basic system but helluva fault

[fridge doctor]

Hi,

Anyone who can figure this out deserves a medal.

I have a basic chinese 9000 system (Mayoki). This is a no frills unit as I will describe. The fault is that the unit reaches 28 (on it's own display board) and cuts out, then back in at 30 irrespective of what the temp is set to. (The room temp IS genuinely 28)

Mechanically everything fully operational, gas charge, outdoor fan running full speed, good temperature at the air off vent and equipment begins to cool the room quickly when running.

First thoughts - temperature sensor faulty. Changed them (pipe and air sensors share a common plug on the PCB). No difference. OK so I think it could be the PCB itself not interpreting the information from the sensors correctly, or even self inducing the timer... So changed the PCB from an identical known good machine - no difference. For good measure I changed the display/receiver board too although I couldn't justify to myself why.

Went back to the job and studied the pattern. The outdoor unit is merely a slave. No electronics, no sensors, just typical 5 wire connection Live comp, live fan, live Rev valve, neutral & earth. It just does what the indoor PCB tells it to do, and is doing that job just fine. Indoor unit has a single main PCB to which is connected the evap fan, transformer, mains, flap motor, etc, etc. When the unit cuts out (early) the relay is heard to drop out, and power to the outdoor is terminated. So something is causing the relay on the PCB to drop out too early. If there was a fault on the indoor fan, this model would show a fault code and shut down, so apart from the previously mentioned sensors what else could cause the compressor relay to operate before it reaches the set temp?

Remote control works fine, all instructions are accepted by the unit, all figures match between remote and display... I am baffled

One last thing, Thinking that the pipe sensor might be picking up some liquid slug or similar, I have moved it from the pipe socket to just hanging in the cool air passage, no difference.

Any ideas please guys.

Trevor

[Brian_UK]

Tricky one Trevor.

I would have suggested dry joints overheating when live but having changed the PCB that went out of the window. Or, is the relay separate from the PCB?

Does the unit have a DRY mode and is it somehow running that cycle instead of COOLING?

[fridge doctor]

Good suggestions Brian - thanks. The comp relay is however an integral component of the PCB and as such must receive it's instructions from the sources connected to the PCB. The system does have a 'dry' or 'dehumidify' mode, but unless the remote is completely screwed up and sending the wrong information - and given all the circumstances I have no reason to suspect this - then I would have to rule that out too. Please keep thinking, 44 years in the industry and a simple AC has got me stumped, I can't believe it.

[Brian_UK]

This thought only applies if there is the chance that "fingers have been playing"...

Is the outdoor fan controlled by the indoor PCB as well as the compressor? Has a wire been misplaced or mispositioned so that the whole unit drops out instead of just the fan ? If you see what I am getting at...

[install monkey]

faulty coil thermistor?

[lawrence1]

I have worked on these chinese no frills and i would say it's the setting on the remote,,,maybe it's on Dry or Auto.
If you have a generic remote try that,,,maybe the remote is locked on Dry or Auto modes>

[wingman]

Is the PCB shielded or just like any wallmount in a plastic box ?

Are there any small electronic transformers of dimmers in the neighborhood ?

If you have a power analyzer, look if there are any fluctuations in power.

Is the unit on a single circuitbreaker ?

Or the light inside the room is interfering with the IR sensor. Any fluorescent lights in the vicinity of the unit ?

If none of the above, replace unit with a mitsubishi heavy

[sumsor]

Hi Fridge Doktor,

is there no button/switch for a test run option in heating/cooling?
That way you could rule out a faulty remote signal.

[paul_h]

OK, I repair and use and own a few of these generic cheap units.
Are you sure it always cycles on a certain temperature?
Like have you been there at different ambients and it's always run different amounts of time but always seemed to cycle at exactly 28-30C regardless?
Also I know some other dogey brands too smart for their own quality that had bimetal switches on the outdoor for high temp.

Most of these dumb outdoor units have an outdoor coil sensor, but I always thought they were just for heating cycle and defrost. But if the run time and temps were always variable, maybe it's that causing a fault/compressor shutdown after a few minutes operation?

