Another Mastertemp 400 question ....

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T

TX-Pool

Member
May 10, 2025
11
TX
Hello Friends, need help with a non-functional MT400 heater.
I've searched online and might have missed somewhere that addresses this specific issue.

Info
- Inground pool built in 2016, Pentair equipment. As mentioned, MT 400 natural gas heater
- Using the Intellicenter with EasyTouch controller, no phone app installed.

Issue
- Sudden failure of heater not igniting, no blower either. Everything worked fine about a month ago.

Observations
- When SPA is selected on the Easytouch controller, both auto valves rotate normally from pool to spa settings
- VS pump changes speed as it has done in the past
- Heater control panel goes through what I believe is a self-check, shows the current software (R13 I believe) and then reverts to the selected heat setting on the panel. This is different from prior where it would indicate the current (before-heating) spa water temperature. I can manually select up or down on the panel to change that number. Before, I could simply observe a steady water temp increase reading on the panel as the spa began to heat.
- No warnings on the membrane touch panel (service system, service heater, etc.) just a green led showing heater on. However, the service system/heater led's do light initially, again, I assume as part of the first start self-test.
- No green "heating" led showing on the membrane touch panel
- Control board has no warning led's lit on the back, just the green operating led.
- Upon initial SPA selection, all red leds on the back of the board light up momentarily, then are dark, I believe this to be part of the initial self-check.
- When SPA has been selected, other than the valve position changing, nothing happens at the heater, no blower, no gas on, no ignitor, all dead.
- When SPA selected, I cannot switch between pool and spa on the panel, just stays in spa mode, maybe this due to the Intellicenter controlling the heater? I have not tried the V (Valves) setting at the Intellicenter itself.
- These results are the same whether I select SPA on the EasyTouch or at the Intellicenter panel.
- I've attached a picture at the molex connector from the membrane pad to the control board. I'm not sure but it almost looks like a transistor (?) may be burned at J6-6 circuit? But I also see some other darkened components, I'm not sure.
1000009552.jpg

What I've done
- over the past several years I've addressed stuff like short-cycling, flue gas, water flow switches etc. The heater has generally run fine and I keep the pool filters clean. I have a general understanding of the guts of this thing.
- A couple of years ago I changed out the membrane pad on the heater
- checked that gas inlet valve and gas valve are on
- verified 24v out of secondary transformer
- verified main power to the heater and to the ignition module and related connectors at the power box inside the heater
- verified continuity on both sides of a glass fuse at the firemans switch
- verified heater-related connectors are ok at the Intellicenter board in the panel itself
- verified wire integrity inside the heater itself, no frayed/chewed wiring
- verified that there are no red trouble leds lit on the back of the control board. Since they all cycle initially I figured that they are operational.
- unplugged membrane connector at control board, jumpered J6-5 to J6-6 to see if heater would activate, nothing.
- Even though no trouble led's were lit, I jumpered a few of the switches, such as flow, flue gas, etc just in case they impacted this no-heater issue, no change at all.

Any help would be appreciated; I'm suspecting the control board but wanted to ask more knowledgeable folks first.
thanks!
 
Welcome to TFP.

The first thing to do is restore the jumper on the fireman's switch so the heater is controlled locally from its control panel and not through the EasyTouch.

Work on getting the heater to work locally before connecting it to the EasyTouch.

Do that and take a video of the heater control panel startup. Post the video on YouTube, mark it Public, and post a link here.
 
Thank you.
A silly question, when you say to restore the jumper are you meaning to remove the fuse that's in place?
I'll try to look up info on the fireman's switch jumper - I think that's the description I saw for heaters as shipped?
Appreciate the help.
 
Considering your extensive list above I assumed you knew your way around the heater.

The "Fireman's Switch" is a poor name for the 2 wire 24 volt AC remote control system of the heater. The Fireman's Switch is a 2 wire control that can connect to automation such as the EasyTouch or IntelliCenter or to a timer. The current is less than 500 milliamps (0.5 amps).

The firemans switch is the two center bottom terminals with the yellow wires on the block below the Fenwal box. The right terminal has the fuse. You remove the jumper and run a wire from one fireman's switch terminal to the one connection of the J19 terminal in Easytouch and then a wire from the other J19 terminal back to the other fireman's switch terminal or similar connections in other automation controllers.

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com www.troublefreepool.com

675px-Pentair_MasterTemp_Firemans_Switch.jpg
 
I just realized my mistake, see pic.
I jumpers the 2 lower leads as mentioned but then realized that these are simply control leads to the panel.
I'll redo this but it seems that it's the top 2 connectors (yellow wires) that need the jumpers.
Doing what I just did didn't accomplish anything, panel was totally dark.
1000009572.heic.jpg
1000009573.heic.jpg
 
I'm a fool, I see that I need to simply jumper across those two lower terminals, sorry.
Heater doesn't ignite, I can turn the unit on and off and there's a set point of 102 displayed, water temp is way below that, in the 70s.

