I bought the new alternator from Motor Parts Direct and they used the registration number to look it up. However, I'd already looked up the WAI alternator number myself and they got the same result as I did. Thanks for the information. I'm so fed up with it, I've not done anything about it yet. I'll get back on to it soon.

@TinyTim What is the reg number, or VIN? (Private Message me if you don't wish to publish such info for all to see).

I can look up with VIN, and I can find VIN with reg number, but I assume you're pretty sure the 03F903023E is factory original?

I've had the car for 9000mls and 18 months and the 03F 903 023E has been on it all that time, with no trouble until a few weeks ago. It doesn't have S/S as far as I'm aware! The VIN is TMBFM25JOE3131706. This listing for an alternator on Ebay shows my car in the compatibility list and cross references to the 03F903023E and the WAI 20588N. https://www.ebay.co.uk/itm/205297702030?_skw=03f903023e+alternator+wai&itmmeta=01JQEJD8GT42H612JB7BCG0TJN&hash=item2fccb2548e:g:jgIAAeSwvolnsx16&itmprp=enc%3AAQAKAAABAFkggFvd1GGDu0w3yXCmi1cpA2lhmjsJtpypgneemvWDtFyWGtzt2WhrcYfD3gufTbvgs%2F5P8QgwwIyrByzIW%2BCCM5Yibj9JWwKxM7etQ%2FGzXlARhNwqMavyYz2v1LOhwglUJMIqIF%2BkaMJZhMnbZHAekRjX2068Y%2BjubdOFZoXEaMA3FqRk9%2BpKtnUgkFlfty7mmFDAmeYVdyTmiceL%2BkJgWvc35G%2BUFVguGDiW1ll1B9VF2cosFeFbryZkhFPfulpTJljjgxmwfiw9Fsz%2F0S7%2FwDsNExYTfTJngjNatvj0JAIp96kcrmaE1jp3dVO5k44NUpgcwxpZe8sr%2F0y0TSg%3D%7Ctkp%3ABk9SR8iItdK7ZQ

The socket for the WAI 20588N is exactly the same as on the 03F903023E and the warning lamp functions correctly.

Here's an Amazon listing: https://www.amazon.co.uk/WAI-20588N-Alternator-Compatible-03F903023EX/dp/B0CYQ9Y77F

Here's another listing: https://www.ak24parts.com/en/spareparts/oe/03F+903+023EX

Are these all incorrect? You obviously know much more about all this than I do, but there are contradictions! I looked up DFM: DFM/M: (Digital Field Monitor/Monitor) This type of connection is connected to the vehicle ECU. Every alternator brand has a different abbreviation for the DFM connection: FR (Field Return); DF (Digital Field); DFM (Digital Field Monitor); M (Monitor); and LI (Load Indicator), but all function in the same way. Also: The "DFM” connection (DF monitor) facilitates recording the alternator's present utilisation status. This makes it possible to react to certain situations, such as raising the idle speed or switching off consumers that are not required. The signal curve of "DF" can be picked up at the "DFM" connection. I also found this: In the context of smart alternators, "DFM" (Field Monitor) refers to a connection that facilitates recording the alternator's current usage status, allowing for reactions to specific situations like raising idle speed or switching off unnecessary consumers.  You can't blame me for being confused!

https://www.waiglobal.com/eu_en/vehicalapp?vehical_type=Automotive&appDropdownCount=7&applicationSearch20=782322&applicationSearch21=799532&applicationSearch22=799597&applicationSearch23=799598&applicationSearch24=799620&applicationSearch25=1222898&applicationSearch26=1222899&page=1

Above is the look-up on the WAI catalogue.

Yep, I can confirm that your car is "without start/stop system, without regenerative braking", and the original alternator it was built with has part number 03F903023E. 👍

If you were to go back to the start of this thread, I'd be surprised if I hadn't explained the DF(M) function.

Many online look-ups have errors and overgeneralisations, so yes, I believe any source that says your car should have a LIN-controlled smart alternator is incorrect.

Typically, I believe that if a smart alternator doesn't receive data over its bus (LIN) connection it will revert to non-smart, fixed voltage mode, so that could possibly explain claims of compatibility, but wouldn't explain why yours is not working.

Probably still worth doing those voltage measurements at alternator and fusebox as described earlier.

Can you remind me why you think the WAI alt is LIN-controlled, please? It's not leaping off the page on those listings?

@TinyTim I'm going to go out on a limb here and suggest that you're wrong about that alt being LIN controlled, and that it is in fact suitable.

So there's another problem in the system.

Please do the voltage measurements to help pin it down. 🙂

At this point I need to review everything we've both written and see if I can find anything that suggests that the WAI alt. is LIN-controlled. I will do those voltage checks when time and weather permit. Thanks.

Yes, you did explain the DFM function. I see that I stated that the WAI 20588N was LIN-bus controlled. That was because I read that in a spec-list, but I don't know where at this point! I'm looking for it.

I give up! I'll assume it is a compatible alternator and do those checks.

There was 14.5V at the alternator output using the alternator body as the earth. There was still 14.5V at the +ve terminal on the battery, using the alt. as earth, but when I moved the earth lead to the -ve post, it was down to 12V. In the end, I found that the earth lead from the engine to the body was high resistance on the engine connection! I cleaned it up and now have 14v+ across the battery. I have learned rather late that a charging fault is not always the alternator or the battery! Thanks for your help. I now have a spare good alternator.