Mastertemp 400 Heater bad temperature reading

acastal

Member
May 12, 2020
8
Northern CA
I have been trying to get my heater working reliably. I have been looking through this forum and was hoping for advice. The temperature in the display of the heater is showing much higher than the actual pool water temp. It more often than not reads above 104 and therefore cuts off the heat. I bought a new thermistor and hooked it up by removing the wires from the installed/old one and putting it on the new one just hanging in the open 60 degree air. The new probe also measures an inaccurate temp of 90-102. At some point there were rats in the unit and some wiring was damaged. I bought a new wiring harness and hooked the new thermistor with the new wires only (I did not replace any of the other wiring) but the problem still exists. Does this point to a bad control board? Anything else I can check?
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
19,661
Northern NJ
Welcome to TFP

I think you have a resistance wiring problem maybe caused by the rats and maybe by the new wiring harness. The higher temperature reading indicates the circuit has higher resistance then it should. Thermistors work by varying their resistance with temperature.

Start checking wiring with an ohmmeter and look for areas that should read 0 and instead show resistance. Rats urine often creates corrosion and high resistance in wire connections.

@Pool Clown @ps0303 @swamprat69
 

Pool Clown

In The Industry
Sep 5, 2008
2,155
Silicon Valley, CA
The wire from the thermistor runs directly to the board. If you have a new wiring harness, a new thermistor, both are OEM, AND the harness is connected correctly, then yes, question the board.
The OEM wiring harness is pretty good with respect to being able to hook it up correctly. Wires are length specific, so it would be hard to hook the stack flue sensor wire to the thermistor and vice versa,
but not impossible.

What temp are you getting on the heater display?
 

acastal

Member
May 12, 2020
8
Northern CA
With the new wiring harness and the new thermistor hanging outside of the heater in 65 degree weather (the old one is still installed since I was thinking to return it if it's not the issue) it reads anywhere between 92 and 102. The new wiring harness did not make any difference in the reading so I put it back away hopefully to return and get my $200 back. If i switch back to the installed thermistor with 90 degree water it reads 115.

I found this odd: the temp on the new not installed thermistor reads 92. I turn off the heater and switch it to the old installed thermistor it reads 115, I turn the heater off and switch it back to the new thermistor and it reads 102?

I bought a new ohm meter which should arrive Friday to check the (old) wiring. Thanks for all your help!
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
19,661
Northern NJ
Last edited:

Pool Clown

In The Industry
Sep 5, 2008
2,155
Silicon Valley, CA
I found this odd: the temp on the new not installed thermistor reads 92. I turn off the heater and switch it to the old installed thermistor it reads 115, I turn the heater off and switch it back to the new thermistor and it reads 102?
Take a hard look at the old temp sensor. Does it have any cracks in it?
If the temp display is jumping around, i'm leaning toward the board. Providing that the new temp sensor is OEM.
 

acastal

Member
May 12, 2020
8
Northern CA
I bought a new from this decade ohm meter and some 10k ohm resistors. I tested the old/installed thermistor and it is reading exactly right. I did not test the wiring but when I install the 10k ohm resistor at the back of the control board (by sticking the resistor in at the connector and removing the clips to the thermistor at the other side) and boot up the heater it reads initially 92 and then ramps up to 100-102. Bad control board?
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
19,661
Northern NJ
I bought a new from this decade ohm meter and some 10k ohm resistors. I tested the old/installed thermistor and it is reading exactly right. I did not test the wiring but when I install the 10k ohm resistor at the back of the control board (by sticking the resistor in at the connector and removing the clips to the thermistor at the other side) and boot up the heater it reads initially 92 and then ramps up to 100-102. Bad control board?
Yes, bad board. 10K resistor should read 77F
 

acastal

Member
May 12, 2020
8
Northern CA
Replaced the board today and all is well. new board is rev 14 and the display does not align as well but it's all working and I am happy and thankful for everyone's help!