When the heater worked, we used the ET remote to turn on amd off and to set desired temp. I could check the MT panel display and see the temp slowly rising.
1000009576.heic.jpg
1000009577.heic.jpg
 
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Show me a picture of the entire electrical box showing the Fenwal module.

You either have a main PCB problem or a Fenwal problem as the first thing after powering up is the PCB tells the Fenwal to start the blower.
 

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ajw22 said:
Follow voltages using your multimeter for the start sequence through the PCB to the Fenwal and to the blower.

Read Pentair MasterTemp Heaters - Further Reading
Click to expand...
OK., I was looking at both the wiring diagram and the fenwal terminal diagram after posting for a logic sequence/voltages - the link you provided is a big help, especially for sorting out circuit relays.
I was also thinking that verifying 24vac at the fenwal would show that the pcb terminal J1-5 has voltage. If no voltage then the pcb would be at fault, I believe.
 
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TX-Pool said:
OK., I was looking at both the wiring diagram and the fenwal terminal diagram after posting for a logic sequence/voltages - the link you provided is a big help, especially for sorting out circuit relays.
I was also thinking that verifying 24vac at the fenwal would show that the pcb terminal J1-5 has voltage. If no voltage then the pcb would be at fault, I believe.
Click to expand...
The F1/F2 relay contacts in the Fenwal should close and start the blower motor when you have 24 volts between "IND" and the 24-volt ground. If you have 24 volts at these 2 points and the F1/F2 contacts do not close, you would need to replace the Fenwal.

Check for 24V between IND and ground when the heater starts up.

The ladder diagram shows that once 24V is on IND and the Fenwal closes the relay to power the blower through F1/F2, it expects the AFS to close, putting 24V on TH, which tells the Fenwal to begin the ignition sequence with the igniter and gas valve.

1747002716692.png
 
Thank you, yes, I've got the same circuit diagrams from my MT manual.

I'll print the troubleshooting info you've referenced and go thru it methodically. One thing that I've learned from automotive diagnosis is test-not-guess.
 
Here are my findings and what I think needs to be replaced.....

Whenever I'm faced with an ECU that doesn't seem to functioning or respondingn to scantool I first check powers and grounds, so that's where I started with the PCB (heater ON).

Note - wiring diagram indicates 24v as transformer output, my measurements with good quality multi/voltmeter show 27v which I think is ok. I've not found many transformers that are spot-on.

PCB Powers and Grounds
Transformer has 27v on one leg, continuity to ground on the other, per the ladder diagram.
PCB has continuity to ground from terminals J1-4 and J7-1.
PCB has 27v on terminals J1-5, J7-2,3,4
Spa ON = LED display lights up, Spa ON green led lights up, red leds on the back run through self-check, after check, green light on back of PCB stays on, no red leds on.
My conclusion here is that the PCB has power and grounds intact.

AFS Check
AFS contacts are normally open until airflow, this was confirmed.
2 tubes to AFS. Info said to gently blow into the tube(s), but I found that suction on either tubes would immediately close the AFS contacts with a solid click.
My conclusion here is that the AFS is operating normally.

Fenwal Module
Spa OFF = F1 has 120v, L1/L2 has 120v, F2 0v, which is expected, since it's the load side of unenergized relay.
Spa ON = F1 has 120v, F2 120v, which indicates that the relay is functioning (I can't hear a click though, but it appears to be working properly).
Shows ground continuity
Spa ON = No red diagnostic lights
Spa ON = IND terminal only has .6v, which is essentially no voltage, confirmed that only .6v from the PCB
My conclusion is that the Fernwal is ok but is not receiving 24v to operate properly

Firemans Switch
This was checked with Spa ON
L1/F2 strip has 120v
The Firemans Switch double strip has 27v and the fuse is intact
Trans/L2 strip has 120v
My conclusion is that the Firemans switch stuff is ok

PCB
Spa ON = PCB powers on, runs through red led self-check, appears operational
All powers and ground are intact, it is receiving 27v transformer power
J2-1/2 (AFS) = .6v both terminals
J2-3/4 (HLS) = .6v both terminals
J2-5/6 (extra switch) = .6v both terminals
J2-7 (AFS) = 0v
J2-8 (AFS) = .6v
J3-1/2 (AGS) = 0v
J3-3/4 (SFS) = 0v
J3- 5/6 (gas valve) = 0v
Also, when jumpering J6-5 and J6-6, nothing happens.
My conclusion is that the PCB has failed and isn't providing 24v to the various sensors, in particular the Fenwal and AFS which seem to be needed initially to get everything else going.


Comments on all this are appreciated, thanks.
 
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