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Testing alternator


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Well managed to get a couple hours on the 90 this morning :)

Brake pads changed and a new battery bolted down and with a bit of wire rejigging it starts again.

Now how do I test the alt is charging. Testing the voltage across the battery posts its sort of 12.3 at idle, slowly rising and with a bit of revs and lights on it pushes up to 13.5 or so. Tightened the belt which seems to have helped but does this sound ok?

If that's ok then I think I must have a small drain somewhere while the trucks standing. Not enough to measure, 0.01A perhaps, but you can see the voltage falling off so pulled the wire for now. Any suggestions on how to track it down or likely causes?

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No, 13.5 is not ok.

I've just been doing similiar work. I've come to the conclusion that i need a new regulator and it's sitting in the garage waiting to be fitted.

After 5-10 minutes of normal driving conditions, still with the engine running, and with everything off, you should see 14.4 across the battery, or near as damn it.

As you start adding load, full beam, heater fan etc, the voltage should remain stable and not tail off.

I've currently got about 13.7 across the alternator, falling off to 12.7 ish under load (about 50A). I've stripped and checked the alternator, and everything looks ok, so i've got a new regulator to fit. £11 from LSUK/Durite.


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I'm not 100% certain, but if a diode in the diode pack fails it will cause drain, i would have thought it would be quite considerable though.

The warning light will draw current, when it's on.

10mA is negligible. pulling fuses and watching the ammeter is the first and easiest way to trace drain, but 10mA will take 7000 hours (~42 weeks)to drain a 70Ah battery.

I would suggest that the battery isn't getting properly charged in the first place, hence the dropping voltage.

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OK Paddocks do a A127 45amp alt, STC229 for 200TDi (non aircon) which is what I have or the A127 65amp unit, RTC5680 for the 200TDi with aircon. For an extra £10 I'm tempted to get the larger unit. Any reason why I shouldn't?

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Strangely enough I've just spent the afternoon replacing the alt on SWMBO's car.

The voltage across the battery with the new alt fitted was 14.3v which seemed a little high. Turns out the battery was also cooked cos the old alternator was overcharging it for a couple of days!!!

With the engine running and new battery and new alt it gave a loaded charge of 13.8v across the battery terminals and an unloaded charge of just over 13v.

So that makes me unsure of what Luke has said - having spoken to a couple of people about the problems we've been experiencing, anything over 14v loaded charging was seen to be not good - on any 12v vehicle! between 13.6 and 13.8v is what should be aimed for, apparently.....

I'm not an electrician, but that's the info I've gathered over the last couple of days, so thought I'd share it...

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Ralph, cheers though I can't afford to get bogged down in making pulleys fit if I'm gonna get this sorted for next Saturday.

Orange, ain't forums great. Now I've got some different views to confuse me consider :D And of course leaving the current alternator in place does save on expense/effort

The battery is new and been sat in the garage holding its charge for a week now. The old batteries died on me but I'm not sure if it was a battery fault or if I've just been slowly draining them over the past few weeks/months. The alt is probably 3-4 years old, ign light comes on/ goes out as expected.

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Trev, Orange, put your voltmeter across the alternator output with the engine running, you should have 14.5volts (ish), any less than 13.5volts and you either have a power drain or a knackered alternator. My speciality is not auto electrics but I am a professional automotive mechanic and this is what we were taught at college - I just discussed this with a mate (also a mechanic) and he agrees

Hope that helps

Lewis :)

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If you are worried about undue battery drain, take the motor for a run, park up, turn headlights on for 2 mins, check voltage on battery. A good battery fully charged should read 12.6 volts.

Leave overnight. Check voltage in the morning, a small drain for radio memory etc should only drop the voltage by a couple of tenths at most. So 12.3-12.6 is fine.

Alternator charging, take it for a run, measure alternator output voltage between alternator output lug and alternator casing, should be around 14 volts (13.8-14.2). Measure battery voltage with meter probes on battery lugs, not the clamps. Any difference between alternator voltage and battery voltage is voltage lost in cabling and connections. Lost voltage should not exceed 0.5-0.75 volts.


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its quite possible that if your diode pack has died then the alt will be trying to run as a motor when the car is switched off, that creates a nice drain.

No - you will get a drain, but it won't try to motor the alternator. It might happen with an AC supply when the alternator may act like an induction motor. It is quite possible for a dynamo to act like a DC motor though.

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Ralph, cheers though I can't afford to get bogged down in making pulleys fit if I'm gonna get this sorted for next Saturday.

no need to just remove the multigroove pulley & fit the V groove pulley for a 65amp alternator, thats all I did & upgraded the wiring to suit. :D

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OK got this from 02GF74 on the subject too which I'll add here for completeness (and peer review):

what model is it? the working voltage varies according to model, older Lucas one throw out 13.5 V; more modern 14 V. as mentioned, too many volts will boil dry the battery - not good - you get the smell of bad eggs.

re: And could I be leaking current through the alternator? I don't get an open circuit when I disconnect it but the resistance certainly increases considerably which I find unsettling

eh? to measure leakage curent (not resistance) you sould put in an ameter in line with the battery. (don;t try to start t he engine or you'll at best blow the fuse in the meter, at worst fry it).

by remvoing the fuses in the fuse box, wather the current change - if all of them are out and there is still a drain, then it looks like the laternat doides are bad.

haynes books go into detail on how to test and replace these;

for the electrically challenges, a scrap yard is a good place - you can fit a bigger altnerantor - mondeao bineg on or as said, paddocks are doing what I thinkg are amazing deals on new alternators (probalbly made in chine form noodles so won;t last but who knows?)

The balance of views seems to confirm duff alternator so I'll set about swapping that this week hopefully and just pull the battery wires when the trucks standing in the interim.

Thanks for all the input.

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Tried to post last night but the broadband was being a pain :angry:

You really want a charging voltage of 14.0 to 14.5V, anything less than 13.8 and your battery isn't getting properly charged, anything over 14.7 and it's getting boiled!

With the lights on it should not sag very much at all as already said.

Check the output at the alternator (from the output terminal to the casing) you should not be losing more than 0.5V max between the alternator Voltage and the battery Voltage.

The figures I work to are:- 0.1 V max drop between the body of the alternator and the battery negative post, 0.2V max drop between the output terminal and the battery positive post. I get more problems with bad earths than anything else. I aim to get volt drops as low as possible especially if you have lots of extras, every little bit helps.

Alternator output Voltages vary by make I've seen some as low as 13.8V, Bosch are usually very close to 14.0V, Lucas are often a bit higher. If you've got one with a lowish Voltage then obviously you can't afford to lose very much getting it to the battery!

Silly point, check the belt is tight :D

Slightly O/T but when wiring spotlights it's worth a thought to connect spotlamps etc. to the output terminal of the alternator (if you can get to it) and not the battery, after all this is where the power comes from. You are then not putting so much current down the feed wire to the battery and you get a slightly higher charging voltage at the battery terminals.

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I've just replaced the regulator on my alternator tonight,


An un-bent paper clip does the job of holding the brushes in perfectly.


I decied to swap the regulators over with the alternator unbolted but still in the 90, you could probably do it with it still mounted, but 30 seconds with the air ratchet had it like this, so why not, make it a lot easier to see what you're doing.

Charging voltage is up at 14.15 with a few revs, not taken it out for a run yet.

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