If it's not that, then maybe the controller sending a 'dry' signal instead as mentioned.

So i'd i) check outdoor thermistor, plug, cable, ii) try a universal remote that you know works on that brand.

If no joy, well then the units only cost a few hundred anyway, so not worth any extra trouble.

I consider myself a repairer and am loathe to replace units. I used to take pride in the fact I would correctly diagnose faults and repair, while others recommended replacement because they were unable to diagnose a faulty capacitor even. But with what I have seen in just a few years on my own, you have to learn to pick your battles. I spend a bit of time on decent systems that have available manuals and parts, that are well known, that are expensive to replace like ducted. But anything else like the unit in question, it is cheap because it's made to be disposible, I've lost so much time and money bothering with generic stuff, never get answers, never get paid for work done unless I perform a miracle, but even then charge minimum for these minimum priced units and in the end not worth it.
It would be better if no one bought these systems in the first place.

[Aardvark]

Hi, i've also worked on and installed Mayoki systems, How old is the unit? If it's more than 2 years old it's actually a TCL and if it's under it's actually an Aux. I've never come across this fault but if you've already changed the pcb i would suggest trying another remote.

[Aardvark]

also is the condenser fan running or running slowly?? could try checking the fan capacitor, with the temp the way it is at the moment if you haven't got a good flow of air that could be your fault

[momo]

Assuming that you are using it to cool (at present!!!) check if you are getting air recirculating into where the return (intake) temperature sensor is this will stop it sensing the correct room temperature. Remotes can also do weird things if sending corrupt messages (low batterise - a favourite) to inadequtely programmed gadgets because they most likely do not have error correction or checksums.

[fridge doctor]

Thanks for all your replies guys, unfortunately I am still no nearer. Paul makes a very good point about not bothering with cheapo systems, it's just that it's so simple it has to be fixable !!

The wiring between the units is correct and good. No-one has been messing with it previously. No external transformers, dimmers or flo lamps nearby, nothing interfering with signal. There are no switches of any description on the outdoor unit, it is dumber than dumb, the outdoor unit stops when the indoor PCB relay is heard to drop out. No question, it is the relay coming out that is stopping the unit, nothing else. Re. Dry mode, customer has a second unit the same. Tried remote from that system - no difference. Aardvark, it is a TCL.

Many thanks,
Trevor

[fridge doctor]

One thing I forgot to mention. If the unit has stood idle (say all day). When it is first switched on it will reach it's set temperature (eg 24). It then stops but doesn't restart until the display shows 30, at which point it then cycles 29 - 30 -29 - 30 ... So it reaches temp just once, the first time.

Let me run this by you. Suppose the transformer was maybe 50% over voltage and perhaps producing say 16 volts. Most components would easily cope with quite a wide tolerance but would the thermistor sensors? Could the transformer (not replaced) have screwed up the original and my replacement thermistor sensors? Even so it doesn't explain why it works fine the 'first time' as I have written above ???

[Temprite]

One last thing, Thinking that the pipe sensor might be picking up some liquid slug or similar, I have moved it from the pipe socket to just hanging in the cool air passage, no difference.

Any ideas please guys.

Trevor

Have you done this with the ambient sensor?

Once had a cheap unit do a similar thing but on heating. It would heat up and cut out, then it would take forever to cut back in.

The ambient sensor was retaining heat from the coil after the compressor had turned off and was remaining warm even after room temp had dropped dramatically.
Relocated sensor away from coil and it ran perfectly after that.

Cheers.

[Aardvark]

Trev,

Did you check the condenser fan and fan capacitor??

Jono

[Aardvark]

Trevor,
give me a bell if you wanna bounce some idea's around

Jono 99056246

[fridge doctor]

Ha Ha, thanks for the offer Jono... I already have your number, in fact I passed a couple of Limassol calls on to you just days ago. I will call you as soon as I can - Cheers for now. By the way condenser fan full speed, capacitor no problem.

