Focus AC problem

The AC on my 1.8 TDi is not working, I have checked the pressure and it is fine but the compressor does not engage. I have also changd the compressor.
The light on he dash comes on to tell me that the AC button has been pushed.
Any suggestions as rto what might be happening.
Thanks
Frank
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The ecu controls the compressor clutch. There is a relay in the engine bay fuse box, also a fuse. The low pressure switch cuts out the clutch if there is not enough R134a in the system. There is a superheat switch on the body of the compressor.
Any of the above could cause non-operation, you have plenty to go on!
Tim..
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Anybody can tell me wath is ECU ? because i see this frequently in this newsgroup.
Thank you.

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is that wot or where?
the PCM/ECU is the large silver module located in the drivers footwell(on the uk verisons anyway)its the 'brain' that controls the engine.
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if your sure the pressure in the system is ok then the fault must be eletrical,just cos the little switch lights up, doesnt mean that the A/C is working time to study that wiring diagram. (the look on the apparentices face at work when i tell them that is so funny.....)
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