Alternator light on but alternator ok?

[Cwatters]

2006 petrol fiesta.

Alternator light is on but alternator seems ok as I puts out 14.3V at idle. Voltage doesn't seem to vary much with load or rpm. Has been driven like this for perhaps 100 miles so pretty sure alternator is ok.

Have heard it could be Issue with "sensor" wires on the alternator corroding. Is there a wiring diagram available anywhere so I can buz out the loom?

Have also read that it could be a temperature sensor issue? The ambient temperature display on the dash is displaying --- but it's been like that for a  few years so unlikely to be causing this problem? Is it easy to replace the sensor?

Would it be worth me getting the Haynes manual to help sort this? Does it have wiring diagram?

Would a Diagnostic port check be likely show the cause?

Thanks for any advice.

 

 

[540itouring]

often the alternator is still at fault even when it is still charging ok. I have found when a wire brakes is will not charge at all.

[Tdci-Peter]

13 hours ago, Cwatters said:

Alternator light is on but alternator seems ok as I puts out 14.3V at idle. Voltage doesn't seem to vary much with load or rpm. Has been driven like this for perhaps 100 miles so pretty sure alternator is ok.

The Ford charging system uses a two pin connector between the alternator and the ECU. Normally the ECU can control the charging, and it can know about ambient temperature, and electrical load. So it can optimise the charging a bit (Smart Charge, some call it), though it usually works much like a normal charge system.

(Later Fords, like the Fiesta 2008 on or Mk3 Focus, have a one wire serial data bus (LIN) connection instead of the two wire connection.)

If this two wire connection is broken somewhere, the alternator should revert to standard, constant voltage charge. So it will still keep the battery charged, but the ECU will put the battery/alternator warning lamp on.

Problems can be in the alternator, in the connectors or wiring, or even (but unlikely) in the ECU. It will need a garage who understands the Ford smart charge system to diagnose properly, or Google Ford smart charge if you are prepared to use a 'Scope and multimeter as diagnostic tools.

The in car diagnostics, even with a good system like Forscan, are unlikely to be able to separate wiring problems from a problem inside the alternator.

Though you can quickly visually check the the small connector in the alternator is ok, not corroded, and no visible broken wires.

Haynes should show the wiring, but it should be possible to trace it through from the alternator.

Since the ambient temp sensor is an option, and I assume its failure pre-dated the alternator light on, I doubt if this is related. I think the ECU gets temperature information from the coolant temperature sensor, and roughly guesses the battery temperature from this, and time.

[stef123]

Without a scope you ain’t going to be testing much.

you should have a 3 pin connector on the alternator, this gives;

Pin1 - feedback 

pin2 - command

pin3 - reference voltage (you should see battery voltage here) 

[Tdci-Peter]

15 hours ago, Cwatters said:

Alternator light is on

 

1 hour ago, stef123 said:

3 - reference voltage (you should see battery voltage here) 

Yes, I forgot about the battery voltage connection on pin 3. My memory is getting more and more like a famous Swiss cheese, one that is being slowly eaten away by mice as well!

There will be a fuse on this battery supply, but it probably powers other stuff too, so is unlikely to be blown. The other two are sometimes called Alt Mon & Alt Com. They have PWM signals, so are best viewed with a scope.

Buzzing through any wiring to the ECU is a pain, since there should be a security bolt that has to be drilled or ground out to remove the connector.

 

[stef123]

If that’s that same bolt as the focus uses it’s a pain in the backside lol

[Cwatters]

Thanks for the replies. Have got it booked into local garage. Due for a service anyway.