407 Hdi 2.7 Alternator voltage drop only after a long run

[407]

My Hdi alternator has developed a voltage drop issue after a long run. Voltage is normally around 14.5 volts but after a long run (say 100kms), it will drop to 12 volts and continue downwards, suggesting its has stopped charging.

If I spray the alternator with a hose or even a water bottle, it jumps back up to 14.5 volts and will run fine for a few weeks before again dropping voltage. Given the regulator and diode pack is at the back of the alternator, I doubt the water would get to it to cool it down.

Battery is new (Dec 2025), I've changed the overrun pulley and belt is not slipping.

I suspected that maybe the brushes were sticking (eg build up of carbon and maybe oil - had this happen on the petrol 2.9 version) ) but when I took it off yesterday the brushes were very clean.

The only other item I spotted was that the nut on the bolt connecting the alternator positive pole to the battery wire was not very tight (but did not look scorched or burnt from arcing).

I've bought a new regulator (IB6011, F00M346011) and fitted it ready to put back but thought I might ask if there are any other things I should check (given refitting will be worse than pulling it out).

[mickthemaverick]

Having found your loose connection on the charge stud I would check all the connections to the battery and around the fuse boxes before you refit the alternator.

[ozvtr]

My Hdi alternator has developed a voltage drop issue after a long run. Voltage is normally around 14.5 volts but after a long run (say 100kms), it will drop to 12 volts and continue downwards, suggesting its has stopped charging.

If I spray the alternator with a hose or even a water bottle, it jumps back up to 14.5 volts and will run fine for a few weeks before again dropping voltage. Given the regulator and diode pack is at the back of the alternator, I doubt the water would get to it to cool it down.

Battery is new (Dec 2025), I've changed the overrun pulley and belt is not slipping.

I suspected that maybe the brushes were sticking (eg build up of carbon and maybe oil - had this happen on the petrol 2.9 version) ) but when I took it off yesterday the brushes were very clean.

The only other item I spotted was that the nut on the bolt connecting the alternator positive pole to the battery wire was not very tight (but did not look scorched or burnt from arcing).

I've bought a new regulator (IB6011, F00M346011) and fitted it ready to put back but thought I might ask if there are any other things I should check (given refitting will be worse than pulling it out).

Sounds like you have everything under control.
The cap over the brushes should only be half on. This will hold the brushes out of the way while you install the regulator. Then push the cap all the way down to release the brushes.
How are the slip rings?

[Peter.N.]

I suppose the alternator belt is not slipping?

Peter