Daikin Altherma Freezing up

[roboskip]

Hello, first time poster in need of help.
I have a Daikin Altherma LT split. EHBX08CB inside and ERLQ008CAV3 outside. It worked great for about 3 years then for the last 3 winters has been frosting up when the weather gets below about 5C.

I have had 2 engineers out, checked the refidgerant, checked for leaks, regassed and then scratch their heads and eventually the weather improves and repeat the next year. The last guy installed a buffer tank and as soon as the weather turned this year it's done it again.

The indoor unit reports a 89-01 error and the outdoor heat exchanger has about 30mm of ice all over the back. I have been watching it closely and i have never seen it go into defrost??? That is I have never seen the frost on the heat exchanger melt in any way.

This got me investigating more.

Going into the installer menu i can see the warning errors logged and noticed that 89-02 was coming up which appears to be a problem with the 4-way valve not reversing. So last week I took the outdoor unit apart and following the service manual I found online checked the 4-way valve, the coil and the PCB.

I found that the coil measured 550 Ohms in resistance and not the 1-2kOhms that was specified and the connector S80 on the PCB always seemed to read 0V on 220-240V. So I ordered and new Coil and PCB. I have since fitted these and it still doesn't defrost interestingly the new coil also measured 550 Ohms so i think the manual is wrong?

While i was waiting for the PCB I did some investigating. The coil is energised but the PCB via the relay MT04 which is a 12V relay. So I soldered wires onto the 12V terminals of this relay. I set up a datalogger to record the voltage on these terminals expecting to see it change from 0 to 12V for a few minutes every 30 mins or so but got nothing. Over about 2 hours never did the relay try to energise the 4-way valve coil to reverse the 4-way valve and defrost the outdoor unit.

As I said I have since replaced the PCB and it still doesn't work so i conclude that for some reason my system never tries to defrost the unit. I have checked the thermistors that measure the outside temp and that seems OK but not sure where the other ones are to test? And I don't know where else to go with this? Why is it not trying to defrost?

Attached are some photos of how icy the outdoor unit gets.
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Please help...