300Tdi cooler fan relay gone crazy

[JFD]

The first symptom was the cooler fan running as soon
as engine was started in the morning. Usually, switching
ON then OFF A/C blower fan was enoug to shut down the cooler fan.
Now the cooler fan relay is triggering like mad.

I've been looking at electric diagram, the activator relay
circuit goes from fuse Nº 15 to relay then to A/C switch
fan. How this circuit knows when engine is hot ? If i find
the relation beetween engine temp. and relay circuit, then
may be i'll find the fault.

Any A/C electric knowledge to share ?

Thanks :)

[JFD]

Find what is controling relay, it's a switch
on thermostat housing. Anyone know the
part number ? It seems not to be on the
part catalog.

[JimC]

There are two switches that should be checked here - both the thermo switches on the thermostat housing and the trinary switch on the AC receiver drier. The trinary switch takes binary temperature input and binary pressure input from itself to decide 1) whether or not the cooling fan should be on and 2) to shut off the compressor as a high temp or low pressure failure mode.

[JFD]

Jim, i have no switch on the A/C receiver-drier, nothing
electric at all, just 2 pipes :eek: ...

I have 2 switchs and 1 sensor on the thermostat housing.
One of those switch seems to be wired to the relay, same
wire color and size and i checked for continuity. The earth of
that switch seems to be all right also. My only explanation
so far is that this switch is defective, i think i will have to take
it out just to check part number.

[Stmpede]

Have you checked the relay? If the switch was telling it to go on it should stay on after cycling the A/C switch I think. When relays go bad they have a tendency to chatter or stick in either position. If it is chattering I would check both the relay itself and the ground to the relay. If there is a bad ground to the relay it will cause all sorts of problems.

[JFD]

Yeah, i changed the relay already, same issue.
The ground goes thru the switch on the
thermostat housing, and after the switch
it is all right.

[JFD]

I found the part number of the thermostat elbow,
ERR3738, i'm missing the part number of both
switches under it.

Anyone ?

[JFD]

Got them :)

From the front of the vehicle, first one is PRC3366
and second is PRC3541.

In my case, i think PRC3541 is giving erratic signal
to fan relay, i'll change it and see what will happen.

In order, the old, the new and the elbow:

[323jamie]

do you have an ac? is this what the double sensors are for?