I think my alternator has gone...

I drove over to the 70th Anniversary event at Gaydon at the weekend, looking at my voltmeter on the way over which was showing an ominous 11.5V. Sure enough, I could not start it again when I got there and got jump started by a kind person to get home. The battery has accepted a charge from my mains charger and shows a healthy 13.8V, however when I start the vehicle at no point does the low charge light come on, even when I've only turned the key to the ignition on position.

So is that a knackered alternator? Is there a way to test if it's working using a multimeter?

Vehicle is a 1986 110SW with a disco 200Tdi in it.

Thanks in advance!

Meter to 20v dc.
Connect across battery. Note reading.
Start engine. Connect across battery. Note reading. On my RRC (A127 alternator) I'd be expecting around 14.4v.
With engine running connect +ve probe to alternator main terminal (the one with the thick wire) & the other probe to a good earth (not the alt itself) or the battery neg (assuming neg earth). Note reading. I'd be expecting around 14.4

There MAY be an issue if the bulb is blown so that's worth a check.

That's about the extent of my knowledge & anything else would see me whipping it off & taking it to the local reconditioners - in my case that's Green's in Leicester (256 Victoria Road East, Tel 0116 246 0688, no website) who will check it on their test rig. I've had a few from them over the years & they've always tested for free & their recon ones are sensibly priced. He did tell me they could test on the car.

Hard-Drive said:

So is that a knackered alternator?

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Probably. - but check the charge indicator lamp - make sure it works out of the holder - and check continuity to it's connection on the alternator......the lamp and circuit need to work for the alternator to charge.....

If the voltage across the battery is not rising with the engine running - to 14V ish, then the alternator is not charging ...

Hard-Drive said:

Is there a way to test if it's working using a multimeter?

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Not really, even if the voltage is rising, you might not be getting full output as the brushes and slip rings of the alternator might be kaput - the large current is outside the range of most multimeters, so ....

Guys, thanks for the advice. Had a very quick look at this tonight but not enough time to do it properly. I know the alternator light bulb is OK because I swapped it, and I cleaned the battery terminals and clamps. However it's still not lighting up at any point. I've not tried a multimeter on the battery yet, however I have a digital voltmeter in my Raptor dash and it's showing low volts and no increase when revved.

I will check the charge indicator continuity next...is this simply a case of setting the MM to bleep test, and poking one probe in the back of the plug on the dash, and the other (extended) end down to the alternator? Which wire is it, the thinner of the two going onto the larger 10mm terminal at a guess?

Also, there's a short black spade connector on the back of the alternator that doesn't seem to connect to anything at all...it's just dangling...is this right?

As I said, excuse the noddy questions, I literally had 10 mins to look at this tonight!

+ = big thick wire to battery.

D+ = exciter wire from ignition live, this runs through your battery warning lamp.

W = signal wire for the tacho.

Take the exciter wire off the back of the alternator and short the wire to ground. Then turn the ignition on, if it now lights the alternator is dead, if it doesn't you need to find where the break is in the exciter supply.

Another way to check the alt is make up a wire with a terminal on it and connect it to the exciter wire connection on the alt, with the engine running momentarly (1/2 second) touch the other end to battery + (or use the big stud on the alt if you have one). That should be enough initial excitation to get the alt started and running and your voltmeter should show charging. Do not keep the wire on the + terminal when it is running.

Thanks guys. I have done head gaskets, gearbox and engine swaps, all kinds of mechanical stuff but electrickery really isn't my strong point!

So for the avoidance of all doubt, here is the back of my alt, and the wires (from left to right) are as follows...is this correct?

+ is the two blue/blacks going onto one terminal
D+ is the exciter and is the red/yellow (and will be the one I short to ground for Kwakerman's test)
W is the thin black one to the right

Is this correct?

You could try running a separate positive to the D+ terminal with a lamp in line to see if it excites the alt to charge and if it goes out then you know the wiring is your problem.

Hard-Drive said:

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I keep looking at you alternator power cables and comparing them to the one I just fitted they look like speaker wire.

^ Eek!!!

She's got a 200Tdi in place of the original 2.5n/a (done before I owned her) and a lot of it was a bit of a bodge. I've sorted a lot of it since (temp gauge, exhaust and intake arrangements) Do you think I should beef them up a bit? Where does that power cable actually go to?

Kwakerman said:

Take the exciter wire off the back of the alternator and short the wire to ground. Then turn the ignition on, if it now lights the alternator is dead.

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Tried this...dash light came on straightaway. Cleaned the alt terminals, still no joy, so it looks like the alt is dead. Ho hum. But many thanks for your input!!

Hard-Drive said:

Tried this...dash light came on straightaway. Cleaned the alt terminals, still no joy, so it looks like the alt is dead. Ho hum. But many thanks for your input!!

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Looks like a standard A127 alternator, you can get fairly cheap rebuilt kits which include regulator/rectifier, slip rings & bearings etc. Suppose it all depends upon how comfortable you are with basic electrics and whether you have a big soldering iron (50w+) or go for recon.

Just want to say a huge thanks to all that helped on this thread. I could not be bothered to repair the alt for the sake of £60 off eBay for a new one, just really short of time at the mo! Anyway, all installed, charge lamp and voltmeter back where they should be!!! Thanks again!!!