Carrying out a bit of free diagnosis for a regular customer I have come across a bit of a quandary. His problem was one of unstable idle and occasional stalling plus his headlights were occasionally dimming then getting brighter. Main dealer could find no problems (!) and another indy had also found no faults. I got him to check charge rate and he had 14.0-13.9 which I thought a little low so got him to check AC volts which were 35v. 'Theres our problem' methinks so as there was no improvement after cleaning earths recommended he fit a new alternator. This we did with a genuine Peugeot supplied exchange unit he supplied. We now have a constant 14.4 volts and the idling is perfect and light output stable BUT I still see 25v AC at the battery?
Any thoughts?
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Without wishing to seem disparaging, buy a good quality DVM such as a Fluke or use an analogue meter and settle for lower accuracy. Chinese DVMs cannot integrate the alternator ripple properly and are fooled by the switching action of the regulator.
659.
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Andrew,
I'm sure that Screwloose will be along with a better idea than me, but this is my thoughts.
The altenator will produce power A/C, Ie possative and negative. The regulator will "remove" the negative, so you will only get the possative, DC pulses.
I would suspect that connecting a multi meter that is looking for a +/- cycle will confuse it and it will be doubling up the DC pulses.
But I'm probaly wrong......
Over to you Screwloose. (I'm just dousing the cockpit in petrol now.)
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Andrew,
Interesting post.
Why did you look at the alternator as a cause for this problem?
Also - when you say check the AC volts - what do you mean? On the Alternator itself before the rectifier?
Cheers,
James
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My understanding of Andrew's post is that the AC voltage was being measured at the battery terminals, hence his concern. 25V is a hell of a lot of ripple!
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I don't think he is - he is saying he see's ~14 volts at the battery with the new alt...
update - sorry - he is....
Edited by Javalin on 05/12/2008 at 12:52
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I think the only decent way of looking at an alternator output is with an oscilloscope. Can you still get them?
The last time I had an iffy alternator it clearly showed one of the three phases missing.
Are alternators still three phase devices with full wave rectifiers?
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I think the only decent way of looking at an alternator output is with an oscilloscope. Can you still get them?
Yes, these days you can get fancy DMMs which have a reasonably usable oscilloscope ( 1MHz or so bandwidth) built in.
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>>buy a good quality DVM such as a Fluke or use an analogue meter
Yes, a meter which reads true RMS might get a bit closer to the truth.
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I am using the same DMM as I have used for the past 8 years, cost me £350 then. I always check AC voltage and normally get a reading of 0vAC. From experience a high AC reading will upset modern electronics just doubting my findings a little. So far, touch wood, all is well but I am going to check again to satisfy my curiosity with my PicoScope automotive'scope. I asked him to check output of his alternator because of the lights dimming.
Thanks for the replies.
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>>From experience a high AC reading will upset modern electronics just doubting my findings a little.
Yes, I wonder if what you are reading might not be alternator ripple, but higher frequency interference? - again, the oscilloscope should find it.
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Number cruncher
Any suggestions of the source of the HFI?
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On a petrol, the obvious suspect would be an HT fault, where HT instead of leaking to earth is leaking into the LT, or even into any adjacent wiring.
I can imagine that a good quality meter as you describe would be sampling quickly enough to include the resulting spike in its averaging/integration.
Have you seen anything unusual with the 'scope? I've never used a Pico scope [I use an IOtech Wavebook] - how quickly does it sample, and at what frequency is the input signal anti-alias filtered? How do these frequencies compare with the sampling rate of your meter?
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Going to see it next time I am passing but he's a bit out of the way so may be a week or two. As regards your question re Pico scope I'd have to be honest and say I dont know verbatim, I'll look it up. An Electronics expert I aint!
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PS If it was HT/LT related it would presumably manifest itself on the scope as a primary or secondary HT 'pattern'?
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Yes, you would probably see a spike which would correspond to the "KV" spike - I'm not sure you would be able to pick out any ringing, or the burn time!, and unless you're very unlucky with the speed ratio between crank and alternator, there won't be any coincidence or pattern between the alternator ripple and the spikes.
i.e., if you see interference which is consistently timed with the alternator ripple, then, it's unlikely to be HT.
If you see an interesting pattern, please grab an image, and post it up! (via an image hosting site)
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