No earth on interconnect- u4

[jimbo1979]

Re-installed a Daikin inverter multi 80 which has 4 units on the system. As the system was re-installed in the same site pipework and cables re-joined and one unit goes straight into fault u4. Swapped the signal cable with another system momentarily so the system is cross wired. Fault moved onto other ducted unit. Found after eliminating boards etc the earth has a break in it somewhere on that particular interconnect. As the junction box where it was joined is tucked away in a finished ceiling and I cant run a new earth could I tap the earth from a ducted system that is nearby? Does this missing earth cause the u-4 and how exactly? replies much appreciated

[AndrewB]

Hi Jimbo1979, I don't think the broken Earth would cause a U4 Fault, By swapping Interconnects with another unit, like you have and the fault moves to the other unit, shows that it seems to be something wrong with the outdoor unit, I would suggest a new outdoor PCB would be a good start. However the broken Earth issue still exists, and there would be no problem taking an earth from another indoor unit on the same system as you suggested, I would not take an Earth off another separate system, however you could take a separate earth from a switchboard.
HOPE THIS HELPS

PS. I had this similar problem recently and a new Outdoor PCB fixed the problem, Remember to put fresh heat paste on the back of the components of the Inverter board. Very Important.

[jimbo1979]

Thanks for the reply Andrew. The client says there is history of outdoor board replacement due to salt because it's near water. Will have to check other boards as we swapped the inverter board on the other identical unit and no joy. Heat paste essential I know as I moved a condenser that was in virtually a cupboard that had a inverter replaced and looks as if they just chucked the new one in and weather got hot and killed it. Will post the outcome. Cant remember first two letters on the model number. Its the old daikin 80 multi inverter. Any ideas anyone?

[AndrewB]

Jimbo,

Is it a 4MX80, I think that is the old one.