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Full Version S2 2.5td Cooling Fan Problem

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I wondered if anyone can offer any pointers to possible cause of following problem:

*cooling fan is permanetly on (as soon as ignition is on, before engine is started). I believe it is running at maximum speed, not half speed.

Car is an early S2 2.5TD VSX.

Car has been in independent garage for three weeks now and they cannot work out at all what the fault might be. I know they have changed a relay on the advice of a citroen specialist but this made no difference.

Any good leads very welcome -would dearly like to have my car back rather than driving the courtesy Corsa....

The fan speeds are determined by three relays situated underneath the black plastic apron above the front grille. They are mounted on a bracket above the nearside(right hand drive) headlight. The relays are in turn controlled by an electronic controller called the Bitron box which is located above the passenger (Right Hand Drive) footwell. It has the word "Bitron" moulded into the plastic cover. The Bitron box senses the temperature of the water using the sensor located on the aluminium block at the top right hand side of the engine (looking towards the rear of the car from the front) beside all the other sensors. The one which sends the temperature information to the Bitron box is the one with the brown connector.

Either the Bitron box is faulty, the temperature sensor is faulty or, most likely, the cable connecting the two together is faulty. The first place to check is the connector between the connector and the cable. An open circuit is interpreted by the Bitron box as a very high temperature and so it brings on both fans at full speed to cool it down. If that fails, do a continuity check between the sensor connector and the Bitron box connector to make sure the cable is OK. Next, check to see if its the Bitron box which is holding the relays in rather than melted relays. Lastly, find another Bitron box to swap with your own to eliminate a faulty Bitron.

If I get the chance I'll create a Self Help file to show how it works. In the meantime I've attached the wiring diagram to this post to help.
Relay 813 is the main power relay which is needed no matter how many fans run.
Relay 814 and 815 are energised together.
When 814 and 815 are not energined the fan motors are arranged in series so they run slowly.
When 814 and 815 are energised the fan motors are arranged in parallel so they run fast.
53 is the Bitron box.

The wiring diagram is only one example. There are many more depending on Aircon etc but it shows the principle.

Hope this helps,


noz cool.gif
Thanks for the informative reply.
Unfortunately the actual fault was not traced by the garage despite changing the temp sensor, the bitron box and checking all wiring!

Before bill would get to be too expensive it was agreed to fit an after market system with it own temp sensor and disconnect the original Citroen one.

Not the best of solution but at least the the cooling works again as it should do and I have my car back!
My 2.5 runs at 95ish most of the time rising by 2-5 degrees when
towing the caravan. The temperature control of the water jacket
happens in two distinct stages. From cold, the water is circulated
throughout the cylinder head circuit and does not go near the
radiator at all. Once the c/head is up to operating temperature the
thermostat starts to open. This allows SOME of the water to be
passed out to the radiator. On a cold day, particularly if you are
moving and there's a flow of air over the radiator then the water
will cool down rapidly and if left like this it will sub-cool the
engine below its preferred temperature. So the thermostat, seeing
the lower temperature, will close slightly until a balance is
reached. The radiator fans will play no part in the cooling role
during this time. However, as the air temperature rises or the car
comes to a standstill with the engine still running the water
temperature starts to rise because the radiator can't reject of all
of the heat. The c/head temperature therefore continues to rise
until the thermostat is fully open. If the c/head and radiator still
cannot dispose of the required amount heat then the temperature will
rise even further. Only then will the temperature sensor on top of
the block start the fans via the Bitron unit, on slow speed at
first. Continuing the hypothesis, if the slow fan speed still cannot
dispose of the heat (say on a hot summers day) then the temperature
will rise even further. At that point the Bitron brings on both fans
at full speed. If the temperature continues to rise, you're stuffed -
call the AA !!!. Logically, the reverse procedure takes place as
the temperature begins to fall.

Of course all of the above scenario takes place within a few
degrees, mostly within the thickness of the needle on the dash so
its not very noticable but thats how it works. For the electronics
people amongst us the thermostat will be proportional control only
so you will see the temperature difference on the needle. However,
the Bitron unit for the fan control is more than likely to have P+I
(Proportional + Integral) control and due to its inherent cleverness
will not exhibit the same temperature difference to achieve the same
control, hence the slight movement on the needle over the whole
control range. Anorak bit over ;-(

Blocked radiators or indeed any part of the cooling circuit will
reduce the heat rejection capacity and bring the fans into play much
sooner than they would otherwise. Increased temperatures on the dash
will also be evident.


noz cool.gif
Hi Noz, Serge & All

I thought the correct running temperature for an XM was about 90 and I am surprised that Noz's 2.5 is typically 95. All my 2.5's are around 90 and only the V6 seems to vary from 88 to 95. It is also the only one with brown sludge trouble which citroenxm has identified in another posting as a potential overheating cause.


