GE P7 experts -- need your help!

[robinsondm]

Folks,

 

Ever since we've owned it, my 1966 GE J486 P*7 40-inch range has never done a great job of self-cleaning. I finally decided to measure the temperature during cleaning, using a digital thermometer and high-temperature thermocouple (rated to 1832F!).

 

During a cleaning cycle, the "lock light" came on at 550F, which makes sense, as that's the highest selectable cooking temperature. But then, the oven temperature peaked at 575F-580F and stayed there! As I understand it, the self-clean temperature should be closer to 880F! I see the "oven temp" light change slightly in brightness, presumably as the heating element cycles on and off.

 

The oven temperature sensor was replaced in 2012 with part WB21X158, to fix a non-operational oven, but my problems with self-cleaning pre-dated that fix.

 

What do you think is the problem -- the thermostat? Would anyone have the part number handy? Thanks in advance!

 

Dean

 

[danemodsandy]

Calling Dr. Bombay! Calling Dr. Bombay!

This is a job for John combo52 - come in, John combo52!

 

[robinsondm]

Brief update

I did a bit of online research, and realized the main oven doesn't have a thermostat. Instead, it has a sensor (the part I replaced in 2012) and a "responder." I may have purchased an extra responder back in 2012, when I had the earlier problem. If so, I can try replacing that part.

 

I'll also look on the back of the oven to see if there's a wiring diagram that might be useful. Again, any input from others is most welcome. Thanks.

 

[robinsondm]

Update

Well, I did have an extra oven responder, part number WB21X162. I swapped it in, but no change. If anything, the max temperature dropped about 25 degrees. I'll probably put the old responder back in and save the new one as a spare.

 

I found the range's wiring diagram and schematic tucked in an envelope attached to one of the back panels. I also did some reading on how the self-cleaning cycle works, including one of the original GE patents from the 1960s. Apparently, the "clean" setting connects a relatively low-value resistor into the oven temperature sensor circuit, effectively raising the responder's maximum setting to the cleaning temperature.

 

I think I've found the resistor that does this. It's a potentiometer, with a specified range (on the wiring diagram) of 110-242 ohms. I measured 195 ohms. When I have time, I'll try adjusting the potentiometer during a cleaning cycle, to see if that brings the cleaning temperature up to where it should be.

 

Is it possible the oven was never calibrated to the right cleaning temperature? Somehow, that seems doubtful. Or is it that the calibration needs to be adjusted after parts are replaced, and/or after they've aged several decades?

 

Dean

 

[combo52]

Non Cleaning Early P-7 Oven

Hi Dean, Its broken LOL, sorry I have been so busy to get back to you.

I have seen this problem before but not for many years, it is most likely a bad switch in the latch circuit or maybe a bad limit-lock thermostat, either of these parts will keep the oven from getting anywhere close to the proper cleaning temperature. If you can get your hands on a copy of the service manual for a range of this vintage it is not too hard to troubleshoot. The part it may end up needing may be NLA but I may have a good used or may be even a new part in stock. If you can not come up with the repair manual I can do some digging here but it may take a while, good Luck.

John.

 

[robinsondm]

John (combo52),

 

Thanks for the reply. I truly wish I had the repair manual for this range, but no such luck. Neither is there a relevant GE repair manual available on Automatic Ephemera. If you had a copy of the repair manual for this range, or a similar vintage, that would be awesome. But I know you said it would take a while for you to hunt for one.

 

Would it be useful for you or others if I posted snapshots of the schematic and wiring diagrams? I've been staring at those diagrams, but haven't had a Eureka moment yet.

 

You mentioned it might be a bad switch in the latch circuit, or the limit-lock thermostat. The latch circuit seems to function as intended, but maybe there's something wrong with it that's not obvious. Would the limit-lock thermostat be built into the oven responder? (The responder is sort of a thermostat without the sensor, which is separate.) As I mentioned, I replaced the responder but saw no change in symptoms. Thanks again,

 

Dean

[this post was last edited: 1/19/2014-15:46]

 

[robinsondm]

Another update

Earlier, I mentioned that I'd try adjusting the 110-242 ohm pot to see if that would raise the self-cleaning temperature to where it needed to be. The good news is that this pot is clearly meant to be adjustable. It's mounted on a small board with a cutout in the back for accessing the pot while the board is installed. Even better, the board is marked with a CCW arrow, and the words, "Increase, 1/8 turn = 70<span style="font-family: symbol;">°".</span>

 

<span style="font-family: symbol;">Unfortunately, adjusting the pot had no effect on my oven's symptoms. And the pot only moves about 1/4 turn in total, so there's no way I'd get an additional 300° out of it! I imagine it's a fine-tuning adjustment for the cleaning temperature, but clearly something else is wrong with my range.</span>

 

<span style="font-family: symbol;">As I mentioned above, I'd be happy to post the range's schematic and wiring diagram, if anyone thinks they could make use of that information to help. Thanks,</span>

 

<span style="font-family: symbol;">Dean
</span>

 

[robinsondm]

Found the problem

It was the "lock switch" -- not to be confused with the "latch switch," which seems to be working fine. The lock switch was not quite open, but it was introducing too much resistance for the so-called "bias rheostat" -- 110-242 ohm pot I mentioned earlier -- to do its job of shifting the responder's max temperature setpoint.

