Alternator under charging until accelerator is pushed

[Sir Brixalot]

As above really. My battery light is on all the time. When I start the van the alternator puts out about 11v and the blue wire 5.5 after letting it warm up and blipping the accelerator it goes up to 13.5. Any ideas?

[MidLifeCrisis]

Does the light go out when you have blipped the accelerator?
Are you getting 12v on the blue wire when ignition is on but engine isn't running (you'll need to disconnect the blue wire from the alternator to measure this)?
Do you have a split charge relay and/or fridge relay triggered by the blue wire??

[itchyfeet]

it's not self exciting

It seems quite common but is possibly caused by poor connections,

Does it never charge cold even if revved?

Pages of discussion here about it

https://club8090.co.uk/forum/viewtopic.p ... t=exciting" onclick="window.open(this.href);return false;

[Sir Brixalot]

Thanks I followed that thread at the time although it lost me at times. The blue wire is spliced into another blue wire, when i disconnect one the light goes out. I'd have thought the battery light would stay on.

[itchyfeet]

if you disconnect blue wire, light can't work at all, it works when the alternator is grounding the wire and this happens with no or low volts generated.

if the alternator produces higher volts than battery it goes out because blue is now no longer ground, when both ends of the blue wire are at the same volts ( alternator generated volts) no current flows and no light.

[Sir Brixalot]

Ag I get it, I should have said there is a blue wire comes through the floor under the seaa into the split charge relay

[itchyfeet]

Ok so if light is staying on it could be earthing through the split charge relay, it should go out when volts come up but only if the connetion to alternator is good.

late exciting (i.e late to come up to charging volts) can also be due to poor blue connetion at alternator so check that out.

[CovKid]

Not uncommon - mine does it. I've just got used to blipping throttle when I start now. Fairly sure its the diode pack deep inside of alternator.

[Oldiebut goodie]

My brand new alternators, boat and van, both do it. Blip on throttle cures them. More important things to worry about.

[itchyfeet]

My battery light is on all the time.

this is what needs fixing, it may also mean leisure is not charging if split relay is earthing lamp it's not triggered.

[Sir Brixalot]

Going to try and sort this once and for all when it's light. At saafend today wouldn't start to tired kids in the back, I thought not again. Voltage on main battery at 12.3 then dropped to 11.6 when turning the ignition. Put the voltmeter leisure battery to see if I could swap it, turned the ignition and there was a click and it started. So I reckon you're right IT - it must be the split charge

[Sir Brixalot]

1. Can't start today.From the alternator I have two wires spliced. On goes to the split charge relay, I've disconnected that one. The other blue wire has 7.5v - should it be 12 and does this one come from the ignition.

2. Took a reading from the nut that holds the two red wires on to alternator. 9.5v ignition off drops to 7.5 ignition on.

[ghost123uk]

The blue wire at the alternator end does come from the ignition switch, but also via a diode and a resistor (see Itchyfeet's diagram above). It should have 12V on it.

The nut on the back of the alternator with the two red wires on it certainly should have the same voltage as the battery has, ie. about 12.6 volts, as they are quite heavy wires and one of them goes pretty much straight to the +ve on the battery. Suggest posting what your main battery is reading next.

[Oldiebut goodie]

Battery's flat most probably f**ked. You may have a drain on the battery flattening it overnight or it may have gone to meet its maker.Or both. Your first post mentions 11v to start with so you are dropping more and more.

[Sir Brixalot]

Good point OBG I'm doing all these readings with a battery that flattened overnight. Now charging it up, shouldn't be knackered as it's only about a year old but I'll test it just to check.

Where would I find the diode and resister, just to check they haven't failed.