Hi all as well!
I was going to ask why there is such a difference in running temperatures.My 2.1td runs at around 72 degrees only going up when on a steep hill or in a traffic jam.
What have/do your 2.1's run at Peter?
It seems to be a large difference in temperature for an extra 400cc in the same sized car.What temp should the fans come on?I have never seen mine work as the hottest mine has ever got was 90 degrees in a Periphique traffic jam!!
My water loss seems to have slowed dramatically,I am sure it is no coincidence as the heater blower has not worked at all this week,and I am sure the loss is related to the heater.
Regards Rob
Hi all!

Does someone know the resistance of the temperature sensor in various temperatures? I suspect my bitron module is wrong, because the relays and connections are OK, fans are working when you short the points that the relays would in normal conditions. But I don't know how much has the resistance of the sensor to change. Disconnecting the temperature sensor causes the fans operating (with ignition on).
My car is a 2.1TD VSX Aut.

Thanks in advance!

Regarding the running temperature it was for me at 90 degrees when everything was normal. I think 72 is too low, your thermostat must be stick open.

Best Regards,
Hi All,
the 2.1Td thermostat is set at 80 degrees whichi thought a bit low so ordered a replacmet stat from Citroen and that still ran at 80 Degrees so had a chat with the parts man at Citroen and it turnes out that the auto is fitted with a 80 degree stat whilst the manual is fitted with a 85 degree one so i ordered a 85 degree one and thats what my temp gauge now says, But the lexia now tells me that the engine is running at the wrong temp.


I am from brazil and need your help to identify some question about relay cooling fan.
From left to right I have all relays, and I did make test, all are functioning, but I don't know the correct requisits configuration of each one. Why is the function of each relay?

When I power on the AC the temperature change from 90 to 100 and 110. I think that my cooling system has a problem.

Sorry by my bad english sad.gif
other picture biggrin.gif
The last picture! ph34r.gif
Hi Jef360

I will find and post a typical XM fan circuit.
The relays switch the fans between slow and fast speed.
Each fan also has its own fuse.

It would help if you can identify the XM version, age and its build date.
Build date is a 4 digit number painted between the front door hinges.
VIN number from the plate above the fans will confirm version.

In the UK the cooling fan motors can degrade with age and lock solid.
If either fan locks up this reduces or stops the cooling.
It can also blow the fuses and can also stop the relay circuit working.

The easy check on this fault is to try to spin each of the fans by hand.
The fans should spin freely.

Hi John,

Thank you by answer. biggrin.gif
Build date is orga number 6616. Conform site decoded is December 20, 1994

It's a V6 Exclusive Fase II with Diravi, enginner ZPJ and HP18 Transmission.

Today in the morning I did test the fans, its okay, very fast and power.
I think that one relay is wrong. In the test I reversed the relay 2 and 3 happen improved operation with air conditioner on, but with air off the operation worsened because the temperature goes from 100 degrees.
If I find out which relay is wrong I can buy a new one and maybe solve the problem.

About code VIN I have a little diference: wacko.gif

I placed number 1 and 2 to understand this picture.
VIN number 1 I never try use on Lexia, VIN number 2 I did try to use, but Lexia does not read my XM with this code. My friend that has a Lexia use a VIN universal that function on a lot cars and make Lexia read my XM. laugh.gif
Hi jef360

That tells me your car is Mk2 V6 with prv engine.

I will add here the Mk2 cooling fan wiring diagram.
That is the wiring layout diagram.
The actual circuit is here.

You will see that the 3 relays can all be the same 5 pin types.
The circuit only uses the 5th pin on relay 815 but a 5 pin relay works fine in 813 & 814.
Check Maxi fuses 48 and 49 first.

These diagrams are for a Mk2 2.5TD but the fan section will be the same.

When I get the time I will look up the diagram for your actual model and post if there are any differences.

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