Power Distribution Unit (PDU)

Check the twelve volt battery first, because it is by far the most common cause and costs nothing to exclude. The contactors that connect the pack are held closed by the low voltage supply and the control units that command them run on it, so a weak auxiliary battery produces exactly the symptoms of a high voltage failure. If the twelve volt system is healthy and the vehicle still refuses to close its contactors, the distribution unit or its interlock circuit becomes the candidate.

The inverter contains large capacitors, and closing the main contactor directly onto them would draw a huge inrush current that would weld the contacts. A precharge circuit routes current through a resistor first, bringing the capacitor voltage up until it matches the pack, then closes the main contactor into a balanced condition. The resistor and its relay do this on every start, and either can fail, producing a precharge fault and a vehicle that will not power up despite everything else appearing healthy.

That usually means the system is trying to close its contactors and something is preventing it holding. The candidates are a low twelve volt supply that cannot maintain the coil current, a contactor whose contacts have degraded, a precharge fault that means the voltage never matches, or an interlock circuit reporting open somewhere in the vehicle. It is a diagnostic job for a workshop with high voltage access, since the fault codes distinguish these quickly and guessing does not.

Occasionally at component level by a specialist, since fuses and sometimes contactors are replaceable within some designs, and generally the manufacturer route is the assembly. Where a branch fuse inside the box has cleared, that may be serviceable with the correct part. Where a contactor has welded or a precharge resistor has failed, whether it can be replaced depends entirely on the design. Any of this is high voltage work, so it is a workshop question rather than an owner one.

Because direct current charging usually has its own path and its own protection within the distribution system, separate from the drive path. A dedicated charging distribution box or a branch within the main unit connects the inlet's direct current pins to the pack through its own contactors and fusing, so a fault there disables rapid charging while leaving drive and alternating current charging unaffected. That symptom pattern points at the charging distribution path rather than the pack or the inlet.

Brackets and small hardware are trivial. A direct current charging distribution box is a moderate purchase. The main unit, and the combined modules that fold several functions into one housing, are among the higher value items on the vehicle. Current figures are shown in the band at the top. What decides your bill is often whether the failed element is serviceable. On designs where the branch fuse or the contactor can be renewed separately, the repair is a fraction of a replacement unit, so ask that question before anything is ordered.

It is a housing that combines several high voltage functions, typically distribution together with the direct current converter and the on board charger, in one assembly with shared cooling and a single set of connections. The benefit is fewer high voltage joints, less cabling and lower weight. The consequence is that a failure of one function usually means replacing the whole module, and that a vehicle built with a combined module cannot take the discrete components from a vehicle built without one.

Plan on it. A distribution unit reports current, monitors isolation and controls the contactors, and the vehicle needs to know which variant it is dealing with and what limits apply before it will allow the pack to connect. Fit one without that step and the car typically refuses to go ready and logs an isolation or contactor fault. Some units also carry serial linked data that has to be written by the diagnostic tool. Check that your workshop holds the right access and the current software release before the part is ordered rather than afterwards.

Visual inspection from outside is reasonable: look for damaged orange cabling, corrosion at the housing, missing brackets or evidence of water. Everything else is not, because the unit sits directly on the pack output, its housing contains live busbars, and even after shutdown parts of the connected system retain charge. Opening a distribution box without isolation, verified zero voltage and the correct equipment is genuinely dangerous rather than merely inadvisable.

Only where the OE number agrees and the vehicle identification number confirms it. Internal fusing values, contactor configuration, communication protocol and the presence or absence of combined functions all follow the specific build, and two vehicles of the same model year can differ. A unit that mounts and connects but is configured differently either refuses to code or protects the wrong circuits at the wrong values. Have the reference confirmed before anything is dispatched.

Mostly through contact degradation. Every closure and opening carries an arc, small if the precharge circuit did its job and severe if it did not, and over many cycles the contact surfaces erode or, in the worst case, weld closed. A welded contactor is a serious fault because the pack cannot be disconnected, and vehicles detect and report it. Coil failures and low supply voltage produce the opposite symptom, a contactor that will not close or will not hold.

Yes. Distribution units combine contactors, a precharge circuit, busbars and branch fuses in one sealed housing, and the internal arrangement follows the pack and the charging specification, so they are quoted rather than priced. Ask with the number from the unit and your vehicle identification number. Get the diagnosis confirmed first. A welded contactor, a blown branch fuse and a failed precharge resistor all stop the vehicle in similar ways, and on some designs the fuse is a serviceable item while on others the whole unit is the repair. The reply will tell you which applies.