Alternator under charging until accelerator is pushed

[ghost123uk]

Suggest posting what your main battery is reading next.

Good point OBG I'm doing all these readings with a battery that flattened overnight

If it flattened over night, then either you have a BIG parasitic drain, or as OBG says, it IS knackered.

Now charging it up, shouldn't be knackered as it's only about a year old but I'll test it just to check

Doesn't matter how new it is. It's quite easy to knacker a brand new battery in one day. Just totally flatten it once and it will be knackered.

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

You don't need to check them, it's VERY unlikely they have failed. They are part of the (delicate) blue paper thin pcb behind the instruments.

[Sir Brixalot]

When I say flattened overnight I should have said finished running down. Charge from alternator has been erratic so I suspect it's not been getting charge

[ghost123uk]

When I say flattened overnight I should have said finished running down.

Bit confusing that

[Smcknighty]

I've heard self exciting from cold takes longer the older the hardware involved gets. Not something I've observed mind.

[Smcknighty]

And there is plenty of material about self exciting available on the Internet at large. Covers pretty much every problem

[Sir Brixalot]

When I say flattened overnight I should have said finished running down.

Bit confusing that

I was trying to say that if the alternator wasn't charging the battery properly is would have been progressively run flat. Have charged it, showing 13.18v, it'll be interesting to see what it measures tomorrow. It's my daily driver and shouldn't really have failed already although looking in the window for the green OK display, it's not showing.

[Oldiebut goodie]

That little green eye is only indicating the state of charge of one cell - that cell could be good and all the rest are US. So is a waste of time and just a gimmick. All it is is a small hydrometer.

[Sir Brixalot]

Thanks OBG

Removed shrink wrap from red alternator cables, someone had bodged it so one cable only had a single thread of wire connecting, I put a new lug on so charge is reaching battery.

Had a look at the split charge relay, it looked like its been opened many times, I think I finished it off opening it and the warning dash light came off. I was going to buy a new SRB630 but noticed that Brickwerks are selling this:



It's twice the price but doesn't need a trigger from the alternator. One less wire seems a good idea to me but is there any reason apart from the price that I should stick with the lucas relay and trigger wire?

[Oldiebut goodie]

Do you need 140A? 100A for £30 http://www.ebay.co.uk/itm/Voltage-Sensi ... 3aafb4bd7a
People have reported problems using an alternator triggered wire, I prefer to use a voltage sensing one.

[Sir Brixalot]

I'll go for the brickwerks one as I need a few bits and pieces and they're a good company to deal with.

The split charge is now disconnected but I'm still only getting 7 volts from the blue ignition wire at the alternator. Is it likely to be a duff ignition switch?

[Sir Brixalot]

If the connection is good at D+ and the diode and resister are very unlikely to have failed that must mean that the ignition is faulty or am I missing something. Can i test the ignition or is it just a matter of fitting a replacement. The blue wire has been extended with a block connector, is that the best way or would it be better with a crimped butt connector. Thanks for the advice

[ghost123uk]

It won't be the ignition switch (in this case). The blue wire starts it's life on the thin plastic pcb behind the dash, it materialises at the multi plug under the dash, by your right knee (assuming RHD). That multi-plug is a very likely candidate for a bad connection. You can also test the voltage there.

[Sir Brixalot]

Thanks. I'll check the multiplug today. I've had water getting in through the windscreen which may have affected the multiplug. Reading through other posts it seems running a wire from the leisure battery to the starter solenoid and extending it to the alternator is workaround.

[Sir Brixalot]

I've just fitted the split charge and the readings are (engine running)

Red wires on alternator 14.1v
blue wire 13.1v shouldn't it be 12v-will it do any damage? It was low, now high?
starter batter 13.9v
leisure battery 13.4


ignition off

The problem is that the alternator now reads 12.57 volt with the engine switched off. Checked both red cables one shows 0v at all times the other is permanently live at 12.57 exactly the same as the starter battery.

[ghost123uk]

Those readings are all good (including the blue wire)

With the possible exception of the two red wires.

Checked both red cables one shows 0v at all times the other is permanently live at 12.57 exactly the same as the starter battery.

Most I have seen have two red wires onto the same terminal on the back of the alternator, so they should of course have the same reading (ie battery voltage, ignition on or off). Are the two red wires you are referring to going to the same terminal? If "Yes" then a simple bad connection at that terminal is indicated.