Hey, can anyone offer advice how to proceed with this diagnosis?

My mother's 2007 Roomster 1.6 (BTS engine) suddenly has the alternator warning light permanently on, even when the engine is running. Also displays the "alternator workshop" warning after a moment of running. Zero other symptoms.

Charging voltage at driving RPMs stayed above 13.5 and capped at 14.3. At idle it was around 13.4 at light load, but dropped momentarily to 12.7 with every load imaginable on + wiggling the steering to make the power steering consume even more. Sounds fine to me.

The alternator and battery are a few years old now (last alternator stopped charging altogether), but have not shown any symptoms of going dead, the battery holds it's voltage fine when sitting and starts the car even at under -20°C.

I checked all the wiring related to this and found nothing.

-Battery - alternator postive cable was secure and corrosion free

- Engine/gearbox - framerail negative cable ditto

- Battery - strut tower cable ditto

- All the fuses and connections right on the battery are fine

- The brown/red and later purple/green sense wire has good contact to the engine ECU connector. This had the crap connector near the gearbox, but i redid the wires some years ago when i replaced the alternator. I redid them again now just in case.

- The blue exciter circuit wire from the alternator to the BCM is fine. I traced this all the way to inside the BCM and it's microcontroller. Redid the wire as above.

- The instrument cluster is visually alright. I couldn't figure out which wire carries the alternator signal to it without a diagram. CAN from the BCM?

The BCM had a 01598 "Drive battery voltage signal too low" code, but i am unsure if it is related or historic from sitting in the colder weather some time earlier. Did not return on a ~15 minute drive with the warning light on all the time. No other codes in any of the ECUs i can access with the crap reader i own.

22 minutes ago, xmikael said:

CAN from the BCM?

Yes.

I will think about this situation overnight, and look at circuit diagrams tomorrow.

It seems like you have done a very thorough investigation already, so I'm not sure I can suggest much.

Is the thin brown wire that connects to the negative battery clamp still connected?

43 minutes ago, Breezy_Pete said:

Is the thin brown wire that connects to the negative battery clamp still connected?

There isn't one and i can't recall there ever being one. I saw this mentioned in some other post somewhere and it was questioned there too if only some models had it? Do you know where the other end of that wire leads? I could check if there is any sign of it further down the line.

Thanks for the reply, i appreciate it.

The other end goes to the BCM, same connector as the blue exciter wire I think. Will tell you pin number tomorrow.

10 hours ago, Breezy_Pete said:

Will tell you pin number tomorrow

That thin brown from battery negative is shown for Roomster May 2007 onward.

It is routed from elsewhere (not battery neg) from May 2008 onward though, so I guess maybe this car is later than that. Cars with this later arrangement have a brown/green wire instead of plain brown.

Strangely our 2012 Roomster has the thin wire to batt negative clamp, so they appear to have reverted to this arrangement later.

It terminates at pin 8 of the connector that goes into XS4 on the BCM.

Another wire that may be relevant is a permanent supply from battery fusebox fuse #11 ( 5 Amp) to XS4 pin 5 (thin red wire).

Edited 10 July, 202510 Jul by Breezy_Pete

Thanks, i'll take a new look when i'm around the car next time and post more of my findings here.

You are most likely correct that the car is actually newer than we remembered, i just checked the registration and it's been registered in 9/2008. I'll take a look at the actual paperwork when i can to see if i can figure out the manufacturing date too. Sorry for my mistake!

Turns out the alternator voltage regulator was faulty. Taking a graph of the voltage showed brief periods of low voltage under 12,5V that i didn't pick up with just a real time readout.

A new 20€ regulator and a few curse words later it seems to be fixed. The car may see half a million kilometers after all...

Edited 21 July, 202521 Jul by xmikael