Microwave Surface Light Failure

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I've got a GE Spacemaker microwave (Model JVM1631WB001), which is installed in a configuration over a stovetop. This model microwave has both separate lighting and fans built into the underside of the microwave to both illuminate the stovetop surface and to vent cooking odors (ostensibly). The lighting to the stovetop isn't crucial, but it's sure nice to have.

Some number of months back both underside light bulbs stopped working. Here's the general diagnostics that I've performed.

* the microwave itself still works fine, so obviously the problem was not with the main power source
* the underside ventilation fan is also still working, so no issue with the power source to that function
* I performed a continuity test on both light bulbs (both are fine)
* a voltage test on the actual lighting fixture contacts showed no current present (obviously)
* the switch membrane for the underside lights still produces the audible "beep" when pressed, and the display panel alternately shows Lights On or Lights Off
* nothing I see seems to indicate that the underside lights are controlled or protected by its own fuse or other internal electrical breaker

I think there's been a failure on the main power circuit board. I can purchase a replacement board (which I'm sure I can install myself) for about $115. I'm sure I can't replace the whole microwave for anything equal to or less than that amount. Can anyone find fault with any of my diagnostic approach or suggest anything I've missed. I know nothing is 100%, but I'd like to cover every base that I can before I purchase the power board, since I'm certain that it will be a one-way (non-returnable) purchase. I am heading in the right direction?

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[+3] - CDobyns - 11/02/2008This posting is a little past its prime, but I thought it would beneficial to give it "closure" if I could. After a seemingly very long period of watching for the correct power control...

If you have a schematic or possibly even trace back the wires from the light you might find the component that switches the light on and off. It's possible it isn't even on the main board since it would require 110v to pass through it.

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Good point. I don't have the wiring diagram/schematic, but I've actually pulled the switch membrane panel and traced the circuit from both the switch membrane and the lighting circuit to the power circuit board. So, I know that both the lights and the switch membrane have a common terminus on the power circuit board. Since the circuits from both sources lead to a common point, and I've reconciled what I think are the problems (or lack of problems) evidenced either upstream or downstream from these sources, hence the arrows (figuratively) which point to the failure of some component on the power circuit board. Other thinking or input?

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Most such microwaves have a bottom that swings down, after you unscrew three to five screws. The bulbs are easy to replace after that.

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I think you and parados are on the right track.

You've eliminted the bulbs and wiring as possible problems. The on/off switch still beeps but that is wired through a 12v/5v circuit. The switch itself controls a relay on the circuit card. When you press the switch a low voltage signal is applied to the relay to activate it. When the relay activates, the 120V power should be sent to the bulbs.

I'd look at the card and see if you can locate the relays. My guess is that the fan and light relays would be the same. If they are plug-in relays you should be able to swap them to assist in your troubleshooting. If they are solder-in then you are probably stuck with replacing the board.

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Okay, some good insights from fishin. I'm also leaning toward thinking that there's a problem with the relay switch. I'll take another look to see if the fan is controlled by a similar relay - and to explore whether the two relays could be temporarily "swapped". I need to look again, but I don't think that they can be interchanged, either because they are dissimilar or because they are hard-wired into the board. I can't remember specifically.

And for edgarblythe, you're of course right that the underside assembly does release by a couple of screws. That actually gives you access to the entire wiring assembly to the lighting group. The bulbs themselves (at least on this model) are simply accessed by releasing the one screw that secures the light/lens cover. All that advice was helpful, but you probably already noted that I'd checked the bulbs and they are still fully functioning (at least from a continuity standpoint). Thanks though.

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I misread the question. Thought you just wanted to access the bulbs. Embarrassed

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@edgarblythe,

This posting is a little past its prime, but I thought it would beneficial to give it "closure" if I could. After a seemingly very long period of watching for the correct power control board to become available on the market at a price something less than what a whole new microwave would cost (<$100) - just such a board popped up on eBay for $49. The GE Power Control Smart Board WB27X10257 arrived this week and it literally took ten minutes to unscrew the old board and remove the integrated connectors and reinstall the new board.

The moment of truth came when I pressed the surface light membrane button and voila (I love it when I get to use French), the lights both immediately illuminated! This just confirmed my diagnostic logic in trailing the problem back up into the power board. Best I can tell, some small component or capacitor or something failed, since there was a small discoloration on the metal backside of the membrane panel, like something had arced. Just goes to show you that you most of this stuff can be fixed if you're smart enough and patient enough to wait for the right part to become available - at the right price.

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@CDobyns,

Hi - I have the exact same problem with a Sears Over the range Microwave Model 82509 - works fine but underside light and fan are not working. Can you buy these smart boards at a retailer or just on E Bay. And how tough are they to install. Does the microwave have to come out?

