C3 Charging circuit diagram please Marc :)

[MikeT]

This is where I need help. All I know is the alternator would place excessive drag when starting so is not given the "signal" (whatever form that takes) to generate until engine is known to be running on fuel (ie any speed below idle?). Any excess charge would dumped to earth via the regulator, rather than it being controled by, say PWM to match loads.
But what form should this signal be - voltage? I can see the terminal and can likely backprobe it if necessary.

[xantia_v6]

The actual excitation current to the brushes is controlled by the regulator in the alternator. I don't know how much control the BSM has.

Worn or contaminated brushes can cause similar symptoms, but these days they should last 150,000 miles or so.

The alternator putting load on the engine while starting was not a problem for 100000000 cars built without BSMs.

[EDC5]

This pic here is the starting sequence of my engine showing battery current and voltage that I recorded with diagbox.

From the smooth way the voltage climbs to 14v once the engine finished cranking I'd say that the control signal to the alternator might be PWM or analogue rather than a digital signal?

[white exec]

Alternator drag on the engine at cranking speed will be very small: back-drag at the alternator will be proportional to its output (more output = more work = more mechanical power drawn from the aux belt), and therefore very low at very low revs, and you don't get much slower revs than cranking. Current input (to an alternator) is also small.

I have read that, in one more attempt to wring 0.00000001 mpg out of ICE, alternator output can be 'intelligently' reduced/restricted to shave a few bhp at driving speeds. Typically alternator rpm is around 3x engine rpm, and many alternators don't reach max output until around 3000-4000 alt rpm - hence the need for small alternator pulleys. There is also a max allowable alt rpm, one reason why they can't be too up-geared.

[GiveMeABreak]

As mentioned above, the battery light was out with the engine running.
The exciter signal (or whatever it's called) is provided by the BSI (presumably as a direct logic control?) via the BSM as shown in the diagram (wire 1087 to 1030).

Yes, 1087 is the Excitation command from BSI to Engine Fusebox to Alternator; 1030 is the Charging Indicator Activation Command.

[MikeT]

I just repeated the test, with an additional step.
Key on, engine off - battery light on.
Engine running - battery light out. Voltage ~11.9v

Backprobed 1030 at the alternator (but suspect a basic MM was inadequate to get any meaningful reading) it showed 0.00v-0.01v for a couple seconds before spiking up (presumably full voltage) then immediately dropping to zero again. This was cyclical every two seconds or so.
I then revved the engine to start it generating again (14.4v) and the backprobe results remained the same, cylcing every few seconds from zero to high. So nothing learnt there except it's consistent!

Can anyone translate the alternator diagram? Specifically the role of the two transistors (one boxed with only two input/outputs and the other unboxed showing all three terminals? Are they purely for regulation purposes (dumping excess current to ground) or is it also "switching" the generator on by completing the negative side of the circuit??

[white exec]

Alternator output control is by control of rotor current. No current is dumped, it's just controlled.
Field windings are multi-phase a.c., then producing d.c. after the rectifier pack.

As the rotor current is increased, so is its radiated magnetic field, cutting the field windings, and producing increasing output. As rotor speed increases (up to a mechanical limit), so does the rate of its magnetic field cutting the field windings, and output from the field windings climbs too. The strength of the magnetic field from the rotor also affects field winding output - more rotor current, more flux, more induction into the field windings - and this last is the means for regulating alternator output.

Small rotor current + high rotor speed = substantial output from the field.
It's the high rotor speed that brings power to the party, by providing the means for the rotor's magnetic flux to 'cut' the numerous windings of the field at very high speed. When working hard, the alternator will exert a backward pull on the driving belt; when demand for output is very high (e.g. immediately after starting, AC/HRW/electrical heating on...) this can be enough to produce belt squeal/slippage. A 14v 120A alternator working flat out will theoretically mop up around 1.7kW of engine power - i.e. c 2½hp. For this reason, some engine designers are now actively restraining alternator output, in order to save fuel. My guess is that this could be accelerator related, so on overrun, the alternator is allowed to output more fully.

As said, the residual magnetism of the rotor is often enough to 'self-excite' and get the alternator outputting, given sufficient alternator revs. Beyond that, the small and controlled current input to the rotor controls the units output from very low to maximum. Think of it as one gigantic current amplifier, but with mechanical power input.

On the circuit diagram, the boxed transistor is likely the voltage regulation module, rather than a literal single transistor. The unboxed one is likely directly controlling field current. Alt mfr's site will probably detail all the unit's internal components.

[MikeT]

As said, the residual magnetism of the rotor is often enough to 'self-excite' and get the alternator outputting, given sufficient alternator revs.

Would this then maybe explain why I only get charging output once the alternator is given sufficient revs, in this case 4Krpm engine speed?
Which in turn would suggest the BSI exciter signal is not reaching it's destination?

[white exec]

I think that was what t'other Mike was suggesting above.

[MikeT]

My bad. Sorry Mike!

[xantia_v6]

I don't know whether this is a smart (computer controlled) alternator, or not. All the references I find on-line indicate that smart alternators have 3 or more wires, the wiring diagram for this system shows only 2, which would be consistent with an old-fashioned dumb alternator. There will be a fuse in the BSI for the charging system, have you checked that?

[GiveMeABreak]

I can't see any dedicated fuse in the circuit diagrams at all mentioning the alternator

[xantia_v6]

I was maybe slightly confused.

On the 308, if F21 blows you get a charging system fault indication on the screen, but in my case the alternator appeared still to be charging, although it could be that it continued operating in a dumb fallback mode.

[MikeT]

The alternator itself does have 1 main terminal post and a 2-pin connector (one pin unused), if that makes any difference?

To get at the BSM I had to remove the battery so that's now indoors on charge again.
I tested all fuses shown in the diagram and also tested good continuity on wire 1030 from BSM to alternator connector.

That's about all I can do right now and will resume once battery is better charged, probably tomorrow.

[GiveMeABreak]

There is a specific TSB document that is entitled "The Alternator No Longer Works" - what it will tell you I haven't a clue, but it's one I don't have and would require a charge to access it Mike. Sometimes if I have enough issues I'll buy some access time to make it worth while - but I don't have enough items queued up yet!