I have an older (20 year or so) Brett Agualine EM203T, I have had the spa pack rebuilt several times. The last time just a few months ago. But now I have this problem. The hi-limit trips and of course the heater shuts off. I then find a cold tub later in the day or the next day. Here is what I have troubleshooted so far:
1. Pressure switch, it turns out was wired backward and wasn't engaging the pump correctly. The switch is good and works perfectly. It was once believed to be bad.
2. Heater element, has been bench tested, and deemed good. It does heat the water and shows all the correct ohm/volts/amps.
3. Contactors, have both been checked from a not physical point. They seem to open and close when heat is called for or not. The thermostat operation seems to allow the heating element to receive power when it is called for and disrupt power when it is not needed. I have not taken them out to physically check them.
4. Thermostat, seems to work however sometimes I can turn it all the way up and it does nothing. Just the ready light stays on and not the heat light. Even when the tub is under 90. Then I have to cycle through the functions with the thermostat in off to get it to work again. I can hear the thermostat click when I turn it. When it is working (or seems to be) and I turn it to heat, the pump kicks in and the heater starts to heat. If I set it to a real low setting, like barely above ambient temp, it seems to kick on and off correctly, but if I set it above like towards 100, thats when it seems to have a problem. I had also adjusted the thermostat to shut off sooner (at a lower temp) in hopes I could control it and keep the temp down.
I don't have the time to watch the tub for a day and monitor the temp and see what it does, but I am leaning towards the thermostat. I have pulled it out of the well and put it directly into the tub to see if it was possibly the well that was corroded or damaged, but it made no difference. In my hand the thermostat turns on and off like it seems it should. But that is a simple test and not an accurate temperature test. From my schematics, the thermostat seems to control a relay that allows the pump to operate and circulate the water to heat. It also seems to switch the line from ready to heat causing the heater contactor to close and allowing the heater to run. When I watch my meter on the heater element, it seems to get power when I turn it up and the pump starts, and no power when I turn it down and the pump is off. This indicates the contactors are working, correct? So it must be the thermostat? It is the original thermostat and like I said it is about 20 years old or so.

Thanks for any suggestions or other possible items it may be.

if your high limit switch is kicking off, either your high limit is weak or you actually have an overheating problem within the heating tube.

Well, when I did get out there before it tripped one time, it was over 110, well it was way past the 104 mark on the thermometer in the tub. I am assuming that it is atleast making it to the 120 kick-off.

I don't think it is an overheating problem in the well as I have it in the tub directly and it still has this problem.

I did notice this evening, with the pump off, I hit the hi-limit switch and could hear a buzzing noise. I am not sure if it is the contactor for the high-limit. But I will leave it as low as I can before the heater turns on and see what it does tomorrow.

in light of this new information, the tub going past the thermostat's setpoint, then yes, I think the thermostat would be the place to start looking.

Well, I put the tub into freeze mode, our outside temps have been in the 100's and so the tub isn't being used. I have it circulate at 4 hour intervals for 30 minutes at a time. Today (3 days since the hi-limit tripped) I noticed that it had been tripped. I am going to test the heater again, but I am now leaning towards a contactor being bad. I notice a kind of motor startp whining when I hit the switch with the pump running (not the pump I know that for sure), and a buzzing noise. This I have heard signifies that it is a contactor or a relay.

What is the best way to test a contactor? There should be zero volts when I test it in the off position correct? not even 5-10 volts? Could an arc cause a small leak?

Thanks

Tore the controls down to the contactors (wish they were easier to get to), and found that the heater contactor had small pits, but the pads were discolored. I was able to dissasemble the contactor and file the pads down to a clean shine and reassemble it. I have put it all back into service and it is currently heating up. We will see what it does after this. My next concern is that I bent the thermostat wire to many times and may have broke it. It does turn off and on when I turn the knob, but we will have to wait and see. Current tub temp is 84.

a broken capillary tube on a mechanical thermostat will render it useless... be prepared to have to replace the t-stat

I am pretty sure I broke the line. I smelled a solvent smell, and now I can turn it to just above the cold/hot line. At this line (about 1/2 the scale) it turns on and off. The tub does heat up, but it seems as though the thermostat is now a goner. Does anyone have a good cheap place to get the following t-stat?

SPDT 60" x 1/4" x 3.50"
Has 3 terminals

new problem? I have since played with the contactor, and I can hear it click on and off and no more buzzing noise, however, I looked at it again today and one side was discolored again. The low speed pump wouldn't turn off, and when I do get it to turn off, I still have 120v at the heater. Is this normal? I don't think it is. The heater when on has 240v. When I ask for heat is when it should have power and not any other time. I can also hear a sizzling noise in the control area when the pump is off. When I trace my wires, my incoming L1 and L2 go straight to the contactors. L1 goes to the high-limit contactor and passes to the heater contactor and then to the heater element. L2 goes to the high-limit contactor and then to the heater element. When the high-limit is working it should pull the high-limit contactor closed allowing power to pass through the contactor correct? That should be at any time the tub has power to it from the house correct? Now then, the heater contactor is controled by the thermostat. When the demand for heat is on, the contactor should close allowinfg power to pass through it. Therefore creating the 240v to heat the water. But at no other time should there be power correct?

I am totally stumped and can't figure this thing out. Aside from replacing the spa pack where should I start?

This is a tub wiring diagram for the earlier post.

heater contactor after cleaning