Thanks

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@stoneham,

If this is the exact same problem, my best guess is that there's been some failure on the power control board (PCB) - especially if the rest of the microwave is still working fine. That tells me its not a blown fuse, or at least probably not.

Here's my best advice. Purchasing the PCB from the retailer will likely not be cost-effective - since it will probably sell for about $20 less than you paid for the microwave originally. I couldn't even locate your model anywhere on-line, but I've had good luck on Repair Clinic (http://www.repairclinic.com/), but you could also check Sears.com. The trick is to cross-reference the primary part number, since this same part is probably used in other microwaves other than just Kenmores. After that, you can troll for the board (if you want to wait patiently) to come up on sale (eBay, whatever).

In terms of the installation, that was remarkably easy. The cover, which is just the membrane, keypad and clock window, etc. , likely releases with just two screws at the top of the panel, and then lifts up and away from the microwave. The PCB is actually attached to the back of the membrane cover assembly. Did I mention that it is smart to unplug the microwave? After that there are typically three small screws that attach the PCB to the back of the panel membrane, and I recall there are two plugs - which just pop off. Reassemble in reverse. When I did it, it took me about 10-15 minutes.

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@CDobyns,

I'd just like to know how to remove the bulb! I've looked online and can't find any pictures that look like the bulb I have (w a black base) does the bulb push in a turn or...?

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@pinbuoy,

The model microwave we had, and the bulbs it uses are a threaded bulb base.

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@CDobyns,

Bonus points to CDobyns for keeping up with his own threads.

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@CDobyns,

I have this exact same issue with my kenmore model 721.80593401.. The underside lights stop working,but vent and fan still works. Should I order the circuit board?

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@Mstryingtofixit,

If you know unequivocally that the bulbs themselves are not the source of the problem - my gut instinct is to say yes. Although it would be a little extra work, you could try and confirm the failure on the power control board, by pulling the control panel and disconnecting the existing power control board and examining it (the soldered connector side). On my board, the arcing/scorched problem where the lighting circuit had failed was readily apparent upon visual inspection. That's an extra step, but it'll be up to you to gauge whether that's worth the trouble and the possibility of ordering a replacement board beforehand.

There's no guarantee the circuit failure will be visually evident - but it's literally about a ten minute investment of effort. My best guess, based on what you've described - is that it is the power control board. Good luck.

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I don't have to face this type of problem but I think there is some wiring problem.

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@Dean Haustead,

Interesting. My experience has been that absent some sort of application where wires exposed to movement or vibration, wiring is almost never the problem. And most of these consumer products are so well-engineered any more, that they already consider movement and vibration when they're designed. If a large electrical appliance, like a microwave oven, works for an extended initial period of time - and absent some kind "event" (dropping, falling, folding, spindling or mutilating), the most like cause of a failure is going to attributable to some electrical or electronic component (fuse, resistor, etc.). It's almost never a wiring problem.

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@CDobyns,

Thanks for all the information. I had a bulb that went bad and when trying to remove it the bulb broke. I tried to use a pair of needle nose pliers to take out the bulb base, but wound up breaking the plastic assy. I ordered a new one and put it in and got nothing. Disappointing. I am going to try the continuity test as you suggested and then begin the research to find a board.

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@Curly2,

I am having the same problem, twice in one year the bulb stopped working and when I went to replace it the glass separated from the thread base. My husband says it's a fire hazard to use now but I need my light for cooking! Trying to find a solution to this issue.

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@Agullins,

I've seen this situation, where the glass bulb when it is blown will sometime loose it's adhesion to the base - and pull free. I'm not sure how much of a "fire hazard" this particular situation is (a fire protection engineer at work assured me that nothing was going to burst into flames). That said, it does cause the bulbs wires to be exposed - and that is a (non-fire) safety-related issue, that should be corrected.

Not sure if this is a screw base, or spring-loaded base - but either can be removed from the fixture, with a little bit of effort. With the microwave unplugged, sometimes you can press the bulb back up into its base, to give you a little leverage to either depress the spring and turn the bulb - or to just turn the bulb, if it's a screw base. If the bulb pops off from the base it's not a disaster. Almost any small pliers or even tweezers are usually sufficient to allow you to get a grip on either the outer edge of the base, or by using reverse friction and expanding the prongs inside the base - should allow enough leverage to turn the bulb (yes, counterclockwise). Give it a try, as it's worth the effort - and it's less expensive than just throwing the microwave away (although I'm pretty sure no one would do that . . . ).

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Microwave Surface Light Failure

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