T5.1 New alternator - No output on startup until revs > 1400?

[meza]

Hi people,

I've just had the alternator replaced on my 2012 T5.1 as the previous one on the van didn't have an overrun pulley and was causing aux belt squeal. Squeal has gone now, but noticed something about the new alternator output when using my BM2 battery monitor.

• When the engine starts, the voltage stays low when engine is idling: ~ 11.6 V.
• When I drive off, or if I rev the engine past approx 1400 RPM the voltage increases to 14.4 V, and stays at that voltage until I turn the engine off.
• Then when I restart the engine, the pattern repeats.

Why is the alternator needing to be revved to kick in?
Is this normal? Do I need to do anything about it?

Also does this mean, if the van engine is started while camping, eg to charge leisure batt - that unless it's revved, the vehicle battery will actually be DIScharging, because the alternator hasn't started?

Graph from BM2 app for more detail...

 

[oldiebut goodie]

On old engines this used to happen if the exciter wire was not connected to the alternator and thus relied on induced current to kick-start it. I don't know what you have fitted, you may or may not have one there.

 

[meza]

Thanks for the insight. The garage fitted the alternator, and I was told it’s a 140a Lucas but that’s all I know.

So this behaviour isn’t expected then? So I should take it back to the garage and then to have a look?

 

[oldiebut goodie]

Certainly not as it should be, I doubt that they checked it so closely after fitting it.
I would expect the battery light to be staying on before it starts charging - is it on with just the ignition on, not started?

 

[meza]

Thanks

I double checked this, the battery light on the dash is acting normally; ie on when ignition turned on, then goes off immediately when engine starts.

Not what you’d expect?

 

[oldiebut goodie]

No, I didn't expect that.
It could well be a duff alternator as its not unheard of of new items failing at first use. Definitely a job for the garage to sort though as they replaced it.