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Late 1940's GM Frigidaire Refrigerator ML-77 Starting Compressor Relay Question


Premium Member
May 17, 2021
Holt, Missouri
Model Number
Hi All,

New member here and I had a question regarding my GM Frigidaire ML-77. So, I started working on the fridge a couple weeks ago and noticed the starting relay wouldn't stay engaged. So, I insulated all the breaks in insulation I could find, re-terminated & marked the wires on the compressor, and found that I had loose connections on the starting relay. My relay has a part number of 5516701 and a date code of 10-30-61. Correct me if I am wrong but, this fridge is a late 40's vintage so, I am assuming I am dealing with a replacement part. Once I verified connections and reapplied power the compressor would initiate to start, and that's it. I looked up the relay and cross referenced it to a Mars 21310 solid state relay/ over load. I've read the thread titled 1955 GM Frigidaire Refrigerator New Relay Wiring Problems and found myself having more questions. Is my wiring the same and Cooter's in that thread? Is the relay I had & source insufficient? Any help is welcomed. My apologies if this is a busch league question.

Old Relay: Delco Hot Wire Relay - 5516701 (10-30-61)
Replacement Relay - Mars 21310 Solid State Relay - Overload
Fridge: GM Frigidaire ML-77

Your Frigidaire is a 1949 Master Series. If the wiring diagram that was originally on the back of the refrigerator is gone, here's one for you to refer to. I would go ahead and use the Mars replacement relay. I would also recommend replacing the wiring that goes from the compressor to the relay if it is in poor condition. The terminals on the compressor unscrew to be able to remove the wires. Use the wiring diagram for proper placement of the new wires. You should use 16 gauge wire.


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Rick, it was like the compressor would try to start. Almost like trying to start a carbureted engine not getting gas. I suspected I could have a bad compressor so, I took the following readings (below). Using the drawing OFG supplied, and using the drawing you supplied in an old thread titled 1955 GM Frigidaire Refrigerator New Relay Wiring Problems, I marked my wires as follows.

Originally W = Red
Originally R = Neutral
Originally B = Black

Static Resistance
Red to Black = 5Ω
Black to Neutral = 6 Ω
Red to Neutral = 9.2Ω

Megger Reading @100V to Ground
Red & Black = 110MΩ
Black = 110MΩ
Red = 110MΩ

To clarify, I have not installed the Mars 21310 relay yet, wanted to make sure my wiring was correct before wiring this in. I recently read that a Supco R081 is another suitable relay as well.

Appreciate the help,


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No problem Fish, If there's no start capacitor on the compressor, don't add one. Check the hp on the compressor tag if it has one. RO81 might be too small. Most of those older compressors were around 1/4 hp. If you can't find the hp anywhere on the machine go with the RO41
Rick, both that I am working on have 1/8 hp compressors. Funny, I work in industrial automation doing motor controls 1,000hp and up.. just makes me laugh when I see fractional HP.

I will update when I rewire and have both running. Any suggestions on a replacement door witch? I have read on other sites about using door switches out of cars but, figured, I would ask here. I took the 2 I had apart and made 1 switch out of 2 but, the more I think about it, I would like to have a back up plan if the 70+ year old switch fails again. Lastly, on the motor mount bolts on the compressor, did they have rubber grommets as well as the springs?

Any suggestions on a replacement door witch?
Can you take a pic so I can see what it looks like?
did they have rubber grommets as well as the springs?
I can't remember, LOL. Some motor mounts were rubber and spring but most were just rubber. Looks like yours is rubber with an internal spring?? Hard to tell from the pic.
The plunger on the switch was stuck and the previous owner had the switch wired breaking the hot which, would have never worked. She was crispy.

When I took the switch apart and cleaned the contacts & soldered on new wire, it’s not the most robust. I’m confident it will work just not wanting to roll the dice with taking this thing apart again with the plastic trim pieces

Only markings on it say .75A /125v/GM.

The mount looks to have had rubber at some point.


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There's a lot of light switches that will work. What ever you have the room for will probably work. Search for momentary on/off light switch for refrigerators.
light switch 3.jpg

light switch 2.jpg

light switch-1.jpg

The mount looks to have had rubber at some point.
You can use just rubber. As long as the compressor is sitting on an inch or more rubber you shouldn't have a problem with vibration.

Question- If we go off the original wiring diagram for this fridge, I have a total of 4 wires (2 marriage cables). If I go off the original color scheme, I have 2 black, 1 white, and 1 red. I am really not wanting to tear this one apart. (This is the one that only needs a relay. Just wondering how this will wire to the RO81 when It arrives. Any suggestions?

OK, you lost me. What wires are you referring to?? What are they connected to?
The two marriage cables I am assuming go to the door switch & thermostat. Going to see if I can ring out the wire because, honestly, I don't understand why I have 4. 3 makes sense... 4 doesn't.
Looks like you have an early '49 model. Referring to the original relay, the two wires for the light should be connected to terminals 1 and 2 on the original YT style relay. The wire from the door switch goes to terminal number 1 and the wire from the light socket goes to terminal number 2. The other two wires are for the thermostat and should be connected to terminals L and 1 on the original YT style relay. Look closely at those two wires. One should have raised ribbing on the wire outer insulation. The ribbed wire goes on terminal L and the other thermostat wire with no ribbing goes on terminal number 1.
Hi old guy, Fish isn't replacing the the YT relay with another YT relay. My wiring diagram is for replacing the YT relay with the solid state relay.
Hotwire-Solid State no fan.jpg

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