Erratic temp display on a Max-E-Therm SR400NA pool heater

nickinpalmsprings

New member
Nov 10, 2019
3
Palm Springs, CA
Sometimes from the moment I turn on the power to my 11 year old Max-E-Therm, the temperature value in the display randomly jumps around. It can be pretty stable at times, but when it’s really having this problem the value varies as much as 5 degrees up and down from the true temp with a new reading about every half second. This can happen even on a stone cold heater and before the burners fire. The heater will start up fine, but the moment the displayed value exceeds the temperature setting, it shuts off (as it should) even though the true water temp is nowhere near the target value. I can fool it for a while when heating my spa by setting the target temp to 106 (and it’ll heat without interruption for a long time), but once I set the target to something more reasonable, it’ll shut down once it gets close. Almost instantly after the shutdown, the heater will fire up again, presumably because it’s getting a lower-than-target value, run for a less than a minute (sometimes just seconds), shut down, get happy and fire up again, etc. Yes, this sort of shutoff cycle does sound sort of\kind of like a thermal regulator issue, but this cycling happens even when the intake water is chilly. Set the heater to 78 degrees, for example, and it’ll cycle like this once the water gets to 76.

I had hoped that replacing the thermistor (AKA temp sensor) with a new one would fix this, especially because it’s easy to do and the part is cheap, but I tried that today and nothing changed on the display. Still jumped around erratically, and eventually shows a temp so high that the heater shuts off. I thought that maybe the wires from the thermistor to the control board had issues, but I checked the resistance across the thermistor male electrical tabs and it was 8.88 something (micro ohms? I didn’t really pay attention to the unit display on the meter) and with the wires attached but pulled off of the control board, the resistance at the plug for connecting to the board was also 8.88. I guess it could be a bad wire or a loose plug to the control board despite my test, but doesn’t that usually just send the display to the max temp value when the connection to the thermistor is broken?

The membrane (the thin controller with the transparent window that allows the control board’s LCD numbers to show through) is a replacement Insync Engineering version that I put in back in 2015 and hasn't given me any problems since. But I assume that the membrane is simply an interface between humans and the heater, and couldn’t be the cause of the weird numbers. Is that a good assumption?

So maybe it’s the control board? Doesn’t it interpret the signal from the thermistor and translate those readings into what’s shown on its LCD? If the board was bad, it would explain why the processor thinks the water is at a temperature that it’s not.

Is a new control board in my future? I hate to drop $300 for a replacement if that’s not the case. Could it be some sort of wiring issue, where I’ve got an intermittent ground somewhere on the lines and as a result sending false sensor information to the control board? If so, any ideas of where I should look for such a problem?

Thanks for any advice!

Nick in Palm Springs
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,747
Northern NJ
Welcome to TFP Nick.

The Temperature Sensor is a standard 10K thermistor. The link has a link to the table of resistance it should measure.

It will read 77F at 10,000 ohms. You can get a 10K resistor or another value resistor and substitute it for the thermistor. If the heater temperature is erratic with the resistor in place then you know it is the board.

You can put the thermistor in a pot of water and heat it and compare the resistance readings to a thermometer.
 

nickinpalmsprings

New member
Nov 10, 2019
3
Palm Springs, CA
"The Temperature Sensor is a standard 10K thermistor. The link has a link to the table of resistance it should measure. It will read 77F at 10,000 ohms. You can get a 10K resistor or another value resistor and substitute it for the thermistor. If the heater temperature is erratic with the resistor in place then you know it is the board. You can put the thermistor in a pot of water and heat it and compare the resistance readings to a thermometer."
Excellent info. I checked the old thermistor (both old and new are showing the same problem) and while the values didn't quite match up to the chart (which could be due to many, many things, including my cheap BBQ thermometer and my ancient ohms meter), they were close enough for jazz. Most importantly, the readings were stable at both 75 degrees air temp and 121 degrees in water. The second part of your suggestion will be the winner: placing a 10k resistor across the ends of the wires from the control board to the thermistor (disengaged, of course). If if no variation there, I'll put that resistor across the two pins on the control board. No variation there means to buck up and shell out the $300 for the aftermarket board.

Whatever I do, I'll report back with the winning solution in case someone else has a similar problem in the future.

Nick
 

Pool Clown

In The Industry
Sep 5, 2008
1,995
Silicon Valley, CA
You can unplug the membrane to check to see if that helps. The heater should function just fine without it plugged in. Remember, it will do only what it was told to do before you unplugged it!
Quick question, Was the thermistor that you purchased an OEM part?
 

nickinpalmsprings

New member
Nov 10, 2019
3
Palm Springs, CA
No, the thermistor I purchased was from Optimum Pool Technologies, though it was advertised as part of a "Pentair MasterTemp & Max-E-Therm Repair Kit". Not sure its OEM (or non-OEM) status means much. Both the old thermistor (which was undoubtedly a stock Sta-Rite\Pentair) and the new Optimum Pool one exhibit the exact same problem.

Interesting to learn about the membrane as simply a signal sender to the control board. I'll try that trick to eliminate any sort of role the membrane might be playing here. If I'm still having problems at that point and the 10k resistor test is inconclusive, then it's the board.

Thanks!

Nick