I started the AC, and smelled burning bakelite plastic, nasty smell.
The potential relay never opened the start circuit, so kept blowing the breaker open.

Opening up the relay box, there is a coil. The coil due to back EMF as the electric motor spins up is supposed to pull in and open circuit the start capacitor.
It looks like the way this is made, they rivet the screw lug to the inside lug connected to the coil and the riveting mechanism rusted, built up high resistance and could not connect internally to the coil. This is the 3rd potential relay in about 10 years that has failed. In past times when the relay fails, the start cap would blow, explode, but the last time I bought a 330 vac start cap so it does not blow anymore, but it does get hot!

So I ordered another relay, this one I plan to solder the rivet to the interior lug connection that hooks up the coil wire. And solder the rivets connecting to the start capacitor wires. And also dip the whole thing in engine oil as an attempt to keep it from corroding on the inside.

This is a MARS 65 relay, made by Mars and it is garbage they way they make them, they plan them to blow up on you, that way they keep on selling another part.

So much current flows through the start wires, they overheated and burnt up the bakelite and the interior was severely overheated. Pictures shows damage, and I had started to loosen the screws too. The bakelite fell apart in my hands.

It had worked fine for 2 years till yesterday. The rusted screws are not internally connected, just a neutral connection point.
The 2 screws on the right form the start capacitor circuit.
The double twisted wire connects to the coil, (why it is 2 wires is beyond me, but someone did that)
The internal relay contact points were fine.

No connection except for riveted connections inside the relay box. A fail, from a consumer viewpoint, a success from a manufacturing and sales viewpoint. Overheated screws bake off the zinc plating, then the crews easily rust, the whole thing will rust inside and develop high resistance, high resistance makes lots of heat with lots of amps flowing, and it self destructs, burns up, success! , another sale made, money keep flowing in to MARS, GE , etc....

One year I tried a universal electronic relay, it worked for about a year, then failed some semiconductor device.

 

You can't blame the relay because it is positioned where, along with its wiring, it gets wet. You can't expect an armature that has rusted to allow the contacts to make properly. Power relays are rarely completely sealed because sealing is incompatible with reliability unless special precautions are taken and that runs the cost way up.

The wiring looks like a mess. The fork lug in the upper right corner isn't properly under the screw. I'd have doubts about the quality of the crimps on the lugs. Black wire connecting to white wires?

If I were confronted with this, I'd probably completely replace most of that wiring, then install the new relay with protection against getting wet.

 

It gets humid and warm, it is in a boat in the bilge next to the gen.
The black on the white, well that is the reversing valve, coming from that valve OEM are 2 black wires, so that's the way it is. It is a simple electromagnet.
There is a rain - drip cover, it is off for the photo.

The lug I loosened before the pic, and undid the relay so it is hanging loose from the box.
Those wires lugs overheated and the bakelite burned to ash around those lugs. I took off those wires, ripped off the melted insulation, cleaned them in vinegar, then soldered the wire to lug and tinned lug with solder, they are actually ok. Plan to cover with heat shrink tubing.

The relay is made poorly. As stated, they riveted the screw inserts to the relay frame, when they corrode, they get high resistance and it fails.

I gave some thought to using push on connectors, but that makes me think even worse connections than the screws.

The compressor draws 10 to 12 amps depending on temperatures.

New relay will be here saturday, so likely will have it fixed this coming weekend.

 

That installation looks pretty bad.
For a start, try mounting the relay up higher in the box, in a better location to protect it a bit, and clean up the wiring.
One option that may work is, after you get it all working again, spray the relay with clear lacquer to help waterproof it.

 

Cruisair put the relay low in the box, above the relay is the high pressure cut off switch.
To left of the relay, is the start and run caps. There might be room above the caps for the relay.
Interesting idea to move relay. Being above the caps might help warm it and dry it better.

 

Soldered the rivets, Now the connection is more enduring. And I dipped the entire relay in 5w050 oil, then dried off excess oil and cleaned point contacts.

It should hold up longer now.


Connections 4 and 6 are electrically nothing to the relay, just places to connect wires together. Only, 1, 2, 5 are of internal functional importance. Those control the cut in and cut out of the start windings in series with the start capacitor.
1 and 2 are connected by the contact points, 2 and 5 are connected through the coil. 3 does not exist.

 

Those wires lugs overheated and the bakelite burned to ash around those lugs.

Were these screw lugs? If so, this implies that the screws loosened over time. The increased resistance of the connection causes excess heating.

 

Installed new relay and greased up some bits with marine grease which does not easily wash off.
I moved the relay up high above the run capacitor, that was a good idea, and is plenty of room to do that. High pressure cutt off switch is to right of start relay.

Some Cruisair heat pumps came with super tall run caps and using them would not have fit.
Fixed up all wires and one new lug on one wire running to the magnet coil for the heat pump to AC flow valve.

All 5 wires on terminal 6 are neutral return type wires. For the water pump, the change over valve, the compressor run capacitor, start capacitor, neutral return wire.

Click to see large picture. I feel like this relay with my mods will work better longer.

Runs good

Link to running again
https://photos.app.goo.gl/TryMNiVibETARFTd8

 

Well done