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Air con not engaging

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Hi everyone 


I wondered if you could help, or advise the following:

I have a mk4 1.8TD Mondeo

The air con is not working. 

So far I've eliminated the compressor as the issue, as the clutch will engaged and the compressor will run when I remove the relay and bridge the contacts, the cab get very cold, very quick. 

I had the vehicle re-gassed today, so that is not an issue either, there are no leaks in the system. 

I have a DTC code: P0645:00-6C - A/C Clutch relay control circuit. 

I have searched the forums and found that the purple fan wire is a common one to corrode, now the N/S wheel arch, at the 'T'.

I've pulled all the trunking from the loom and investigated for any signs of corrosion in that area... there is none that I can see. 


I'm wondering if anyone else has come across this, or of anyone can point me in another direction at what this could be?


Thank you in advance 



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Does the fan rotate when you switch air con on? Someone on here recently had the same issue and swapped the Aircon relay for the horn relay to see if the relay was at fault. It was but managed to work sufficiently for horn. 

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Thank you for your reply.

The fan doesn't spin when a/c switch is on, i figure that is because the a/c is electrically shut off due to the fault. 

I have tried swapping the relay, clearing the fault codes, but immedietly get the same fault.

I wonder now you've said that if both the horn and the a/c relay have both blown. My horn doesn't work at the moment. Which is something I attributed to a fuse blown, after checking the fuse is good.

My next port of call.... go to Halfrauds for two new relays. 


Thank you, I will let you know the outcome  

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This purple wire... I think you need to actually check it's continuity from end to end. If you haven't got a multimeter, then now would be a good time to buy one! 

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Hi Nicam49

Thank you, my bat is dead on mine, but I think you are correct. 

Only I'm not sure where the other side of the purple wire would be. I was kinda hoping have a eureka moment, and spot an obvious break in the wire..... but when does that ever happen lol 

Would be any chance know? One side on the fan, the other to...?

Sorry if I sound like a noob, just trying to learn as I go! 

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pcm. thats under and behind the n/s/f wheel arch trim

sometimes the wire breaks inside the plastic so you cant see it. 

have you looked to see if the wirin/connector is still fitted to the evapourator temp sensor. often when servicing mechanics disconnect that temporary when fitting the odour/pollen filter. but that would show up a dtc depending on code reader cappability. 

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I know I've read a thread about this somewhere.. if I find it I'll post a link up. I think if you find the wire where it goes onto the PCM, break open the insulation and measure continuity from there.... 🤔 


Here's that link to a really good thread.... 


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Hi again 

So I spent about 3 hrs last night removing the PCM.... the hard way first, before realising the wheel arch was the way forward.

We traced the pink / purple wire to the PCM port, and load tested between the fan and the PCM. The wire was fine, so I dont think it is that issue.

I also checked under the dash, behind the fuse box for any loose connections, or unplugged plugs, but visually everything looks fine. 


I am at a loss now to know what to do next, it seems as thought the signal to switch the relay is not getting there. I can't see a fault code that would suggest it is a managment issue, although annoyingly my FORSCAN has decided that it will not test in MS mode any more, is there a setting somewhere that will allow the switch, it did work and would pop up to ask me to switch to MS half way through the scan. But that doesn't happen anymore for some reason. Any suggestions on that?


Aside from that, is anyone able to explain in layman terms how to test the port for the relay?

See attached for a basic drawing of what I have.


Once again thank you all for your patience and help! 



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One of the those two connectors for relay should have 12v, the other goes to pcm to provide earth path to energise relay coil. So next would be check for the 12v . It's supplied via fuse 7

Is it auto climate control or manual control 

If you have auto climate control, see below

The climate control and heating system features an on-board diagnosis function which can detect and store both current permanent faults as well as intermittent faults which have occurred during normal operation of the vehicle. It is also possible to read out these faults via the display on the climate control assembly. To read out the fault memory, the ignition key must be turned to the "ON" position and the battery voltage must be between 9 V and 16 V.


Activating the on-board diagnosis function


At the climate control assembly, PRESS the "OFF" and "FOOTWELL" buttons simultaneously for exactly 2 seconds, then PRESS "AUTO" within 1.5 seconds. The on-board diagnosis which then starts lasts a few seconds. An animated display appears in the display of the climate control assembly during this time. Any faults found are displayed on both displays of the climate control assembly in the form of trouble codes.


Example: Left-hand display shows "90", right-hand display shows "B3", i.e. the trouble code read out is 90B3 = short in circuit for right-hand center vents air outlet temperature sensor (short to ground).


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Ok so an update, I bridged the right hand connector to ground and the compressor kicks in.
Obviously this is the same as bridging the to upper poles, so the compressor will not cycle, and eventually blow the gas out. 

So question is, if the relay is working correctly, then it has to be a signal issue. to diagnose this i will need to check the wiring from there backwards to the swith in the climate control module. Does anyone know what colour wire is on the underside of that pole in the relay?

I have amended the drawing and attached it to this post. 

Second question is could it be a problem with the actual switch on the climate control inside the car, or is it likely to be  that the car is preventing it from coming on through a fault?



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Hi again 


I have just plugged the card into FORSCAN again and checked the PID

I have attached a screenshot of the live reading, it looks as though the controller module in the car is not requesting the Air Con to switch on. 

At least that is how I am reading it. 

The PCM1AC is the scope that should read on when I press the button requesting A/C 

Yet it doesn't change from off. 


Am I reading this correctly? 


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Hi all

So I managed to fix the issue.

It was the orange and white wire that runs from the fuse box to the PCM.


I read on another forum that it is a common one to go, the will look fine visually, but if you tug on it (not very hard) it will snap down by the `y` in the loom.

There are two orange and white, so I just tugged both of them separately, the first remained, the second broke away so easily, and you could see the copper was green inside the wire. 

I just cut out the bad wire, replaced it, did a continuity check and removed the code in the DTC and hey presto, it works!


Thank you all for your help!

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