Thread: Alternator low voltage
How easy is it to repair an alternator, the output voltage is low and its drawing current while the car is idle.
Depends on the alternator, but most modern ones aren't worth rebuilding, as you struggle to get parts.
If it's drawing current with the engine stopped, then the diode pack has most likely failed.
If it just wasn't charging, then possibly.
Are you sure the alternator is drawing current when not spinning?
Drawing current is most probably a diode gone short circuit (you may be very lucky and find something conductive has gotten in somehow causing the problems, but it's really not that likely).
You're looking at a minimum of £30 for a diode pack (I've never been able to identify/source individual diodes, plus most are spot welded on now), and that's if you can find somebody to supply one.
There's two ways to check the diode pack.
One involves using an oscillscope with the alternator running to see what kind of waveform it's producing, but with modern regulation/ripple suppresion, it's not that reliable.
Other involves stripping the alternator, unsoldering the stator coils, and checking the diodes (there's 9 of them - 6 charge diodes, and 3 field diodes) using a 12v battery and test lamp. It's not unknown for diodes in alternators to go short circuit, as they're driven pretty hard, with minimal cooling (especially when left idling)
What vehicle is it on?
When the earth terminal is off the battery it's drawing half an amp.
When running it's only putting out 12.2 volts measured across the battery.
It's a 2.0 diesel dti vectra
I've looked on their forums for a high current draw and the blame was put on the alternator
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0.5A draw is not likely to be the alternator. A failed diode would be pulling upwards of a few amps.
More likely to be a failed regulator or worn brushes (most likely built into the regulator), which are usually a bit easier to source than diodes.
Vectra B or C?
Reason I ask, is a Vectra C takes upto 20 minutes for all the modules to shut down, and any spike in voltage/door being opened will wake them all up again (0.5A is what I'd expect from them doing nothing, but not sleeping, as there can be upto 20 something modules in a Vectra C). To test current draw on them, you need to remove the key from the ignition, wait 20 minutes, and without disturbing/disconnecting anything, test for current draw.
Vectra B should be fully shutdown just by making sure the key is out the ignition, and all lights are off.
Definetly a Vectra-C at that age.
If it does turn out to be the diodes, and for some reason you can't replace them then you can allways turn it into a pretty powerful and fast brushless motor.
Just remove the diodes, solder wires onto the armature and stator. Put a few volts on the armature and conect an ESC to the stator.By varying the current through the armature windings you can vary the torque - excellent for starting.
I did it a while ago and it seems to work well. I've not thought of anything useful to do with the motor yet.
Let me know if you/anyone is interested and I'll start a thread with more detail/photos.
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