Mastertemp 400 gas heater blows fuse

[JohnnyD.]

Moved from here.

How are you testing for voltage at L2? One probe to L2 and the other to the jacket (ground)? If so, that is correct. In a 120V connection, L2 is the neutral, and will have no voltage. Single leg voltage (120V) will only be at L1. I don't know what blue wire you are testing.

This is simple. Apply voltage to the heater. Xformer is connected by a plug, 5 wires. disconnect and probe white wire and black wire (supply side), you should get 120V.
If you dont get voltage, stop. There is a problem with the supply voltage. If you do have voltage, reconnect the plug and go to the main board and disconnect the only plug with 4 wires on it. You want to probe the two wires on that plug that were closest to the 5 wire plug that is next to the plug you pulled out, testing the wires on the plug side not the pins on the board side.

No voltage = bad xformer, replace.


Here is the wiring diagram. Hopefully you can make out the wire colors.

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I am having a similar issue and have a 2007 Mastertemp 400 that worked great never an issue until last season and I got no lights or power to buttons etc. Quickly found fuse blown and replaced which immediately blew when power resupplied.

Followed steps in this thread. I do have 240 not 120 setup. Have 120V at both L1 and L2. Have 25v at power side of fuse so assume transformer ok? Tried unplugging 24vac, TH, IND, and VAL from the ICM but as soon as I turned power on with all of those unplugged fuse blew.

Have checked for bad wires ad nauseam and they all appear to be in great shape.

So is ICM bad or control board, what next?

Thanks.

 

[borjis]

Just guessing, maybe check to see if the inducer motor spins freely? I've seen those seize up.

 

[JohnnyD.]

Just guessing, maybe check to see if the inducer motor spins freely? I've seen those seize up.

Yup motor seems to spin very easily.

 

[ajw22]

To be clear, the fuse connected to the Firemans Switch that the right arrow points to is what is blowing?

That fuse is the 24V circuit shown in the ladder diagram below. You likely have a chafed wire with a short somewhere.

Look carefully around each of the sensors. Check for any signs of rodents in the heater who chewed on wires.


Page 41 in https://www.pentair.com/content/dam...stertemp/Manual-MasterTemp-472592-English.pdf

 

[JohnnyD.]

Yes that is the fuse that is blowing. I have continuity checked every single wire to every sensor from the control board and from the control board to the ICM there are zero signs of wire damage.

I have even unplugged all sensors from the control board and left the power from the transfer to the control board and to the fuse only plugged in and the fuse blew again when power turned on.

That seems to me to mean the control board is the issue since I isolated it and blew the fuse unless the transformer can still output 25V and be an issue?

 

[ajw22]

Have you examined the board and looked for any signs of burned out components?

 

[JohnnyD.]

yes I really don’t see anything that looks burned at all anywhere in the heater or on the control board.

 

[ajw22]

I would use the ohmmeter continuity setting of a multimeter and WITH THE POWER OFF see if you find continuity from the fuse to grounds. See if it shows a short to ground.

Otherwise a new board may be your only option. However I am skeptical it is the board but you never know.

 

[wireform]

Usually it's best to check the back of the board for an overheated component.

 

[JohnnyD.]

After closer inspection of the back of the board I do see some discoloration in 2 spots that might indicate overheating. So is the board the likely culprit as I suspect and what keeps the board from just burning up again if the issue lies somewhere else?

If it helps attached are the 2 components that show discoloration on the back side:

 

[Turbo1Ton]

I would use the ohmmeter continuity setting of a multimeter and WITH THE POWER OFF see if you find continuity from the fuse to grounds. See if it shows a short to ground.

Otherwise a new board may be your only option. However I am skeptical it is the board but you never know.

I second this.

You said you did a point to point from all sensors to the board, but that won't necessarily tell you if you have a short to ground.

Just like Allen said, check continuity to ground from the fuse connection, and then work your way through each connector to see if there is a short to ground on that 24V circuit.

--Jeff

 

[swamprat69]

You could try removing the operating control board connectors at J1, J2 and J3 leaving only the 24V input to the control board and the firemans switch return to the control board at J7. Does the fuse blow? If so you have isolated the problem to the firemans switch and its wiring or the control board. If it doesn't blow, attach the connector at J1 and repower the heater. Does the fuse blow? If so, you have isolated the problem to the ICM or its associated wiring. The only connection at the ICM that should have zero or near zero ohms to ground should be the one lableled GND.

 

[swamprat69]

If the fuse blows with only the J7 connector in place, you can remove the J7 connector and check for continuity to ground on the 2 connections associated with the firemans switch. If they read as open, you have a problem with the control board.