UK Car Repair: Mondeo battery light, auto electrician, thin wires

Question
I have a form Mondeo  Ghia  2L 2007
A few months ago the battery warning light started coming on and staying on.
I took it to a garage, they tested it with a computer and said it was the alternator.
They changed it, for couple of weeks it seemed fine, and then the light started coming on again.
I took it back to the garage who said it seemed to be an alternator fault again, the new one must have been faulty. They changed it.
All seemed fine for a couple of weeks and the light started coming on again. I took it back to the garage. Again it seemed to an alternator fault.
They got an outside auto electrician in who checked out the alternator and said that was not faulty and it was still charging the battery even when the light was on.
He could not find out why the light was coming on.
I have since noticed that it seems to do this when the weather is really cold.
Also, the light comes on and stays on a couple of minutes after starting the engine but if I stop the engine and start it again it goes off and stays off.
I have been running the car for 3-4 months now with this light coming on.
Have you ever come across this type of problem with a Mondeo or any other car before ?

Answer
Yes.


It could be the sensor misreading the voltage and "thinking" the alternator is bad when it's not.

Or, the ECU contains the computer that interprets the readings form the sensor - this could be broken.


It's a fairly harmless but expensive to rectify fault, the test procedure involves an oscilloscope to test the charge "quality"(which will be good because it's a new alternator),

then the sensor is tested similarly - if that tests good then its a new ECU, which requires new keys, dashboard parts, ignition barrel..

The 3 pin sensor plugged into the alternator: Three thin wires!! - check this small wiring assembly for damage, it is fragile and may just need splicing/clean the connector.

Otherwise it's a big job and the security system is known to be torturous when replacing the ECU - Verifiable genius mechanics have lived and died trying to get ford immobilisers to recognise new ECU's.   Sorry.


PESROSNALLY: I would get a charge indicator circuit for a kit car and bypass the existing battery light, this would then work like a generic charge indicator - if the alternator is giving 12v or more the light stays off. Make enquiries at your local parts shop.