Hi all hope you are well I’m a little stuck and desperate for any help. Basically it’s a 2011 fiesta 1.25 zetec petrol and what’s happening is when you start the car all as should be ie no lights on and all heated screens work etc after approx 30 seconds the battery light comes on and the revs raise slightly at the same time then returns to idle with the light on and no heated screens now working ??. The battery is charging at circa 14.2 when running  before and after the light comes on.

 

so far I have had replacement alternator and belt. Also new single wire logic connection multi plug and wire tracked all seems ok. Battery swapped from another car and same problem as before 

 

2 garages hit a brick wall with this really fed up and would like to know if anyone has had similar problems?. I have tryed a search but couldn’t find anything I know it’s hard been a newbie and first post is a long winded fault finding post but I’m desperate all the best 

Possible high resistance in the smart charging circuit causing alternator and BCM not seeing the correct voltage.

On the multi pin smart charge plug on the alternator one of the 3 wires is a battery sense lead, it goes to the battery positive terminal (via a fuse normally). If this circuit becomes high resistance it can cause the BCM to think that the battery voltage is low and therefore stop things like the heated screen from working. Sometimes there is a noticeable flicker of the headlights at engine idle.

There was a modification done to some models to fix the problem of the headlight flicker which was a simple piece of wire added between the main alternator positive lead (Big heavy lead held on to the alternator with a stud and nut), and the battery sense lead of the multi pin smart plug.

Thank you for your time and the much appreciated reply Makes sense but mine is a single wire smart charge plug not 3 wires.

with the single smart charge wire unplugged from alternator it’s exactly the same fault as with it plugged in. 
 

I will try check for high resistance how would the best way be to do this ?.

 

regards

3 minutes ago, Kop2381 said:

with the single smart charge wire unplugged from alternator it’s exactly the same fault as with it plugged in. 

That kind of proves the fault is with the smart charge circuit. With it plugged in the battery charging voltage is generally less than 13.6V but when unplugged the alternator should default to standard charging which is about 14.2V.

4 minutes ago, unofix said:

That kind of proves the fault is with the smart charge circuit. With it plugged in the battery charging voltage is generally less than 13.6V but when unplugged the alternator should default to standard charging which is about 14.2V.

Mmm my thinking too but I have tracked the single wire back through the loom and all looks ok. Il go back through the wire at the weekend to see if I have missed anything. All fuses are ok unless there’s a mega fuse somewhere ?? 

The smart charge fuse is normally about 7.5A

Problem could be with the BCM.

Use the software FORScan (it's free) and see what the system voltages are.

28 minutes ago, unofix said:

The smart charge fuse is normally about 7.5A

Problem could be with the BCM.

Use the software FORScan (it's free) and see what the system voltages are.

I will look into it and report back the findings when I get chance with work etc etc