Thanks for replies...
mayer - the RAM was the original and known working RAM. I had taken it out as part of the removal of the logic board. It works fine.
Cees - I powered up without RAM by mistake and noticed the change in fan speed. I dont have the AHT DVD yet - on order from Apple.
Eric - Yes agree the SMC does appear to detect the sensor. I have discovered that the PSU temp sensor can cause this issue along with the CPU and CPU HS sensors. The CPU sensors appear OK and I have ordered a new LCD sensor. In the mean time I will hopefully be able to run the AHT in the next day or so and get some conclusive answers...
?? are you DR Fosgate on the apple support forum?
FWIW the Ambient and LCD sensors operate on a serial bus and the little resistors on the board set the address. It does not matter if you swap the plugs around, they will still work...