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Billy, Trainer/Master tech.
Category: UK Ford
Satisfied Customers: 3498
Experience:  Trainer 8+ years, Master tech. 10+ years, SAE member
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Ford Mondeo: I've recently had issues with my air con on a

Customer Question

I've recently had issues with my air con on a MK3 06-07 Mondeo.
The aircon only seems to come on when the outside temp drops below 18C. I've change the Fan & Control Module, also I've had a new compressor installed.
The AC compression is spot on.. The only thing I've found is the compressor is onlt receiving 4v, but I thought it should be 12v.
Any ideas on where else the fault could be?
Many Thanks
Submitted: 2 years ago.
Category: UK Ford
Expert:  Billy replied 2 years ago.
Hello Dan and Welcome!
I'm Billy and i will assist you with your inquiry.
Feel free to ask if any further info is needed.

Indeed the compressor should get 12 Volts.
Do you have the automatic A/C (climate control)? or the manual A/C?
Customer: replied 2 years ago.

Thanks Brian

I have the Denso touch screen system on the final MK3 face lift model

Customer: replied 2 years ago.

I have also scanned for DTS codes and returned none.

Expert:  Billy replied 2 years ago.
Hello Dan,

I think you mean DTC (diagnostic trouble code).

The most likely cause on your system would be bad wiring, or even bad connectors (should have clean and tight contacts). In fact, this is a common problem with ford, i will include the bulletin:
  • AC/Heating
  • Electrical
  • Engine cooling
  • Engine management
  • Starting and charging

Affected vehicles

  • All models, →2010.


  • Poor performance or failure of AC system.
  • Engine will not start.
  • Failure of one or more electrical systems.
  • Engine coolant blower motor inoperative.


  • Open circuit/high resistance in engine wiring harness due to water ingress.


Description Part number Quantity
Wiring harness repair kit 1 736 825 1
  • Carry out electrical, mechanical and component tests (as applicable) to eliminate any other possible cause(s).
  • If no fault(s) found: Inspect engine wiring harness. Locate damaged wire(s), repair as required using engine wiring harness repair kit.

However, since the problem you have only affect the A/C compressor, and having checked 4 volts on the compressor (should be 12V) i will include the wiring diagram getting to the compressor, so you can check the related wiring, clean or even replace as needed. click here for details

Kindly get back to me with your findings, or if further assistance is needed.



Customer: replied 2 years ago.

Thanks, ***** ***** not explain why I would get AC when the outside temp drops below a certain temp (i.e. Early mornings).

Expert:  Billy replied 2 years ago.
Hello Dan,

Most problems with wirings, or wiring connectors or relays (at the A/C module, at the central electronics, at the relay or even at the compressor) are very temperature dependent, as the wiring would expand and retract with temperatures, and water would evaporate causing humidity or condensate leading to short circuits.

This does not mean, that the cause is confirmed. This just means, that these need checking first, given the symptoms. As they would be the most common cause (especially on your model) for your problem.

Customer: replied 2 years ago.

Hi, I have now check over the wiring. I cannot find any faults relating to the wiring.. I've tried shorting the low pressure sensor to see if the compressor clicks, and it doesn't.

I can 12v to the fan, if I connect this direct to battery it works. What I'm not getting now is any volts to the compressor.

Expert:  Billy replied 2 years ago.
Hello Danny,

Let's try jumping the compressor clutch relay (with engine on, and inside blower on, setting at cold), see if the compressor works. If yes, we would know that the problem is between the relay and the command (engine control and A/C control) so we proceed with the checks. The compressor clutch relay should be in engine bay, near battery. However, following the wire from the compressor should also lead to this relay.

Kindly get back to me with your findings, so we proceed accordingly.

Customer: replied 2 years ago.

I've now managed to test the compressor by connecting it to the battery. The compressor clutch engages as expected. The fan doesn't start, I didn't notice any colder air coming into the cabin tho

Expert:  Billy replied 2 years ago.
Hello Danny,

is it the blower inside not starting? or the fan for the condenser outside (in the latter case it would be normal).
In this case the compressor is engaging all the times needed. Actually you should hear it click even if you don't get the cold you need.
What pressures are you getting at the A/C circuit (as you mentioned those being spot on)?
Customer: replied 2 years ago.

The inside blower was on, I meant the condensing fan had started. I don't have the tools to test the pressure. When I was with the A/C engineer he was reading pressure on both high and low pressure valves.

With the compressor wired to the battery the clutch remained engaged for the entire time. When it's connected back to the car, the clutch doesn't even try to engage, you can see the clutch does nothing.

Expert:  Billy replied 2 years ago.

So when the compressor is working (powered directly by the battery), do you get cold air?
if yes, we need to check the wiring to the compressor (relay, wiring to and from the relay, till we get to the command).
But if, even with the compressor engaged, and inside you have the settings for maximum cold, with blower set on minimum, recirculated air, you don't get cold air, then check the freon line at the low pressure it should be freezing if not, we need to check the pressures.