Alternator waveform ripple and noise

Ask for and share advice on using the PicoScope kit to fix vehicles here.

Alternator waveform ripple and noise

Postby liviu2004 » Thu Jul 06, 2017 9:05 pm


Few days ago I've went ahead to check my car's alternator. One of the test was of course AC ripple test. What I was looking for was this type of waveform, as published under Example Waveforms.


This is what I got, a bit of filtering added to the channel:


At a zoom in, waveform looks like this, also a bit filtered:

What intrigued me was the periodic noise shown. I discussed it with an work colleague, we did a short math and concluded some noise had a frequency of 2 kHz. Then I realized I might know what that is, anyhow, it looks like an inductive load being switched on and off.

This is a capture I've done in the past with (now the back-up) scope, showing a 2 kHz signal:


And this is coming from the throttle body actuator. It is PWM controlled with a fixed frequency for a fixed position, by the ECM.

Went ahead today and did a second test, this time recording both AC ripple on channel A and throttle body actuator signal on channel B. It is clear that the AC waveform is distorted by the inductive load but also, not all noise present is at the moment identified.

A bit of filtering on channel A:

throttle body_3.png

More filtering:

throttle body_1.png

And what you see here circled in orange is the throttle body actuator being energized by the ECM, what is circled in green I haven't yet confirmed, maybe waste spark coil, injectors, fuel pump:

throttle body_2.png

I've uploaded an waveform in the Library and hope above was not very boring for you.
Posts: 46
Joined: Sat Mar 19, 2016 10:23 am
Location: Netherlands

Re: Alternator waveform ripple and noise

Postby STC » Thu Jul 06, 2017 10:42 pm

Sometimes the Pico can be Victim of its Speed, Accuracy & Sampling Rate. The Alternator, a big spinning Magnet connected to the power source (battery) with a big thick wire, when scoped, is going to show all power hungry actuators and coils collapsing. It is not noise, it is real !

From what I can see your alternator ripple is acceptable ?? That is only ever one side of the story, An Amp Clamp on the - Battery Cable will indicate what has the potential to be stored - assuming a good accumulator ?
User avatar
Posts: 790
Joined: Sun Sep 13, 2015 4:10 am

Re: Alternator waveform ripple and noise

Postby FioranoCars » Fri Aug 04, 2017 2:36 pm

HI liviu2004 !
Great to see your analysis, and explanation of the overlaid signal(s) (often misunderstood and called noise or interference!), so well done for identifying the source ... and of course it might indicate a poor engine earth strap or inductive pickup from close routing of cables or chaffing ... though probably not in this case, but well worth keeping these things in the back of your mind!

The A/C ripple looks good for idle, both rectification and amplitude, however from experience doing this test at idle unloaded is not going to fully test the alternator, and we do a loaded test too, both at idle and at circa 3,000 rpm.

As we have spare channels, we use the following, and it's a follow over from a battery/starter test, then Alternator Output, before finally the AC rectification so we move least cables between each test, and this final test is purely for the rectification part ... with an 8 channel scope (6 is all that's needed, but 8 would be good :D ) we could do both Alternator output and ripple in one go! Even using the extra channels to segment volt drop at battery leads!;
Channel A = Battery +/- = +/-20v DC scale
Channel B = Amp Clamp on battery (either +/-) = 600amp scale
Channel C = Positive terminal of alternator (or as close yo it as is accessable - closer the better) =
+/- 2v AC

Channel D = Amp clamp on a Carbon Pile Battery Tester attached to the battery terminals (either Lead) = 600 amps scale

The test is started with the Carbon pile tester at 0amp loading, car idling, Part 2 then loads 30-50amps (a level that will exceed maximum idle output but not the total output of the alternator) while idling and finally applying 3,000rpm.

You could apply loads using vehicle based devices, but measuring the load with an amp clamp gives more accurate results and better later analysis.

The initial part would mirror your test, vanilla AC ripple, the next part confirms alternator can output REAL amps (some can't output anything more than a few amps and fall over), while Amplitude of the ripple might rise a bit, the rectification should remain intact.

The final element shows that the alternator can increase output with increased rotation, and meet the (not unreasonable) demand you placed on it. Again the ripple should be intact but amplitude will increase further, though not excessively (experience will tell you what's good and bad - but over 1v is clearly an issue, at idle with no load 0.2v is normally the cut off, fully loaded and at 3k we expect 0.3-0.5 but make/models vary).

Having a amp clamp on the external load device ensures you can compare the output to demand, and can play at say 10amps load, 20amps etc to see response yet still look back at a saved waveform and know the demand loaded and outputs achieved.

Probably about half the failures we spot pass an unloaded idle test, while a vehicle generated load might catch a far number of these it would not catch them all, and this is a simple test to setup and perform.

I hope that sparks a few ideas!
User avatar
Posts: 377
Joined: Thu Dec 13, 2012 1:26 pm
Location: London, UK

Re: Alternator waveform ripple and noise

Postby victor2k » Fri Aug 04, 2017 6:00 pm

Yes the new tools is a blessing and a curse sometimes.
With the old 4423 I didn't see this interference.Look at this post:
topic15201.html-the floating ground give as a headache
The zero level for clampmeter or capacitive probe for secondary ignition voltage don't go to hell,no interference with signals from other channel... :?:
Last edited by victor2k on Sat Aug 05, 2017 2:55 pm, edited 1 time in total.
Posts: 165
Joined: Thu Jan 28, 2016 9:17 am

Re: Alternator waveform ripple and noise

Postby STC » Fri Aug 04, 2017 8:56 pm


with an 8 channel scope (6 is all that's needed, but 8 would be good :D ) we could do both Alternator output and ripple in one go!

I digress from alternator ripple to 8 Channel Scopes, perhaps a new thread is appropriate !!!

I will start one.
User avatar
Posts: 790
Joined: Sun Sep 13, 2015 4:10 am

Return to Diagnostic discussion

Who is online

Users browsing this forum: No registered users and 0 guests