Mitsubishi Thermistor Fault

[matt.yelland]

Hiya all,

I was working on a Mitsi PUH-3VKA unit today. It keeps shutting down on LED 7, which is the Thermistor probe. The unit is running in cooling at 20 degrees C....

Can anybody tell me any faults as to why the unit would shut down on this please?

Mant thanks

Matt

[Brian_UK]

Perhaps a faulty probe ?

[matt.yelland]

Thanks, but changed the thermistor....

[Thermatech]

LD7 on the outdoor unit is when outdoor coil liquid line temp sensor reads over 67 deg C.
As you are running system in cool mode is hot gas going staight through outdoor coil not condesing out into liquid so that liguid temp sensor gets so high ?
Faulty fan motor ?
Air recirculation at outdoor unit ?

Another alternative is circuit board faulty & reading incorrect data from coil temp sensor.

If you read up the service manual you will see that you can display the coil temp sensor data on the outdoor circuit board LED's.

SW3-1 off & SW3-2 on. SW2 to change to output state.
The LED's display in binary & each LED displays a number of bits.
LED1 = 1 bit
LED2 = 2 bits
LED3 = 4
LED4 = 8
LED5 = 16
LED6 = 32
LED7 = 64
LED8 = 128

The sevice manual has a graph which indicates the temperature at any given binary total for the LED's

67 deg C is about 215 bits on the graph so atleast LED 8, 7 & 5 will be on.
8 bits = sensor open circuit
238 bits = sensor short circuit.

Using this method you can monitor the coil temp sensor data at the circuit board & check with your own electronic temp meter at the same liquid line pipe location.
If the circuit board is reading much higher temp than real temp then suspect circuit board problem as you have already replaced temp sensor.

[matt.yelland]

Thanks for the reply Thermatech.

[matt.yelland]

Thanks for the reply Thermatech,

I dont have the info with me at the moment, but the LED is solid indicating that it is calling for cooling. I have an air off of +10.2 degrees C when set at +19 degrees C. The unit will run for around 5 mins, the stop working on LED 7. The condenser fan motor is running, not on a speed controller though. I have cleaned the condenser with OFN and water

[Thermatech]

On the old R22 K control you can use the SW2 to alternate the LED display between check code & output state.

With the circuit board displaying check code LED 7 will be blinking for the overheat thermistor fault.

With the circuit board displaying output state LED7 will be permanantly on(not blinking) to indicate the bypass valve is open. When the bypass valve switches off the LED 7 goes off.

Each of the 8 LED's give an indication of what the outdoor units it doing when set at SW2 for output state & the LED will light not blink.
The LED's only blink to indicate a fault.

For cooling mode the output state display will be
LED1 = compressor ON command from indoor unit.
LED4 = compressor ON
LED5 = outdoor fan ON
LED7 = bypass valve ON

The service manual has full description of the operation stratagy for the bypass valve.

If you are using the system for cooling in low ambient then indoor unit could be stopping on indoor coil frost prevention.

You can check to see if this is happening by watching the LED1 compressor command from indoor unit.

On these very old systems there was no data communication between the indoor unit & outdoor unit.
The indoor unit could only send compressor & 4WRV on/off commands & monitor indoor coil temp. If no coil temp change after a few mins then stop on P8 fault (outdoor unit not operating)

You really need to get a service manual. They are FOC to download from the Mitsubishi uk website.

[matt.yelland]

Thanks for your help. Im just downloading the manual.

Matt

[perlfather]

I have the same thermistor fault. Can somebody point me to a place where I can purchase this part on line in the US (mitsi R01 885 202)?