Sounds a strange question but here goes.  2019 Ford Mondeo Titanium.  The battery light came on and the heated screen stopped working.  The alternator was producing a low level of charge to keep the car running but was not increasing the output for high demand.  A fault code indicated an open circuit, eventually traced to corrosion in the wiring loom under the front driver side wing.  Replacing the wire fixed the battery light and the alternator now outputs the full 14.5V.  

On my old Mk3, rather than turn on the blower to clear a little fog on the inside, I would turn on the heated screen.  I noticed that this does not work on the Mk5 and immediately blamed the recently fixed wiring loom for the problem (BTW, both fuses are fine).  To confirm my suspicion I started the car and listened carefully to the engine tone.  When I turned the rear window heater on and off I could here the tone change as the alternator demand increased.  However, when I did the same for the front screen heater, there was no change in tone.  This confirmed my suspicion that the screen, or one of its control circuits, was blown.  The screen has two 40A fuses to the single 40A fuse on the rear window, so it should demand a lot more power.

At this point I was sure the screen heater elements were gone.  However, yesterday was the first frosty morning of the season and I was just getting ready to scrape the window when I noticed that the ice was melting.  The same thing happened this morning.  There has been no miracle fix because there is still no change in engine tone when I switch the heater on an off, which would indicate an increase in demand from the alternator.  

There are more superfluous electronics on the Mk5 than my old Mk3, so I wonder whether Ford have done something really dumb and programed the computer to restrict the output of the screen heater when the outside air temperature is only marginally cold?

I would be grateful to know how other newish Fords behave.  Is there something wrong with my heater, or is the functionality just worse than the old Mk3?  Nothing surprises me about new car.  There is just more to go wrong and keep you going back to the dealer.

There are a couple of things that have to be met before the heated front screens (there are two separate half's) will operate.

Firstly the State Of Charge of the battery must be at least 70% or better.

Secondly the outside ambient temperature must be 7^oC or less 

Edited November 26, 20232 yr by unofix

Hi Unofix,  Thanks for the info.  This morning when the ice melted and the air was about 1C outside, the engine tone did not change when I switched the heater on and off.  This suggests that the front screen heater is drawing less power than the rear window heater.  I might still therefore have a fault.

I would be grateful if some of you guys out there would listen to your engine tone tomorrow if it is another cold night.  Start the car, turn off everything.  Then turn the rear heater on and off.  Then try the front screen heater.  Let me know what happens.  I'd like to sort this out because I'd like to go back to the Ford dealer and tell them they did not fix the problem when they charged me £700 for fixing the corroded wiring loom.

Many thanks, Ronnie.

2 hours ago, Ronnie123 said:

This morning when the ice melted and the air was about 1C outside, the engine tone did not change when I switched the heater on and off.  

The heated front screen and the heated rear window are controlled by the BCM and operate in different ways. This is to minimise emissions, save the dolphins and above all else keep Greta happy. 🤣