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What is your EDC from the VIN.

 

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PEUGEOT 308 1.6 HDiF, DV6C (9HR) 2010 - 2013

DTC U1218 Communication with the power supply control unit
Haynes is depressing after that about this one, showing only this for E6 which is usually the slave unit (engine fuse box) and commination is usually from BSI, but could be with Engine ECU or ABS even (don't know at this stage without the wiring diagrams)

Which means Peugeot Wiring Digrams and whole VIN is needed, the DAM/RPO would also help.

Before we go to this path, talk to me about the voltage fluctuation(s) you have, what are they, where are they measured.

 

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Well, PM or publish the VIN and look into this tomorrow, heading to bed. That does sound strange, can you post a photo not too close of the engine near the alternator.

 

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Okay so the photos were just for me to be sure, we did don't have oil, or oil like products, entering the alternator, this kills them (it appears no oil). Does not rule diode pack however, these usually kill the battery from out-of-phase charging, which can be problematic, if you try new battery, and this problem exists, the battery light *may show very faintly when driving. When a diode blows fully, the battery light will be fully on. The diodes don't always blow, but they start to allow voltage through the diode rectifier circuit, the wrong way (also called wrong gate) and hence the name, out-of-phase charging is given.

*you tend to be able to notice this, best when at nighttime.

Given it is a smart alternator, this maybe (seems like it would be) within normal operation as well, you may have removed the wire that controls the feed of voltage, in which case defaults the operation, not stops it. You would have needed to remove B+ wire, and doubt you did this. Refer to 1021 in the diagrams below.

VF34C9HR8BY063540 (ssg.asia) this gives you OEM part numbers and exploded views.

Vehicle characteristics

​ Model: 308 Body: FIRM 5 DOOR SALOON Engine: DV6C TURBO DIESEL Transmission: STT 6-SPEED PILOTED MANUAL GEARBOX

PEUGEOT 308 1.6 HDiF, DV6C (9HR) 2010 - 2013
DTC U1218 Communication with the power supply control unit

Engine (with stop/start) Starting

PSF1 (Engine Fuse Box)

BFDB

1031

PEUGEOT ALL MODELS WIRING DIAGRAMS - GENERAL
PSA Electrical Diagrams - Wiring & Symbols Codes Index - French Car Forum

 

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PEUGEOT 308 1.6 HDiF, DV6C (9HR) 2010 - 2013
DTC U1218 Communication with the power supply control unit

PSF1 con't

So I believe Haynes (E6) will be talking about internally the relay in PSF1 - these are not serviceable units (not easily anyway), usually you replace the whole unit (PSF1) if defective. Communication between PSF1 to BSI are the blue, twisted-pair of wires in the previous post talking about engine starting.

I can't tell you where the problem is, it could be wiring connector / pins or the unit. Even if this is over-your-head, an auto-electrician/mechanic, will find the above very useful to fixing/finding, the problem.

Hope you find this useful.

 

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Peugeot Workshop Manuals - Sedre. Took me a while to figure it out and where to find information, not like any manuals from non-French manufacture. Generally speaking, the Japanese brands win for their layout vs french way ~imo.

other fault

The fault recorded is to do with control of PSF1 (slave) to BSI (master) you could try clearing it and watch to see if it comes back. A low battery or high battery state can sometimes trigger this or other DTC, just wanted to rule that out.