Thread: 300tdi alternator output

Hi Kevin and those voltage levels indicate your alternator is not working.

Probably a diode gone, and this could also be the cause of the drain on your battery.

Are you still running a defender alternator or a disco item, mine with defender altinator with the heater fan/wipers and headlights on would sit on 12v on the vdo guage and back up to 14v with those accessories off, replaced with disco alternator and never looked back main battery at 14v and 2nd battery at 13.5 with fridge running 24/7[this is all with running motor]

Originally Posted by redrovertdi

Are you still running a defender alternator or a disco item, mine with defender altinator with the heater fan/wipers and headlights on would sit on 12v on the vdo guage and back up to 14v with those accessories off, replaced with disco alternator and never looked back main battery at 14v and 2nd battery at 13.5 with fridge running 24/7[this is all with running motor]

disco alternator and pulley

Originally Posted by drivesafe

Hi Kevin and those voltage levels indicate your alternator is not working.

Probably a diode gone, and this could also be the cause of the drain on your battery.

its been like this for months......the SC40/80 that i have flashes for a while after the first start of the day...not sure if this is normal

Is that last figure with revs up? I assume not... I would have said they were OK. My Disco alternator can't output enough power at idle to run headlights on high beam and AC on and cuts out - the voltage will drop like yours has. You need some revs up to enable it to generate that much power.

I would have said if it's outputting over 13v at idle, then your alternator is working.

TIm, what makes you think a diode has gone? How is it generating over 13v in some cases, but not under load?

Originally Posted by Judo

Is that last figure with revs up? I assume not... I would have said they were OK. My Disco alternator can't output enough power at idle to run headlights on high beam and AC on and cuts out - the voltage will drop like yours has. You need some revs up to enable it to generate that much power.

I would have said if it's outputting over 13v at idle, then your alternator is working.

TIm, what makes you think a diode has gone? How is it generating over 13v in some cases, but not under load?

From my limited alternator understanding, a dead diode will effectively remove one phase?? from delivering current, so at low load the alternator will still produce its set voltage, but as the load increases it can't keep up and the voltage drops.
The original 40A alternator in my County would take about 8 hrs of driving before it could get the voltage up above 13v if the batteries were drained from camping for a couple of days. The 120A one I've got in there now sits smack on 14.6v from just above idle - regardless of whether lights are on or batteries drained .

Steve

Originally Posted by steveG

From my limited alternator understanding, a dead diode will effectively remove one phase?? from delivering current, so at low load the alternator will still produce its set voltage, but as the load increases it can't keep up and the voltage drops.
The original 40A alternator in my County would take about 8 hrs of driving before it could get the voltage up above 13v if the batteries were drained from camping for a couple of days. The 120A one I've got in there now sits smack on 14.6v from just above idle - regardless of whether lights are on or batteries drained .

Steve

Makes sense I guess. Alternator being AC, not DC, so only half the AC wave is being used? Maybe my alternator also has a dead diode. Without going and looking (stupid rain ), can I unsolder the diodes and check/replace them?

Lead acid cells have and emf of 2.2v each so a 6 cell 12v battery needs at least 13.2v applied to it to even begin charging, a good alternator should be able to put up to about 14.5v at the battery terminals and that should only drop slightly if the accessories are within the current capability of the alternator.

As you are seeing up to 13.4v on the battery it suggests that the alternator is working but unable to deliver the necessary voltage or current, this is typically caused by high series resistance which could be caused by loose or corroded cable connectors, faulty diodes (typically overheated) or poor brush connections to the slip rings inside the alternator itself. It could also be a fault with the regulator electronics inside the alternator that limit the voltage to prevent battery overcharging.

Hope you find the problems and manage to fix it.