1950 Frigidaire MM-76 too cold

catflopmop

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Portland, OR
Model Number
MM-76
Hello all! I've been scouring this forum and read a lot of posts that seem close to what I need, but I figured it wouldn't hurt to post my own.

I recently purchased a Frigidaire 1950 MM-76 from a dude's crane warehouse. From what they said and the looks of it, it was an employee's drink fridge at one point but had been unplugged for years.

I was very careful with transport, never tipped on its side or back. I cleaned it up and repainted before plugging it in and turning on. It roared to life right away. My questions:

1. It gets really cold. Like freeze ice in a glass cold, regardless of the setting. From my reading here this seems like an issue with the thermostat, yes? Or possibly the attachment of the sensor on the evaporator? I have attached pictures.
1a. The freezer compartment door is broken off. I've kept it set over the opening, but it isn't attached to the spring hinges. Would this affect the temperature sensor?

2. Sometimes it short cycles. So it'll start up, then turn right back off. Sometimes it does that several times in a row. Sometimes it'll do that a few times then keep running. Sometimes it starts up and runs with no issue. Is this a relay switch issue? Or? How much of a concern is this if it's staying cold?

3. This thing is noisy. I noticed that the compressor is attached by two big springy screws and the noise was mostly coming from vibrations. I added some felt on either side and tightened the bolts so there is no more give in the spring. But... is there supposed to be give? Seems like it's springy for a reason. Or was tightening it down the right move?

I attached pictures of all of this. Thank you in advance!

IMG_20210914_143015262.jpgIMG_20210912_162015685.jpgIMG_20210914_143005358.jpgIMG_20210914_142940916.jpgIMG_20210914_142926245.jpgIMG_20210914_142853929.jpgIMG_20210914_142828260.jpgIMG_20210914_142814787.jpg
 
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rickgburton

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the thermostat, yes?
Yes
The freezer compartment door is broken off. ...... Would this affect the temperature sensor?
Not if the door is over thee opening.
it'll start up, then turn right back off. Sometimes it does that several times in a row.
Sounds like the amp draw is opening the overload or hotwire relay depending on what that compressor uses. It could be a weak relay/overload or it could be the compressor start windings are wearing out. On those older systems, the compressor can't start against high head pressure. If it's been running then turns off it won't restart until the two pressures equalize, 30 seconds to 1 minute. Unplugging the machine and plugging it right back in without waiting a minute can eventually damage the compressor.
How much of a concern is this if it's staying cold?
Depends on what it is.
noise was mostly coming from vibrations.
Replace the rubber grommets. The ones on there are probably as hard as a rock.
 

catflopmop

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Location
Portland, OR
Thanks Rick! Some clarifications/questions:

Is there a way to test if it's the thermostat or the sensor? From the photos, does it look like it's in the right place and should be sensing correctly?

I haven't ever unplugged and replugged to get it going. Just left it plugged in and let it do its thing. When it turns on and right back off, it does always wait 30-60 seconds before starting up again. Is there a way to test if it's the relay or the compressor start windings? Is there a sound to be listening for?

So tightening down so there's no spring is okay, once the rubber grommets are replaced?

Thanks again!
 

rickgburton

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The thermostat and capillary is one unit. The capillary is filled with an inert gas that expands and contracts against a diaphragm inside the thermostat. If the capillary gets pinched or broken the thermostat contacts stay closed all the time and the thermostat won't cycle off. The end of capillary mounts against the evaporator with an aluminum or steel bracket that has grooves in it so the capillary doesn't get pinched. Here's one style:
Cappilary bracket t-stat.jpg

Is there a way to test if it's the relay
What style relay/overload does the compressor use? Use an amp probe and check the amp draw when the compressor starts. The start windings should measure between 3-11 ohms and the run windings, 1-5 ohms.
So tightening down so there's no spring is okay,
The springs are for the vibration. Most of the mounting bolts I've seen have a long shoulder and maybe a 1/2" of threads so it doesn't compress the spring and grommet when it's tightened. Without the "give" in the spring and gromit, the things going to be noisy. But hey, they're just mounting bolts so do whatever you can.
 

catflopmop

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Location
Portland, OR
Hello again! After reading a bunch of other threads and comparing part numbers, we bought an RO81 relay. We're working on wiring it in, but we're struggling to figure out which wire connects where. I read in other threads that wiring it incorrectly can fry the whole compressor, so, being cautious.

Here's a picture of what we think is right:

IMG_20210919_105951197.jpg

The packaging on the RO81 shows 3 wires going to the compressor labelled R (run), C(common), and S(start). The old relay had a different set of labels for the wires connected to the compressor: L, M, and S, with no clear decoding. How do the old relay's L, M, and S wires translate to the new R, C, and S?


IMG_20210919_110742573.jpg 16320749067292356247213758938450.jpg 16320749535585177359864501635878.jpg

Also, the new relay has two wires labelled 'line'. Where should these connect relative to the old relay? Thank you so much for your help!
 

catflopmop

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Messages
5
Location
Portland, OR
Thanks, Rick! When we labeled the original wires before removing them, L was not connected to the compressor, but to either the door switch or thermostat.

IMG_20210919_102634532.jpg


So following your L=C, this what we have hooked up:
IMG_20210919_133225358.jpg

So that means that black wire from the grouping of three wires at the bottom of the new relay is connected to the old 'L'. So now we're confused about where the two wires on the top of the relay (labelled 'Line' in the new relay's diagram) should go. Do these just connect to '1' and '2' from the old relay? Thanks for being patient with us!
 

rickgburton

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No, don't use that wire on the compressor. Take the three wires between the old relay and compressor off and discard them. The three wires on the new relay will connect to the compressor. Don't go by color either. This is what you have, correct?
R-Early relay.jpg


This is how you're going to connect the RO41: I made the pics so you can't go wrong.


RO41 wire.jpg
 

Old Fridge Guy

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During normal operation the two compressor bolts should be loose and allow the compressor to float on its mountings. They should be tightened down to ship the refrigerator to a new location. Such as if you move to a new house. Normal setting on the cold control dial is number one. The A, B, and C settings are vacation settings for if you'll be away from home for a few days. Safe temperatures will be maintained because no one is opening the door during that time.
 
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