reason we thought the ecu was bad was because when we removed the temp control sensor, the fan was not coming on.
the engine temp gauge showed nil when out, and engine temp when in, so we knew the sensor was working.
we checked all the relays, interchanging them and they were all fine. the earth is attached to the body just ahead of where the relays are - we sanded it back and ensured that the connection was good and tight, plus we verified with a multi meter. we checked all the relays verifying live and earth and everything was ok. what was happening is we were getting an earth as soon as the fan kicked in.
lesson for today is assume nothing. because we had a 'live' and an 'earth' at the fan connection (near centre of the fan) we assumed the resistor was funtional.
turns out it was failing under load - further examination showed the resistor to be pretty much corroded right through. i connected the 2 low and high speed fan wires together and removed the resistor altogether to be replaced at a later date so it only comes on 'full', removed the temp control sensor from the engine and the fan came on - so the ecu is ok - huzzah!
fan also kicked in when the engine reaches temperature - huzzah!!
glad to get to the bottom of the issue without a costly but unneccesary ecu change that would not have fixed anything