by bridging the 2 larger connectors for the relay (presumably the 2 earths) the fan comes on
does this narrow it down a bit??
yes it does, the 2 larger connecitons are the live from the fan and the earth for the fan. bridging them should bring the fan on.
so we can safely assume the issue with the fan is due to the ecu not switching the fan on.
that does not mean there is an ecu fault, it simply means the ecu is not getting a signal to tell it to switch the fan on. since the main component involved is the coolant temp sensor we need to rule out the possibilty of a wiring fault between the coolant temp sensor and ecu. at the same time it would be wise to test the wiring from the ecu to fan relay.
for the first test get a multimeter, disconnect the coolant temp sensor wiring connector, switch engine on, and then check for 5vDC between 2 of the coolant temp sensor's connector pins. on the coolant temp sensor's connector there are 3 pins. 2 are right next to each other. these 2 are connected to the ecu and are the ones to use in the test. in the attached pic you can see the conector, the bottom 2 pins are the ones to use in the test.
for the second test get a multimeter and check for continuity between the 2 smaller pins on the relay connector and the 2 corresponding pins on the ecu connector (to check for wiring damage). each of the smaller pins on the relay connector will lead to a pin on the ecu connector. unfortunately i dont have the info available with me to determine which pins on the ecu connector should be used. i'd need my haynes to work that out, which is at home. if you have a haynes manual the pin numbers will be given on the wiring diagram.
