Need help with diagnosing a serial comm problem

teckau

Hi everybody,

first and foremost THANK YOU for ANY responds. I don't take that for granted but highly appreciate help you try to give.

I have had a power problem some months ago in my house in Spain. When I called a neighbor for help he started disassembling my AC because he thought that the problem for sure would come from that machine. Turned out to be wrong, I have found the short somewhere else a day later.

But he introduced problems to the AC though. It did not work after his trying to help me. The first problem that came up was a "motor did not start". I measured that the main IC was sending signals to the fan motor but the motor did not turn and also did not respond to the IC. Found myself a motor, replaced it. Motor fault gone, fan starts.

Now I get a serial communication fault (red operation LED on, green timer LED blinking once per 1s; pressing "test" I get 0.1s red and 2x 0.5s green → Manual says:
When a signal is not read continuously for 10 secs from power relay ON immediately after operation starts Cooling 0 code transmission, automatic reset When a signal is not read for 20 secs thereafter
Trouble display continues and power relay OFF
Permanent stop)

I can see there is no power distributed to the outdoor unit. So it makes sense that the indoor unit complains about no communication from above. Checked the relay and it's working.

Disconnected "3" and measured in DC against ground. I see a ~3V fluctuating voltage from indoor and a close to nothing from the ourdoor unit. Again: This seems to me "ok" in that way that the indoor PCB seems to want to talk to the outdoor device which is not powered on.
But I have no idea whatsoever why it's not powering on. I am too stupid to understand the schematics either (normally I can read them, but I seem not to get it in this case): The relay - according to the manual with the schematics - swtiches "L". N goes straight to the outdoor unit, N+L enter the main PCB indoors and the reply switches L to turn on the outdoor part. But I cannot see where "the coil" part of the relay is shown in the schematics. So I don't understand what to do next here.

Anybody who can give me a helping hand? Given what I've written, any idea what to do next to revive this old AC before ultimately deciding to go for a newer one? Would love to just repair as it was in working condition before my neighbors help :)

(The problem the other day that led to this was an old cable from my chimneys fan. So totally unconnected and thus the AC became damaged for no good reason…)

teckau

Sorry, I forgot:
Applicable model : ASY9LSBCW, ASY12LSBCW

Tried and talked about this, too:

https://ilovemyfujitsu.com/images/techtips/serial-signal-troubleshooting.pdf

ratio

The first thing that comes to mind is that the wires to the indoor head are swapped—that's a common problem on Mitsubishis—or possibly a blown fuse on the communications line. Pull the wires off the outdoor & indoor unit, & check them for opens & shorts (both to each other and ground), then verify the terminals when you reland them. Ignore the color & ring them out separately, electricians are known to change wire colors arbitrarily.

teckau

Thanks a lot for your reply. First of all: You were right. When my neighbor reassembled the machine he obviously exchanged L and 3. But after finding out and correcting this the problem persisted.

I have found it anyway: P35 of IC1 is supposed to trigger the relay (which itself is bound to 12V and the 12V is there and stable). D5 (part of IC13) is good. The relay is good, too. Anyway I don't get 220V on the output side for the outdoor unit. I guess P35 of that IC went bad, too.

Gave it a try and bypassed the relay: Detached all power, attached cables, connected them and reattached power. No failure after 2 minutes. Went up: Outdoor unit working. Came back: Room started cooling…

So my question now would be: Given that the rest is not defective and just and only P35 went bad, would it be ok to attach a physical switch to turn the machine on and off PHYSICALLY while using the remote only to control the temps and stuff? I guess the relays only function is to NOT have the outdoor part powered all time, right?

ratio

You should never make alterations to non-servicable parts of equipment like that. The only repair is to replace the bad board.

What is the number of the failed IC? Even the finer-pitched surface-mounted chips can be replaced without too much trouble. Unless it's something programmable, I'd look into that.

I'm not sure it'll work right with just a switch on the compressor. There's a bit more than on/off being communicated, without that info the compressor may not be able to respond. Worth a try at least, though.

teckau

I agree that this is not the best way to repair things. But I've tested it and I am absolutely sure that this is a modification I can live with. The relay is in deep only switching the outdoor unit on and off. Power is permanently applied to the indoor unit to make it possible to use the remote to turn it on. But the outdoor unit also has to be powered and not constantly. So the relay is doing that job.
I put a Shelly in between. With the mobile I can switch on the whole thing "on" (apply power) and use the remote to turn on the AC like normal then. I have tested heating and cooling and because the Shelly also gives me now readings about the drawn current and Watts I can really see how it's working exactly the way I am telling it using the remote.

Then I detached a wire during operation. Within 10 or 20 seconds I got the "serial comm error". Does not matter if I detach N, L or 3. Its all the same and brings back the error. That's proving what I've seen from the other said looked at it :)

I might replace my "repair" with a better solution in the near future: When turning on the AC using the remote the red Power On LED is lid. This is the very moment when the IC1 would issue that missing signal using P35 to make the relay switch. Unfortunately the relay is fixed to 12V and the IC is switching between open and ground. But with a transistor or another relay I could use the red LED to switch the outdoor units relay. Making it effectively working 100% identical to the original state.

But for now I am happy. Thank you for pointing me initially to the wrong cabling as I would not have went upstairs to my roof to check this after my neighbors help. He's an AC technician :) So I would not have thought in this… He recommended to replace the PCB but that's >300 EURs used. I can live with my fix for now and will make a perfect repair later using 2 EUR components … THANK YOU!

ratio

Ok, I finally looked at the model numbers you sent. I've never seen a mini split at feeds the outdoor unit from the indoor unit! In any case, I see what's going on now. Switching the outdoor unit on and off with a Shelly should work fine.

I wonder what would be the result of just leaving the outdoor unit on, aside from increased power usage? Can the indoor unit stop the compressor when it's satisfied via the communications link, or does it need to turn it off via the power relay?

That IC1 seems to be a flavor of the old NEC 78K/0 line of microcontrollers. It claims to be in-circuit programmable, but the data sheet I found didn't mention sucking the program back out of one, so getting a replacement for that might be hard to do without a donor board. The buffer IC13 (looks similar to a diode symbol, but it's actually a logic symbol) on the output should have protected the μC though, maybe check it again. It's an inverting buffer, so the output is will be the opposite of the input signal. I didn't see a part number for it on the schematic, but it should be a pretty standard part, google the part numbers on the larger (16 pin) chips, look for an "octal inverting buffer". Come to think of it, it seems to be powered from the same 12V as the relay that turns on the outdoor unit. I wonder if that's significant.