Would help if you told us which engine you have. Anyways, on all Foci the
cooling fans is controlled by the ECU, which grounds one or both coils in a
relay in the engine fuse box. This starts the fans at low speed, and if the
ECU sees the temp rising further (measures it through a single sensor- the
CHT or CTS) then it will pull in the other contact in the relay and switch
the fans to full speed.
With the a/c engaged, the ECU grounds the slow speed position to run the
fans all the time, unless it receives a signal from the thermostat in the
air con evaporator saying the evap is cold enough (4 deg C) in which case it
releases the air con compressor clutch, and 10 seconds later switches the
When the thermostat calls for more cooling, the clutch pulls in, and 3
seconds later the fans start again at slow speed.
So, if your temp gauge is rising past 12 o'clock and the fans arent running,
then I suggest firstly you check the fan control relay and the 2 fuses in
the engine fuse box.
I'd then put the dash in DTC mode and see wot temp the ECU actually thinks
the engine is running at- your CHT / CTS may have gone out of tolerance- and
sending a signal back saying the engine is cooler than it is. However in
this case I would expect the dash gauge not to reach full temperature.
Tim thanks for the info. The engine in my focus is the zetec. I checked the
fuses but dont know how to check the relays if i can. When the engine
idles for a long period of time it the temp gauge goes in the red and i
have to shut it down , but if i am running the a/c the fans work and there
is no problems with overheating. is there an temp sensor for the computer
that i should be looking at ?
thanks in advance
The temp sensor is fine- this is cos your gauge is working fine- the gauge
gets its signal from the ECU, which gets its signal from the one and only
CTS (under the alternator at the back) Your problem would seem to lie either
with one of the fan fuses, or the primary fan relay. I've buttered up on the
wiring diagrams, and there are 3 fan relays on a/c cars, one for the a/c and
2 for fan actuation when the a/c isnt on, all of which the ecu controls. You
must have a problem with one of these.
If your not reasonably conversant with vehicle electrical systems, then I
would suggest you get a dealer to check and rectify the problem.
That said the Zetec E ECU is very cunning and can avoid an overheat
situation for whatever reason by at first reducing engine power (wont rev
over 3000rpm) and then cycling the engine onto 2 cylinders at a time (to
allow the other pair to cool off) and finally it will shut the engine off
alltogether. Obviously the dash will be lit up like a christmas tree during
all of this, but you can at least drive the car a short distance to a place
where it is safe to stop. The handbook details all of this.
I have an extremely similar problem. I am very observant though and have
done a few things and noticed some differences. The fan running while the
ac is on is definltely right. I have noticed, however, that if it starts
to overheat, and gets to about the 1:30 position, if i free rev the motor
a few times to 3000 rpms or so, i heara the fans click on and the car
cools down. as long as i watch the temp gauge and tap the throttle when
it starts to overheat the fans come on and cool it down. The other
symptoms are exact. Do you think this could be a faulty relay also?? How
would i determine which relay is bad? thanks in advance.
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