This is a subject I've been trying to get a clearer picture of. In Halderman's book, "Automotive Electrical and Engine Performance", 7th Ed. on pages 276 - 277 he discusses "Testing AC Ripple Current" and says that the AC ripple current shouldn't exceed 10% of the alternator's rated amperage. Then, in Figure 20-9 on pg. 277 he says, "AC ripple current should be less than 10% of the DC current output."
The second statement seems to indicate to me that we're talking about 10% of the currently measured output of the alternator. For example it one measures 30 amps DC current, then the AC current should be less than 3 amps.
This second statement makes allot more sense to me since unlike testing for AC voltage, where the voltage output is relatively fixed between 13.5-15 volts and we say anything above 500mv AC average is bad, the current output is hugely variable and I would think 10% of the current amperage output would make allot more sense.h
I've had trouble measuring alternator AC voltage with my multimeters. Apparently, they don't do AC coupling and expect pure AC. They measure wall mains AC just fine, but if I put them on any alternator they just freak out, with the reading jumping all over the place. Suppose I could just put a cap in line like I do for my scope, but my clamp seems to handle the AC current without any issue.
A multimeter on AC voltage measures RMS, root mean square. It's a calculated value (kind of). It's a DC equivalency(ish) It's peak to peak voltage on an oscilloscope, divided 0.7 if I remember correctly. Google away.
Anyway, I was always told 50MV max on AC @ the alternator. Never found that in a service manual of course. I had one today the measure 98MV, about double what I normally see. Not sure how much is too much..... grey areas....
Halderman says in his book that anything less than 400mv AC voltage is OK, and anything over 500mv means the diodes are bad. I think SD says anything over 300mv is bad. I've never heard of 50mv being a problem. Talking about average or RMS values, not peak to peak.
SailorBob wrote: I've had trouble measuring alternator AC voltage with my multimeters. Apparently, they don't do AC coupling and expect pure AC. They measure wall mains AC just fine, but if I put them on any alternator they just freak out, with the reading jumping all over the place. Suppose I could just put a cap in line like I do for my scope, but my clamp seems to handle the AC current without any issue.
My multimeter also freaks out when measuring AC in autorange mode. So I just put in on manual range and it works fine. :huh:
A cheap multimeter will show an average AC voltage, not RMS. An OK mulitimeter will calculate an RMS value which is OK for nice smooth sine waves like your mains. A good 'true RMS' multi-meter take many thousands of measurements per second and will do a good job with arbitrary wave forms like the pulsating DC from an alternator.
I'll try it on manual range and see if that solves the problem. This is a Uni-T 61E which is supposed to be a 20,000 count true RMS meter, so hopefully it's just the auto ranging that's screwing it up.
OK, so it was the auto ranging that was screwing up the reading. Works fine if I set the range manually. This particular car is showing between 350 - 400mv at the alternator post with the brights, rear defrost and cabin fan on at 2000rpm.
OK, this guy named Louis Bernstein from IATN gave me this amazing article on ac ripple in alternators and thought I'd share it here as an attachment to this message.
Hi,
This document seems to be a bit outdated. AC Ripple is something to keep in mind considering the modern vehicles with all the computers installed. AC is not being used in cars or trucks, so all that energie dissipates itself in heat. In the example of scannerdanner it will dissipate 3 Amps of heat somewhere. on the other hand all ecu have filters built in but never the less computers dont like ripple cause it messes up there internal clock timebase. Also its degrading there components and reduces the lifespan. Modern cars have quite a different way to charge the battery then 10 years back. so AC ripple becomes a very interesting topic not to forget.
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