interrupted by our pinched harness. This diagram shows each harness as a single line. Individual wires only separate from this harness on the diagram when they are headed to a load or a switch.

To follow any particular wire, you have to note its code and then follow the harness line to all its various terminuses until you see that wire code reappear. A bit tedious, but doable. For example, if we're interested in following wire YX, then we could trace it like so:

Screenshot_2024-09-30_122359-2.png

 

How Do We Use this to Troubleshoot?

We know that the wires going to the lights weren't broken -- the lights are on all the time. So the break must in another wire in this harness, and we know that the pinch was along the way to the door lock assembly.

What about those switches in the door lock assembly? Come to think of it, we didn't see those anywhere in the original schematic we looked at. What is the function of those switches, and what might happen if one of their circuits was broken?

To find our answers, we have to look at yet another part of the tech sheet. For whatever reason, GE's draftsmen decided not to give us all the information about this wall oven on one schematic. They have the ladder diagram that we looked at above, and then there's a separate control board schematic with other info. And on that diagram is the final piece of the puzzle...

Another Schematic from the Tech Sheet

Screenshot_2024-09-30_122304.png

The circuits circled on the right are exactly those switches we were looking at on the wiring diagram. We now see their functions: one switch is the door lock switch, and the other is the door position switch. These switches give the board information about the state of the door and the door lock. The door lock switch tells the board if the lock is activated or not, and the door position switch tells the board whether the oven door is open.

Switches and Sensing Lines

Notice that the wires from these switches are connected to the DC side of the computer board, not to the loads (door lock motor and oven lights). Used this way, the switches make what's called sensing lines that the board uses for information, not power control. Sensing lines are used all over appliances and it's good to recognize them when you see them. When the oven door is closed, the door position switch is also closed (meaning it completes the circuit). Conversely, if the door position switch is open, the board interprets this as meaning that the oven door is open. The board senses the state of these switches (open or closed) and then makes software decisions about which loads to activate. 

We now have Enough Information to Know Exactly What Happened!

When the wire harness got pinched, it broke a wire in the door position switch circuit. The board would see this as the door position switch being open. Based on this faulty information, the board does what it always does when it thinks door is open: it turns on the oven light and doesn't allow the door lock to be activated.

Our problem turned out to be a pretty simple one, but we had to bounce nimbly between three different diagrams as part of the troubleshoot. Only with info from the entire document could we arrive at the solution.

Want to learn how to read schematics and form troubleshooting plans like a pro? Click here to check out the Core Appliance Repair Training course over at the Master Samurai Tech Academy and enroll today.

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williamg1916

williamg1916

Posted

This says a lot about checking for bogus sensing inputs and wiring before figuring on a bad board (or other things). Not a criticism of our fellow appliantologist, but giving the board the benefit of the doubt.
Story: I tore a front load washing machine down (my own) and replaced the spider and bearings, only to discover that the computer was right, and the problem was a broken (burnt) wire in the harness. If I'd first checked the resistance (sorry but the only way) of the windings back at the connector at the control board, I would have seen the weird resistance reading on one leg of the BLDC motor. I would then have checked the motor itself and seen normal readings and I would have gone to the harness; all of which I did eventually. The mainly broken wire with 16kΩ resistance, compared to 11Ω explains how the board kept thinking there was an unbalance error and a motor error. I thought the moans and groans were bearing symptoms, but I think now it was the motor getting momentary connection through that bad wire. Repaired the harness and it ran perfectly.

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Far field

Far field

Posted

Here’s my rub with running this down. How is an appliantologest to know about 3 different diagrams? Then to go back and forth across pages that he had to discover some how?   Scott how did you know there were 3 diagrams ? What knowledge and how did you acquire this? Is it your Navy back ground? Some - nay most of us are logical but not wise enough to determine not enough detail here.  And then have to logically go to another diagram and find our LOI And then another LOI on a 3rd.  Basically we have to know the logic of the wiring before looking.  So one of these diagrams has to be labeled ? 
the first 2 any headers or footers are not seen so maybe there is the solution it may all be on one sheet but I just don’t see how I can magically denote any thing with out 100% quiet and concentration on finding the magic golden egg, if you catch my drift. 

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Son of Samurai

Son of Samurai

Posted

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@Far field Good clarifying questions!

