Fuel Pump Noise From Low Voltage

I have been suffering a hot soak start stall (see the V12 category) so it wasn’t a big surprise to hear the fuel pump become noisy while on a long drive on one of the hotter days we have had lately- I figured that the problem had identified itself. Then I noticed the voltmeter was somewhere in the 11v area. By not using the a/c or lights, it crept back up to maybe 12v and I was able to get home.
Next day I checked the alternator output and it was under 11v with everything on, but climbed to 13v at 1500 rpm with everything off, indicating a failed diode or two. I ordered a Marelli 100a alternator and continued to use the car around town, keeping the accessories off.
Today I restarted the car after a short drive and noticed the voltmeter was normal at idle; in charge area even with a/c on, and fuel pump is quiet. So alternator is charging normally…drove a couple of times and still normal. I will go ahead with the Marelli but wonder WTF?

Your pump noise is not related to your Alt failure.
I have an upgraded Alt and 13v at idle but had the exact same ting, pump becoming noisy after a long drive. A new pump is in order.

Your alternator set voltage should be between 14-14.2v on idle. If you have 11-12v with everything off - you have lost your charging. With mileage at 100k you usually need to change alternator’s regulator or only brushes (which is highly recommended if you have genuine Bosch).
Have a look here for more info:

Also worth noting that brush-out scenario will make it work randomly. With one diode out it won’t work permanently (not sure how you’re calculating voltage drop on one diode or two ;D)
Another thing worth noting - if your pump is noisy it may be simply burn on coil, still spinning but heating up/consume massive amount of amps (not possible if you haven’t messed with fuses)…

Has a remanufactured Lucas 75A, which I replaced about 2 years ago. I thought about rebuilding it, but couldn’t find diodes available. Based on previous experience is why I thought diodes. You get some output if one or two fail, but not enough for heavy loads. As I have added a larger condenser fan, and E code headlamps, the 75A is marginal anyway. Considered upgrade to Bosch, but didn’t feel cost plus changing the bracket worth it, so went with the Land Rover 100A, which I will install once I figure out the pulley (different shaft diameter).

Well, I found my issue with the intermittent charging and low voltage. A couple of years ago while replacing my mechanical fan drive, I dropped one of the fine thread nuts, which never hit the ground. I looked for an hour or more, even had my wife help looking. Fast forward…
When I removed the old Lucas alternator it rattled, and after some shaking a 5/16” fine thread nut dropped out, with some arc erosion. When the nut fell, it had went past the alternator cooling fan in to the housing, against the field winding; vibration wore through the varnish insulation and was intermittently grounding the winding.
New Marelli 100a on there now. Moral to the story is don’t lose your nuts…or if you do, don’t stop until you find them.