Any issues using a mains oscilloscope to test car alternator for AC ripple? - Page 1
Forum section: electronics / general

author: 2e0eow
author_rank: Newbie
timestamp: April 18, 2023, 03:58:03 pm
content: Hi, please, if anyone has some information to impart it would be much appreciated :-)As in the title, are there any issues I need to be aware of in order to carry out an AC ripple test on my car alternator using a mains powered scope ?For example:Is there an issue with the car floating and the scope being mains earthed ?As the probes on my scope share their grounding point, would it be ok to use channel one 'active' probe tip to make the connection to the the alternator output and the grounding part of a 'not active' channel 2 probe to make the ground connection to the car chassis (Just because it will be easier for me to achieve on my particular car layout) ?Thanks

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2023, 04:06:07 pm
content: The car may be floating on the tires, but it won't mind being grounded and there shouldn't be a lot of current flowing as it equalizes with ground after you connect it. Overall it should work fine but your grounding idea may result in additional noise in the signal due to the huge loop it may create. If you are probing at the alternator output, you can connect the ground directly to the alternator body or something nearby, using a short wire if needed. What are you trying to see with this arrangement?

author: 2e0eow
author_rank: Newbie
timestamp: April 18, 2023, 04:18:43 pm
content: Hi bdunham7, thanks for the reply."your grounding idea may result in additional noise in the signal due to the huge loop it may create"I was wondering about the potential for noise with the loop. I was considering this set-up because I need to be in the car doing stuff and the alternator is underneath and at the rear of my car (Smart roadster). Makes it difficult to see the scope and perform my tasks."What are you trying to see with this arrangement?"I want to see if there is any excessive ripple being generated by a misbehaving diode or winding. the reason I am curious is because I have been experiencing a clutch adaptation issue that has so far managed to remain a mystery as to why it is occuring. I was wondering if the signal the ECU was recieving from speed sensors and electronic clutch actuator etc was being corrupted somehow.

author: bdunham7
author_rank: Super Contributor
timestamp: April 18, 2023, 04:30:58 pm
content: That would be a huge loop if you and the scope are in the car. If you can run the probe cable back, surely you can attach a ground somewhere back there? Also, if you are going to be checking the power or other signals at the ECU with the scope as well, don't do both at the same time, or at least only connect one ground at a time. So if you wanted to check the ECU B+ terminal for example, you'd disconnect at least the ground part of your alternator probe and connect the ECU probe to a local ground. I think the alternator check can pretty much be a one-time pass/fail--if no problems are found just disconnect and retrieve your probe and move on to other things. Alternator noise issues are relatively rare and tend to be fairly severe under load--and often audibly noisy as well.

author: David Hess
author_rank: Super Contributor
timestamp: April 18, 2023, 04:53:29 pm
content: I have measured alternator ripple at the battery using an oscilloscope before, but an AC voltmeter is usually a better choice.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2023, 05:04:14 pm
content: No problem at all from an electrical standpoint, the car has a fully isolated electrical system. Do make sure the scope has a secure place to sit where you will not trip over any wires, and be mindful of where the probe cables are going too so they don't get caught in the belt, fan or other rotating parts or melted by anything hot.

author: 2e0eow
author_rank: Newbie
timestamp: April 18, 2023, 05:24:08 pm
content: Thank you all for the speedy and informative replies. I'll come back in a day or two to share what I find, if anything ;-)

author: pdenisowski
author_rank: Super Contributor
timestamp: April 18, 2023, 09:37:36 pm
content: I'm actually reading this book right now:https://www.amazon.com/Automotive-Oscilloscopes-Waveform-Graham-Stoakes/dp/0992949262Pages 77-78 have some good information (including traces) about testing alternator ripple.[I don't know the author or have any interest in the publisher: I'm just someone who really likes reading books about test equipment]