I was checking my Corolla (92 DX, 4AFE) and saw there is
another sensor-like stuff at the bottom of the radiator
on the passenger side.
Assume the temperature sensor to the ECU is located
near thermostat housing, just can't think of what this
Not likely, the following two figures show 5 sensor-like stuffs around
the thermostat area. The fan switch is actually behind the red handle,
i.e. the white plug. Unplugging it will keep the fan running.
The other four plugs are start inject time switch, temp sensor to ECU,
temp sending unit to the gauge and BVSV.
I really have no idea why another sensor is needed on the radiator
( to double check the temperature?)
This is not documented anywhere from either Haynes or Chilton and
not seeing anyone mentioned it on this forum.
I haven't tried to unplug it, but I did try to unplug the one on the
thermostat (behind red handle). It controls the fan. Also this is
confirmed from multiple sources that the fan switch is on the thermostat
and it should only contain one wire!
But the one at the bottom of the radiator has two wires, so it is more
likely a sensor just like the other sensor on the last figure above.
Anyhow, it seems one of the components is duplicated, could someone
toyota has set up the cooling system in a fail safe way, if any sensor does not send a signal then the radiator fan automatically turns on to prevent worse case senario of overheating, this is why unplugging a switch will "control" the fan.
Just let you know that I unplugged the one on the radiator today, it actuated
the fan immediately.
So, I think this is the second fan switch. The reason it has two wires instead of
one wire is because the two are cascaded together, the one on the themostat
housing then connects to ground. As long as one of the switches is open,
the fan should be turned on.
This is not documented on the electrical diagram of Chilton manual, so I
suspect it is optional, shorting the switch (in case your radiator doesn't have
a place to put in the fan switch, possible?) will leave only one switch functional.