Technical The 20 second Alternator Light

[AsdaPricey]

Hello

Brain picking time.

My 2008MY X250 Ducato van conversion camper has thrown its latest gift at me.

The alternator light goes immediately out upon start up but count to 20 and it reappears.

I isolated the habitation battery and solar system.

It is the same whether loaded or unloaded - adding load such as blower and headlights combined do not change the time frame.

There is 14.38v showing from the alternator upon start up which dips to 13.5v when loaded but instantly recovers.

The battery at rest for 30 minutes held 12.6v. It’s is only 14months old. I have not load tested it. The van fires into life with no hint of a problem.

There has always been a 0.02a drain which is well documented across the Ducato platform. The solar panel maintains the battery.

I have previously replaced and added extra earths.

Didn’t have time to properly conduct test today - the alternator is in a pig of a location & I was only checking it started having been stood for 6 weeks so I put it back in storage.

Could this be a failing diode in the alternator or has someone had the same and found something specific…..?

 

[rayc]

"The connection between connector B (D+) of the A010 alternator and pin 25 of connector A of the M001 Body Computer allows the performance of the alternator itself to be diagnosed (in the event of an insufficient recharging level).Once a fault has been discovered, the M001 Body Computer, through the CAN network, will light the "insufficient battery charge" warning light on the E050 instrument panel"

Therefore you need to check the D+ voltage. this is the same D+ that switches your fridge to 12v when on auto and engine running and split charging. Do these work when the battery lamp is on?

 

[AsdaPricey]

"The connection between connector B (D+) of the A010 alternator and pin 25 of connector A of the M001 Body Computer allows the performance of the alternator itself to be diagnosed (in the event of an insufficient recharging level).Once a fault has been discovered, the M001 Body Computer, through the CAN network, will light the "insufficient battery charge" warning light on the E050 instrument panel"

Therefore you need to check the D+ voltage. this is the same D+ that switches your fridge to 12v when on auto and engine running and split charging. Do these work when the battery lamp is on?


View attachment 480094

@rayc thank you for the comprehensive reply and diagram.

I didn’t check the fridge on 12v - I will when next with it.

You say in the event of insufficient charge level but to add to my confusion the alternator is charging…..

 

[rayc]

@rayc thank you for the comprehensive reply and diagram.

I didn’t check the fridge on 12v - I will when next with it.

You say in the event of insufficient charge level but to add to my confusion the alternator is charging…..

It wasn't me saying it, I was quoting the explanation in the document. The D+ output of the alternator inputs into the body computer which 'interprets' it and switches the lamp on and off in accordance with some, to me, unknown criteria..It is the B+ output from the alternator which charges the battery.

 

[deejays]

Hi @AsdaPricey , you said:
"There is 14.38v showing from the alternator upon start up which dips to 13.5v when loaded but instantly recovers."

When you say it dips to 13.5V when loaded, do you mean it remains at 13.5V when loaded, and when the load is removed it instantly recovers, or do you mean that when loaded it drops instantaneously but then recovers to maintain above 14V whilst still under load?
If the output voltage drops below 14V on a normal load, then the alternator has failed.

 

[AsdaPricey]

Hi @AsdaPricey , you said:
"There is 14.38v showing from the alternator upon start up which dips to 13.5v when loaded but instantly recovers."

When you say it dips to 13.5V when loaded, do you mean it remains at 13.5V when loaded, and when the load is removed it instantly recovers, or do you mean that when loaded it drops instantaneously but then recovers to maintain above 14V whilst still under load?
If the output voltage drops below 14V on a normal load, then the alternator has failed.

Hello @deejays

It’s the latter.

The extinguishing light that returns regardless of load is what has me confused.

 

[deejays]

Thanks for clarifying. The constant timing is odd - perhaps the BCM has a timer built in if there is a fault - like maybe one phase has gone - which would be consistent with your suspecting a failed diode. As rayc said - need to check the voltage on the D+. If you have a current clamp meter connected to a Picoscope (or similar), you could check the charging waveform - that would clearly indicate if all three phases are working correctly.

 

[AsdaPricey]

New alternator cured it.

The old alternator was intermittently charging, with the BCM illuminating the warning light because this was outside the system parameters. Unfortunately this fluctuation was not picked up by my bargain basement digital meter. If only I’d used my analogue meter.