Hi, hoping to get some advice.

The battery light on the dashboard turns on intermittently on my Fiesta and have ran some tests with a multimeter. Previous owner took it to a garage and they couldn't find the fault (alternator voltage, wiring was within spec). They replaced the battery and spark plugs last year (Lion 063 Car Battery) but didn't fix the issue. If it helps, the airbag light was on at the same time and was resolved by replacing the sensor but it probably isn't related.

Battery terminals and leads were cleaned and sanded a day before all tests.

 

Voltage Measured at Battery (red to battery positive, black to battery negative):

• Car Off - 12.6v
• Starting Car Min Voltage - 10.5v
• Without Accessories On - 14.40v (14.36v with battery light)
• With Accessories On - 14.22v (14.20v with battery light)

Voltage Measured at Alternator (red to battery positive, black to alternator case)

• Alternator Voltage - 14.4v (same with battery light)

Voltage Drop Measured Between Battery/Alternator

• Battery Negative to Alternator Case - 0.04v (same with battery light)
• Red to Battey Positive, Black to Alternator Positive - 0.01v (0.08v with battery light)

 

Also, the battery light doesn't come on immediately after starting the car. After a few minutes of idling, it sounds like the radiator fan turns on then off after a few seconds then the engine revs a little and the battery light turns on after that.

There is 1 purple wire attached to a connector on the alternator and from what I understand this can be the problem (related to smart charge I think?). I want to inspect the wiring and connector but I can't unclip the connector to get it out. The Alternator positive cable is also in the way. How else can I test the smart charge system?

Any advice is appreciated. I think everything looks like from my multimeter tests. Maybe apart from that 0.08v alternator/battery voltage drop with the light on. I have no clue what the problem could be. 🙏

6 hours ago, landfall said:

There is 1 purple wire attached to a connector on the alternator and from what I understand this can be the problem (related to smart charge I think?). I want to inspect the wiring and connector but I can't unclip the connector to get it out. The Alternator positive cable is also in the way. How else can I test the smart charge system?

........ and there you have the answer.

You are going to have to remove the connector from the alternator one way or another.

There's a problem with the alternator, that's why you're getting the light.

Just change it

21 hours ago, unofix said:

........ and there you have the answer.

You are going to have to remove the connector from the alternator one way or another.

Managed to get it unclipped. Smart charge connector and purple wire appear to be fine and not corroded. Does anyone have wiring diagrams for this circuit (which I assume goes to a connector to the PCM) so I can check for continuity in case the purple wire is broken somewhere else? If there is another way I can test it please do tell.

12 hours ago, DaveT70 said:

There's a problem with the alternator, that's why you're getting the light.

Just change it

Alternator is charging at ~14.20v with load, I don't think that is the culprit. Others have also replaced both alternator and battery but still have the battery light.

With the connector removed the alternator should have a fixed charging voltage of 13.95V which is the standard default in the event that the Smart charge system has a fault.

Your connector is damaged. Has probably been taken apart before for some reason.

Apologies for late reply. Ordered one and waiting for it to arrive then I'll fit it. Bit difficult to find one of these in the UK.