Nissan Repair: Nissan Serena fans not working, radiator fans, nissan serena

Question
I have a 1993 diesel Nissan Serena which over heats in traffic. The radiator fans and the engine fans don't function. I've checked the fans, fuses and relays and changed the sensor on the engine and even the ECM unit. There is 4.5v on the sensor which is located near the thermostat housing. An identical vehicle has the same voltage, and when you remove the conector off the sensor the fans start, but with this vehicle nothing happens. There is also power to the relays, but no switch power to them. So the only thing I can think of is there is something between the ECM and the relays under the bonnet? Many thanks for your help.

Answer
Ed,

If the relay is good you could ground it and the fan should come on.  If not the fan is defective.  If the fan comes on then check the temperature sensor and the ECM.  There is a micro relay that grounds the fan relay in the ECM.  If it goes out the fans will not come on.