Mitsubishi Heavy multi thermistor replaced but fault flash code returned next day

[langy]

Got a call from a customer, their SCM125zj outdoor with 5 x SRK 25 Zj indoors had stopped working, with indoor run light flashing continually, timer light flashing four times.
Discharge pipe thermistor faulty! I ordered new thermistor set, popped down, fitted, ran, tested, fine no apparent problems, cool suction and suction pressure about 110psi which seemed ok. All 5 units running and rooms down to temp.

Next working day, system ran for a period and stopped with same fault flash code.
Refrigerant charge seemed ok when I fitted thermistors, could a very slightly low gas charge give high discharge pipe temp causing the thermistor to go in to fault, :mad: Will the new thermistor be damaged and need replacing again? Any other ideas ???

[al]

Pull the charge first, short of gas will give high discharge temps, thermistor is probably fine.

[Brian_UK]

You could always monitor the pipe temperature if you want confirmation, but do as Al says will confirm things.

[Tayters]

If you are unlucky and the unit has been running for ages at full whack in a high ambient then discharge temp can creep up to it's limit (100*C or so) and trip.
Would like to think the new probe will not have been damaged but with the quality kicking about these days you have to wonder. Best monitor it and see if reading from controller/resistance match with your own pipe reading.
Slightly low gas charge could cause the fault. Pressure might be fine with a low charge as comp speed might also be low so can be deceptive. Can only weigh it out to confirm. Perhaps no extra charge was added from day one?

Only other thing I can offer it move it further along the pipe. :-)

[langy]

I ran system yesterday afternoon with thermometer probe attached to discharge pipe at same point as the thermistor, the temp didn't rise above 60 c , it was a cheap thermometer and not sure how good a contact but I think it was good enough. It's only infrared remotes so no checks via wired remote available.
Ive left thermometer attached and will call in and check it today ( it's got maximum temp memory)
If I decide to pull the charge, the only point I can get into the system is via the suction service valves. What's the best way to get the 6+ kg charge back into the unit?

[narkom]

What is an exact error indication on indoor unit display panel? If it is an E39 error code than "–25ºC or lower is detected for 5 seconds continuously 3 times within 40 minutes after initial detection of this anomalous temperature". So it is not a high discharge temperature but loosing connection or PCB damage.

[langy]

Continuous flash of run light and four flash of timer light on indoor units.. no wired remote.

[narkom]

15269
This is what the manual says.

[langy]

I've got a thermometer with memory attached to discharge pipe so if it trips again and there's no discharge temperature trip point reached, it could well be the pcb.... good call

[langy]

Ok I've been back to unit with the same trip fault (discharge pipe thermistor faulty) I changed the thermistor a few weeks back but it's tripped again. I left a digital thermometer with memory attached to the discharge pipe. As it has tripped again and no high pipe temp taken it points towards a faulty board. Chris at Mitsubishi tech thinks it's a dry joint and repairable. I'm not sure if I want to piss about trying to spot anything that needs repairing and risk getting called back again if it doesn't work. Has anyone in the northwest used a pcb repair company for a similar problem? A new board is over £200 to me, so maybe it's worth getting it checked. Any ideas chaps any pcb repairer recommendations?

[al]

At 200 i'd replace the board, no question.

[ozairman]

Before you shell out on a replacement board, the following may help.
The Run light Flashing Timer 4 flash error is not just triggered by the E39 Discharge pipe sensor error logic that narkom explained, it is also triggered by the Discharge pipe sensor disconnection protection control.

This control is trying to detect if the discharge pipe sensor has become disconnected from the discharge pipe itself. The logic is complex but in a nutshell it does this by monitoring the discharge pipe temperature and outdoor air temp and looking at the rate of change the rate of change of the discharge pipe temperature sensor when the compressor is running at various speeds. If the discharge pipe sensor temp does not rise to the target level the unit stops with that error code, so what does this mean in reality?

There are a couple of other scenarios which can trigger the error namely liquid floodback from crossed interconnect wiring and is the first thing that you should rule out (even if its been in for a while, I have come across this on systems that have been in for years). Run each indoor unit on cooling mode for 10-15 minutes and confirm that it's indoor coil is the one that is actually getting the refrigerant flow and not some other indoor unit is starting to frost up. Running them on heating to test is no good as the EEV's in the outdoor for each circuit are partially opened on heating mode.

Once that is ruled out the other scenario can be from low load operation with only one head running. Check with your customer what heads they have running when the error happens, there may be a way around this but I don't have access to the tech manual for that model at the moment.

HTH

[langy]

Before you shell out on a replacement board, the following may help.
The Run light Flashing Timer 4 flash error is not just triggered by the E39 Discharge pipe sensor error logic that narkom explained, it is also triggered by the Discharge pipe sensor disconnection protection control.

This control is trying to detect if the discharge pipe sensor has become disconnected from the discharge pipe itself. The logic is complex but in a nutshell it does this by monitoring the discharge pipe temperature and outdoor air temp and looking at the rate of change the rate of change of the discharge pipe temperature sensor when the compressor is running at various speeds. If the discharge pipe sensor temp does not rise to the target level the unit stops with that error code, so what does this mean in reality?

There are a couple of other scenarios which can trigger the error namely liquid floodback from crossed interconnect wiring and is the first thing that you should rule out (even if its been in for a while, I have come across this on systems that have been in for years). Run each indoor unit on cooling mode for 10-15 minutes and confirm that it's indoor coil is the one that is actually getting the refrigerant flow and not some other indoor unit is starting to frost up. Running them on heating to test is no good as the EEV's in the outdoor for each circuit are partially opened on heating mode.

Once that is ruled out the other scenario can be from low load operation with only one head running. Check with your customer what heads they have running when the error happens, there may be a way around this but I don't have access to the tech manual for that model at the moment.

HTH
Ive just received the replacement board, planning to fit next week. I actually installed the system about six years ago and not had any problems with it. I'll do the checks before I fit the new board. Can you come back with your "way round this" please mate?