 

After I manually jumpered the lock switch contacts, the oven continued to heat beyond 575F (its previous maximum temperature during a cleaning cycle). As I mentioned above, the bias rheostat is adjustable for fine control of the cleaning temperature, and I set mine for a steady-state temperature around 882F. The oven is now running a 3-hour self-clean -- finally at a proper cleaning temperature!

 

John/combo52: I'll call GE later to get the part number of the lock switch, in case you might have one in stock. I'll also see if I can extract the switch, when I have time, to see if it might be repairable.

 

Dean

 

[whirlcool]

When our P7 oven was in self clean mode about 1/2 through it a fan would come on I guess to send the smoke and fumes up the vent pipe to the outside. We had the double wall oven and it was connected to a vent pipe to vent out the roof.

 

[danemodsandy]

John (combo52):

A copy of the repair manual on Automatic Ephemera would be really, really cool and helpful to a lot of people here.

If you ever get time....

Jus' sayin'.

 

[robinsondm]

It's clean!!! And, thanks.

Post-cleaning cycle report: the oven is beautifully clean! Spotless! Gorgeous! I can't believe I've lived without this feature for so long!

 

I must say thanks again to AW.org for making me realize that the self-clean feature SHOULD have worked better than it did.

 

John/combo52: thanks for your tips, which led me to keep staring at the latch/lock circuit diagrams until I had isolated the problem.

 

Now I just need to repair or replace that bad lock switch. I don't want to leave a jumper in place, or manually jumper it each time the oven needs cleaning.

 

[combo52]

YAY Dean

Was it the little round black switch in the rear of the lock assembly at fault?

I remember running into a 1967 GE P-7 oven that was behaving the same way yours was over 35 years ago and I ended up going to the GE service center and ordering the service manual in order to figure out the problem.

GE changed the entire control system about 1968 to a little solid-state board which had a whole new set of possible failures, and failures were fairly rare on these early ovens, except for bad oven sensors so we did not get to repair that many of these ovens.

Then by the mid 1970s they went a dual range hydraulic thermostat that controlled both baking and cleaning temperatures. This last system was the most reliable but I don't feel if maintained quite as responsive temperature when baking as the two earlier systems did.

Now all SC ovens employ a completely electronic oven temperature control system for both cleaning and baking. We find that now a days we get VERY FEW complaints about oven temperatures being off, and the ones that do complain just don't know how to properly take an oven temperature and usually don't know how to cook in the first place, LOL.

John L.

 

[robinsondm]

Yes!

John,

 

Yes, it was that little round black switch in the back! GE called it the lock switch. I'll call GE later to get the part number. Thanks again for your help!

 

Dean

 

[whirlcool]

It's been my experience that the P7 ovens are probably the best self cleaners out there. The one we had would burn off anything!

 

[robinsondm]

Fixed!

My oven is fixed! Below is a picture of the guilty switch. It reminds me of the switches GE used as door-opening sensors on my 1966 fridge. The switch tested okay - open when the plunger is in, closed when the plunger is out. But as you can see, the contacts were very dirty. I couldn't tell if it was corrosion, or carbon deposits, or both. Anyway, I cleaned the contacts, and shot a bit of contact cleaner inside the switch for good measure.

 

I reassembled everything, started a new self-clean cycle, and... success! The oven is now holding a cleaning temperature of 880°F! I actually had to tweak the temperature setting again, because it initially settled at ~900°F, probably because the newly-cleaned switch presented a lower resistance than the jumper clips and wire I used yesterday. But would you believe GE added a grommet-insulated access hole on the back of the oven, so I could use a screwdriver to tweak the cleaning temperature without removing the back panel? Yes, they did!

 

So, John, I don't think I'll need any parts this time. It WOULD still be great if, as Sandy and I mentioned, you could retrieve the service manual for this range (1966) and allow it to be uploaded to Automatic Ephemera! Thanks,

 

Dean

 

[swestoyz]

I have some service lit from around 64/65 for the P7 cleaning system. I could publish some of them here, or send them off to Robert to add to AE.org.

Ben

 

[robinsondm]

Ben,

 

Thanks for the offer to make your 64/65 P*7 service literature available. I'll leave it to the site owners/moderators to decide, but I assume uploading to automatice.org is preferable, as that would control distribution and generate some revenue to help maintain the site. Thanks again,

 

Dean

 

[danemodsandy]

Dean:

What Robert does with ephemera is to scan it himself, do any needed cleanup and then upload it. People send him ephemera, and Robert sends it back when he's through.

That's why the literature available on AE is so immaculate; Robert's pretty obsessive about how it gets scanned.

This is not to say that Ben couldn't do a helluva job....

 

[robinsondm]

Sandy,

 

Regardless of who scans the manuals, I thought it would be preferable to make them available on AE.org, rather than just uploading pages to AW.org. Is that right?

 

Dean