Howdy Diplomat Freezer owners!
I have the same model freezer as you all - and the same fault! Being a electronics field engineer (but on completely different products than domestic goods) I have been able to identify the failure.
Put simply, there is a relay on the Thermostat Board which fails to close. This is because it is a 24 volts D.C. relay BUT the voltage available to operate it is only 12 volts! (although a measured 18 volts is available on-board)
Despite being only half of the rated voltage, most times this is sufficient - in fact running a relay at the full voltage over extended 'on' periods can lead to overheating of the core. More sophisticated circuits seen elsewhere will have higher closing voltages, but with a lower 'holding' voltage in order to prevent this overheating issue.
So, 12 volts in the long term is good for the relay but pretty weak in order to get it to operate correctly initially. Quite why this fault does not seem to show itself when the appliance is new is a bit puzzling. It could be down to 'component drift' which can happen with age, but of all the components that I have so far measured, they are all easily within manufacturing tolerance.
To discover whether this undervoltage condition applies to your situation: while the appliance is switched on and the red alarm light is flashing - try tapping sharply the black plastic relay housing (it is marked with the name 'Omron' and Identified as 'RL1' on the circuit-board). If your relay is struggling to operate, this may be all that it needs to actually close the contacts.
If successful, you should here the compressor-motor and the front cooling-fan of the appliance start up; the red alarm led and audible will continue to sound as the cabinet temperature is too low, that is, at least until the compressor-motor has been able to reduce the internal temperature to within limits. The alarm can be turned off by pressing the 'mode' button: the yellow 'fast freeze' or 'continuous' led will illuminate; the red led will continue to flash.
I have yet to finalise my component fault-finding but as a Kwik-Fix (a Bodge!) for now, I have adjusted the centre switch-contact thus: the middle, 'common' one on the relay-actuator to be slightly closer to the 'on' contact when in the normally-closed, at-rest position. This enables the actuator to be attracted more easily to the magnetic core as the distance between them is now significantly closer.
This can be achieved very simply by forming (bending!) the brass arm holding the normally-closed contact until the centre, common contact is closer to the normally-open contact. One should still be able to see a marked air-gap between the two 'on' contacts when viewed when 'off' (too close and you may encourage the mains voltage (240 a.c.) to arc across them when they should be ope - also possibly welding the contacts together - not good!)
My relay also had somewhat burnt contacts from its efforts over time in starting the compressor-motor. I removed the black carbonising and polished out the 'pit and crater' using the finest grade wet and dry abrasive -paper that I had - 1200-grade grit.
All is well for now but I really would like to get to the bottom of this odd fault: one option may be to simply replace the fitted Omron relay with one of a lower voltage-rating; ie:12 volts (15volts?) - which certainly would work ok but it would be necessary to ensure that no overheating of the core occured during the 'fast-freeze/continuous' mode as the relay is continuously enegised during this time.
Thank you for reading this far :-)
TBC...
Bark Chip
July 2011
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix freezers.