Alternator/ charging ripple - busted diode?

[Alias_Knagg]

Hi

I have a 2003 1,4 petrol Multispace.

I recently had the starter out to clean the brushes and commutator.
It had refused to turn over at all on a few occasions.
During refitting I initially forgot the positive cable from the alternator, and had myself quite puzzled for a while cratch:

Now everything is back to sane voltages, except one detail..

I'm measuring about 30v AC across the battery when idling!
Quite alarming, as I've read that 0,5v should be max.
A clear case of a busted diode one would think.
But the DC is a quite good 14,4v

And then I have this old scope.. ( a Tektronix 314 )
and I tried to have a look at the actual waveform, and as far as I can tell it looks good.
A series of little hilltops a bit like this. http://www.drivesys.com/DCpic2.jpg
About 0.5v peak if I read the scales and multipliers right.

What do you people think?
Have I read the scope wrong? I'm not well practised..
Is my multimeter to old or busted?

Is there any other test I could do, or other symptom I should see if a diode is in fact busted?

[Rustscrat]

Hi, I'm presuming you are using a digital multimeter.

These are very sensitive and will give dodgy readings when measuring AC across a DC source. Try and get hold of an analog meter (one with a needle) give it a measure, see what you get.
The ripple will come from the Alternator as it produces raw DC. (no smoothing)
I'd trust your scope on this personally. If you are pumping Ac in to the battery it's not going to charge up and so no start.
I've been caught out with these meters many a time at work so I always carry an old analog (AVO) around with me !!!
Cheers alut:

[Alias_Knagg]

I tried both actually. :-(
The digital jumped about quite a bit yes, but gave essentially the same result as the "dial meter".
I've got a clamp amp meter in the drawer as well. I suppose I could give that a try. I expect it's inductive, so it will only pick up the AC component.

Tomorrow I'm going to give the scope another try. I might have looked so closely for the ripple that I missed the 30v spikes.
The picture I got was way more messy than what what a google for "alternator ripple" will turn up.
http://www.google.com/search?q=alternato...e&tbm=isch

But that might be my use of the scope..
( why are those manuals only sold by those dodgy looking ebay shops )

But anyway, as you say, if I really had 30v AC RMS going into the battery, shouldn't it be pretty shot by now? And not showing 14,4v DC either?

Hm.. a scary thought..
I left the positive cable form the alternator dangling for two short drives before I figured out I had missed it. That seems to be the one and only positive connection point form the alternator according to Haynes.
I was pretty sure I had got lucky and it had rested on this plastic
tube above the starter, but what if it had actually shorted while I was driving?
Could that have killed a diode, or busted the rectifier somehow?

[andy-womble]

Charging voltage sounds about correct for alternator, I wouldn't worry too much about it. If you had done any previous lasting damage it would have revealed itself by now.

[Alias_Knagg]

I may be needlessly worrying, but I just realized that some odd symptoms I've had earlier may be consistent with a "pre existing" rectifier failure.
One of the first google hits for "alternator rectifier failure"
http://www.alternatorparts.com/alternato...oblems.htm
claims that:
"You can tell your bridge rectifier is going bad when you first start a cold vehicle and it charges better at first compared to after it warms up"

I've noticed sometimes the "battery light" staying lit for 5 to 20 seconds too long after start, but only when starting again after a short drive and a short rest. Like after shopping at the local supermarket.
When I took the battery in for charging while I worked on the starter, it only had 12.21v on it as well.
Some sort of "low level" rectifier failure is starting to look likely...

[Rustscrat]

Hi, usually if the alternator is faulty you would see the battery light on all the time.
There are also brushes inside and if it's a high mileage vehicle these might be worn out causing the slow extinguish of the light.
Diodes can easily be checked but it's a bit of a job to strip the alternator down to get to the diode pack.

[Alias_Knagg]

I did some further testing today, and the alternator is definitely not seriously damaged.
I'm still measuring > 30v AC however..

But have a look at what I was able to coax the scope to show.
Just different timescales and gain.
Probably different numbers of "passes" on screen as well. ( don't know if this scope can do single shot captures yet)
I guess I should be happy that idle rpm fluctuations makes the tracks separate and not blur together.
But I don't really like the look of those spikes..

   
   
   

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Just had a look with the clamp meter, and it showed 3,4-4A at the battery!
Does not that seem a bit much if it's only the AC component?

[Rustscrat]

I'll take some readings & measurments from mine tomorrow then we can compare. cratch:

[andy-womble]

From the results on the ECG it would appear that the patient is in Atrial Fibrillation and in need of a cardioversion to correct this irregular heart-beat and to restore regular sinus-rythm.....tudy:tudy:illyme:illyme::whistle::whistle:

[XTR]

From the results on the ECG it would appear that the patient is in Atrial Fibrillation and in need of a cardioversion to correct this irregular heart-beat and to restore regular sinus-rythm.....tudy:tudy:illyme:illyme::whistle::whistle:

Yeah,what he said.cratch: