mits elect split fault

[paul_h]

I've got a fault I haven't seen before on a mits MSH-09NV. It's a small conventional split that has an outdoor control and de-ice PCB.
The indoor runs as normal until the compressor is supposed to start. It briefly sends 240V to the outdoor unit, then cuts it after 2 seconds, then the indoor turns the fan off.
I went out to the outdoor unit and checked compressor, capacitor etc, and retested the system.
240v goes to the outdoor, but the comp relay just chatters on and off ~ 6 times, with no attempt by anything to run (fan, comp etc) Same result if I remove the relay output to the compressor, so no power going to comp, no load on the circuit, just 240v to the outdoor pcb makes the relays chatter and the indoor shut down.

Anyone have any ideas with mits electric?

[expat]

Paul, I found this.
Hope it can help.

http://es.mehvac.com/UploadedFiles/R...0059123856.PDF

[Temprite]

Faulty solder joint on PCB at compressor relay??

[paul_h]

I removed the board and checked solder joins, all seems OK.
Thats the main reason why I removed the compressor as a load on the relay, also can't figure out why that would stop the indoor working (as I said, supply to o/u is cut and indoor fan stops).
While the outdoor relay is chattering away, normal non fluctuating mains is being supplied to the unit from the indoor.

edit: thanks for the link expat.

I guess my thoughts on the matter at the moment are: Is this supposed to be energize the outdoor unit longer if the comp relay is out there and there's a comms line? if so indoor board is playing up.
Or is this supposed to only power up the outdoor when the compressor needs to run? If so the indoor is shutting down due to maybe bad comms from the o/u when it sends power to the outdoor, and the outdoor board is playing up and is the culprit.

I had a look at the troubleshooting guide in the linked manual, nothing seems to cover my senario, and the outdoor unit isn't powered up long enough to do any voltage tests on the outdoor PCB, the indoor cuts power too quickly.

[nike123]

Did you checked indoor PCB relay (52C) DC voltage for same "chattering" when relay chatters?
Use analog voltmeter!

[nicolacozma]

Could be a problem related with the driver circuit of 52C. But in this case you have to replace entire deicer PCB.

[Brian_UK]

Do you have a good neutral ?

[Thermatech]

Low voltage comms cable from indoor unit to outdoor unit going through outdoor isolator ?
contacts on isolator = high resistance ?

suggest make comms cable direct straight through.

have not looked at M series for a loooong time
but
this one often resolves problems as any resistance on the low voltage comms line makes a problem.