All of the diagrams that I've put in this post are taken from the same tech sheet. So you're not bouncing around between different documents -- you just have to study the entire tech sheet and compare all the various schematics/wiring diagrams to each other.

As for how we came to the conclusion we did, all it took was focusing on the circuits for the door lock and the oven lights in the upper oven and analyzing their circuits. Annoyingly, GE decided to spread out the information about these circuits across multiple diagrams. But by looking at the information that all three diagrams provided on these circuits, we got the full picture.

It's not magic -- anyone can do it if they know how to read a schematic and how to apply basic troubleshooting principles.

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Far field

Far field

Posted

Ok so if we have the key, we can deduct the lock of the problem. Now it comes to getting the key (sheet with all the info) and 1 hope the answer is there on it. 2 logically follow the path of our circuit of interest 3 experiment or deduce the existing actions against the desired outcomes. I bow to you superior luck in finding all unknowns on the sheet.  Is there a way to deduce that a particular sheet has all the information needed? 

Far field

Far field

Posted

Ok son one more question about the diagrams. How can we lable these diagrams? And know this one is showing ? The compressor by control or heating on ovens or gas valve is the main board usually but are there exceptions to this? And the next diagram how to lable it? And the first one is called? Are they labeled by mfg as to their use ? Why do mfg want to play guessing games? So we will give up and the customer goes buy a new one! I knew that. 

Far field

Far field

Posted

The reason wires are shown in one stretch of wires I guess is because they are installed at the factory that way. Makes sense but a key telling where the numbers go to what next connection has to exist somewhere at one time. Again mfg, on full knowledge how this will fair in a repair issue, keep us searching. I guess I’m just dumb by environment.  I understand some models don’t use some features so the wire is left open. That option could be in dashed lines but that would warn us it may not be there. And with that said they would not go on into an option not built in. So if it fails replace the option. Like a rotisserie in an oven. And I’d agree about that. 

Far field

Far field

Posted

Le

t me know if I’m being difficult! I don’t mean to be. I just want a clear way to proceed that any dummy like me can recall and follow. 

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Son of Samurai

Son of Samurai

Posted

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On 10/19/2024 at 2:46 PM, Far field said:

 Is there a way to deduce that a particular sheet has all the information needed? 

You simply take all the information you can get. Sometimes the manufacturers give you plenty, and sometimes you have to work with scraps. You just do the best you can with what you've got.

If a tech sheet has a schematic, a wiring diagram, diagnostic mode entry steps, error codes, and component specs, then you've pretty much got everything you could hope for as far as troubleshooting goes.

On 10/19/2024 at 2:54 PM, Far field said:

Ok son one more question about the diagrams. How can we lable these diagrams? And know this one is showing ?

The first diagram I show in this post is a schematic. The second is the wiring diagram. The third is a second schematic which is focused on the control boards. This is somewhat non-standard. Normally, one schematic shows the whole picture of all the circuits in the appliance, which is why I found this tech sheet to be worthy of a blog post.

]They decided, for whatever reason, to show some loads and switches on one schematic, and some on the other schematic. By looking at both together, we get the whole picture. But normally, you're not going to have to do that, because everything will be laid out in a single schematic. 

Far field

Far field

Posted

Ok I get what you’re saying.  My question is how can a tech surmise the different boards on a sheet. You used your logic to deduce which diagram sent you to the next one … how or what point gave you the start of the trail? Obviously the main board has the power to it, but is that always the case? I believe going to the motor board then feeding to the main board may be at work in some logic somewhere? Then going on down to the next board? How did you jump to the second board ? And finally how did you go to the 3rd? It’s sure simple when you have the legends or like you, you understand which one comes next and how you know it does. 

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Son of Samurai

Son of Samurai

Posted

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@Far field I think a lot of your questions will be answered if you look at the tech sheet yourself. All the diagrams are right there in the same document. There's no special, secret information that I had to know here.

At every step along the way, I just looked at the diagram that showed the component I was interested in examining.

First, I looked at the ladder schematic, since that shows both the lights and the door lock motor.

Then, based on the information about a pinched wire harness, I looked at the wiring diagram.

On the wiring diagram, we noticed the switches by the door lock. Those weren't showed in the ladder schematic, but they are shown on the board schematic.

By looking at the board schematic, I could tell that those switches are door lock motor position switches.

 

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