VE Commodore V8 Alternator light, no charge p0621


Holden Commodore VE SS V8, Series 1


The alternator light comes on on dash board that warns you that the alternator is not charging, The late model vehicles have ECU controlled, charging circuits and no longer just use the battery for sensing, the alternators are pulse width controlled by the ECU for charge rates from the alternator. When the alternator lights comes on you now have to ask your self is the battery not being charged due to a faulty alternator or is the Engine ECU not sending a signal, or worse both,


The Alternator light was on in the instrument cluster, how ever they still will hold a fault code, so it is always best to check the fault code number to get a clearer picture on what the fault is.

Once the fault codes where checked we found

  1. P0621 – Generator lamp L control Circuit.waveform-ve-commodore-alternator-L-circuit-bad

Great news as it was meaning that after checking the fault codes we did not understand if the alternator was faulty or the ECU was not sending a pulse width modulated signal to the alternator, before we just got stuck in there and ripped the alternator out with out testing we decided to check the circuits on the scope, there is two control wires orange/white for the L circuit and Purple/white for the F circuit, having the P0621 Fault we scoped the Purple/white wire first and found the quite bizarre pattern, being a duty cycle or pulse width modulated line we would have expected a square wave pattern, alas we found a signal that looked almost analogue, having a manual that showed what a good pattern should look like we then quizzed our selves, could the circuit not have enough ground, could the regulator just be faulty internally and the ECU is sending out the right signal which is then distorted by the internal voltage regulator? from experience, when working with ecu controls, you either get a signal or you don’t, we decided switch of the engine and unplug the regulator and connect the scope without any load, this produced a normal signal.

Bingo the Problem was found, but not fixed just yet, we removed the alternator and installed a new unit.

then re tested the pattern. we wherenot out of the clear yet as one peice of information was not yet certain, the waveform showed a nine volt square wave, ours qaure wave pattern only showed 4volts, after the installation of a new bosch alternator we then re tested the L circuit and F circuits just to show a working alternator.



P0621 fault code, scope to test circuit integrity,

Replacement Bosch alternator


After spending a few hours on testing and preparation of learning the way GM had decided to build this system and work out that they also have a battery current sensor located in the boot to measure current draw over a time period, we where confident that the testing method we had used was the right way before just installing a new alternator, rather than fit and find out way later.