[Brian_UK]

You say that it works OK once....

Is there a timer function that is doing something it shouldn't do?

[mikeref]

Might be a long shot but i'd be measuring compressor amps on initial run then compare when a/c cycles. might explain why a/c appears to run fine first time( cold compressor), then screws up when hot...mike.

[fridge doctor]

Thanks again for your continuing support ....

Mikeref, The compressor isn't shutting down on overload. It is definitely shutting down because the relay on the indoor PCB tells it to.
Brian, I was thinking along those lines, but remember I have changed both the PCB (timer module) and exchanged the remote control, so can't be that.
Temprite, Yes I have... no difference.

Remember that the outdoor has no sensors and cannot pass back any info to the PCB. Also that the outdoor is comfortable and happy to work when ordered to do so by the PCB (relay).

On the indoor side, the only things that have not been replaced are:
Fan motor, flap motor, transformer. Baffling or what?

[mikeref]

O.K Fridge doctor, i'm not finished yet, knowing the evaporator pipe sensor would be monitoring for extremes(cold/warm) on cooling cycle, and if these are breached, then pcb times out and compressor is cycled off. You said the customer has a second a/c the same, swap over its pipe and air sensors. New parts can sometimes be faulty...mike

[fridge doctor]

Mikeref, that's exactly what I did. All the parts I have exchanged have come from the known good donor machine in a room next door - which the customer is not bothered about having cooling in.

[Brian_UK]

You swapped all of the parts, including the remote controller? at the same time or one by one? (Sorry, just going for a memory refresh).

[nike123]

Did you actually fitted that "faulty" PCB to other unit and than checked that other unit have same bahaviour. OR Did you checked that other unit works normally before PCB exchange?

[nike123]

Mikeref, that's exactly what I did. All the parts I have exchanged have come from the known good donor machine in a room next door - which the customer is not bothered about having cooling in.

Is this checked by you?

[nike123]

To me it looks like frost prevention function. Try to hold heat exchanger sensor with hand and see if that has any effect on behavior of unit.
Could it be that exhaust air on indoor unit recirculate for some reason?

[mikeref]

Is this an existing or new unit? also, it would be reasonable to assume after swapping components over, the "donor" a/c did not run.. No? worked fine? (Nike123 is thinking along the same lines). NEXT, transformer warmes up and supplies low current and can't hold in compressor relay! Brian UK's maintenance comes to mind.. mike

[fridge doctor]

Brian, The items were replaced one at a time... and thanks for maintaining interest.
Nike, Sorry, but I must admit that I did not check the donor unit, I took the customer's word that it was OK, and nor have I re-installed all the 'faulty' components back on to the donor system - lack of time! And yes I have hand held the frost prevention sensor but no difference. But honestly what are the chances of teh donor unit having the same problem. Plus, the pipe and temp sensors were not replaced from the donor unit, but from an unopened pack I was carrying.
Mikeref. Point taken re the component changeover.... the system is 4 years old. Quite like the transformer idea... just need some time to get back and try a couple of things.

Thanks all. Trevor

[fridge doctor]

Guys I still have no clues on this one. Maybe I should ask 'Reptile' how to fault find by text book ??

[lawrence1]

fridge doctor,

i like it

[mikeref]

HI fridge doctor, taking the customers word for it, naughty, you may be going around in circles assuming the donor unit worked, but i guess the grey matter has figured this oversight out now.< DID this part come this one or that? OH MY! >

[Lesley]

Guys I still have no clues on this one. Maybe I should ask 'Reptile' how to fault find by text book ??

I've got a mayoki too - can anyone tell me please what the message E6 on the display screen means?

Thanks.

[Gary]

You seem to have eliminated all of the possible control faults. Perhaps there is a legitimate system fault the controls are correctly responding to, such as high/low pressures, airflow problems, undercharge, overcharge, restriction, etc.

[back2space]

Does it operate on other operation modes such as dry mode, heat mode?

You should swap pcb over from other unit and see how you get on.