RDspaguy

From your description it is likely a flow or sensor issue. Is the tub actually getting too hot overall? Bold statement. How was this determined? Remove circ filter (the closest to the pumps) and test. Check circ pump for obstructions. Check circ for loud or hot operation (you should barely hear it and should not jerk your hand away when you touch it). Test thermistors (unlikely but I'd do it anyway).

The white wire determines component voltage anx connects at the main power terminal block for 240v or 120v. Just follow the other white wires to find them.

The heater should not be effected by a leak unless electrical components are wet.

What is that red wire supposed to be connected to? Does the motor start in low and run fine, only having an issue when you switch down from high, or will it do the same starting in low?

Sounds like a stuck/shorted button. No way to fix it that I know of if it's not wiring or board connection. Check back of board for discoloration at the connector. Brush lightly with wire brush.

Maybe. Post pics of your circuit board and wiring diagram.

Those are usually air intakes for the jet air control valves or manifold. Either way, it should go up under the shoulder above water line. This becomes pressurized from damaged or improperly installed jets. Under normal circumstances this is an air vacuum line.

What kind of a racket? Test voltages. If the board is switching correctly it's the pump 99%.

Check the controller wiring for mice damage.

⭐️ Good God, man! Get a new one! Just kidding, new motors are crap compared to that one. Take it to a shop for a rebuild. So, pump manifacturers don't make motors, and you're looking for motor parts. Shop by motor brand (century, emerson, ect.) and mod# for motor parts.

There are 3 led lights on the board, which are lit?

Definitely pull the filter. Filters can collapse under high pressures, greatly reducing flow. There also may be a check valve in the system that could be damaged and closing under high pressure. Most SDs from that period are plumbed such that a collapsed filter would effect all the jets on that pump, not just the heat return. Post the pics.

Post pics of equipment area, circuit board, and wiring diagram. As I recall, that circ pump pulls off the pressure side of the jet pump, so turning on the jets would force water through the circ pump and heater. This is how the circ is optional, as the main pump will feed the heater too. Which makes this even more weird than you think.

Turn on power and check the 3 led lights to the right of the "3rd board". Which are lit? If none, turn off power, unplug controller, and turn on power. Now which are lit?

Where does it get power? Does it heat, or just tell you it's cold? In Tahoe, we would run a separate 20 amp gfci circuit to the spa and mount a plug in the equipment bay for a space heater. If running, the equipment bay will be warm, but if it gets cold the space heater kicks on. If you get power from the spa pack a gfci trip still screws you, so the extra circuit is crucial. Of course a power outage will still be an issue, but you can't have everything.