Weekly Waveform #2 - Voltage Drop testing the starting circuit

[@billyyoder8171]

Thank you Chad and Val. Good job. Word of caution to all and learning from my mistakes. We learn and remember from our mistakes, the best teacher is experience and making mistakes. Over 20 years ago, as a younger tech, I had a truck that had 7 VDC drop from positive battery post to positive starter post during cranking. Complaint was slow cranking. So I made a new cable and installed. Same result. LOL. Then I load tested the battery, it failed, dropped below 9.5 VDC under load. Installed new battery, same result, 7 VDC drop across battery positive to starter positive. Logically, replaced the starter. Fixed. Moral of the story. Weak battery and dragging starter results in slower turning starter motor. Which results in more amps flowing down the battery cable. Which results in more heat build up in the cable. Which results in resistance in the cable. This heat resulted in my 7 VDC drop across battery positive to starter positive. Lesson learned. Since then, battery load test first. Visual inspection of cables and connections. Then starter amp draw test. Lastly voltage drop cable. The things we remember LOL. Have a blessed and safe week to you and your family.

[@ThePracticalMechanic]

Wow. Thanks for information Billy. Nothing can replace experiences. We haven’t added accessories to the scope or we would have used an amp clamp as well. That will give us the last piece of the puzzle and hopefully help us determine where our problem lies. Excessive amperage in our current waveform like Joe’s Auto diag had the other day better make us stop and look for the issue. Seized accessories, tight engine or failing starter will overload the starter cables. This truck had a .320 voltage drop on the negative cable after replacement but unfortunately the customer decline the positive at this time. Thanks again for sharing your lesson and now I’m on the hunt for a case study.

[@Tek_Nician]

Thank you for doing this series. I needed this. I’m also learning more about the scope functionality of my Zues

[@ThePracticalMechanic]

No problem. I enjoy using the scope even for simple test because it can give us so much information. I’m already learning new things based off others doing the weekly waveforms. It’s amazing how many ways we can perform a test and interpret data. Thanks for coming along.

[@JustinDowDIYcentralhighway]

This was really cool. Thanks for sharing your experience. Very well explained & I learned something new about GMC & Chevy Cables. Thanks Brotherman 🙌🍻

[@ThePracticalMechanic]

No problem Justin. We are going to working our way through as many test as I can possible think of as well as ideas from the community. I think the cables on the newer cars are getting worse and worse. Gm cars actually has a TSB for their replacement because the voltage drop and noise was causing all sorts of electrical faults.

[@mike-yp1uk]

Great video I have the Zeus and i enjoy it's versatility. It's compact and clear enough plus component repair is a nice feature as well.

[@ThePracticalMechanic]

It’s super nice to have component information built in. I like running split screen with scanner and scope as well. Makes bidirectional controls easy to access and watch the outputs on the scope side.

[@juanmgomezb]

Excellent explanation. A very useful information.

[@markferraro5250]

thanks PM great stuff keep them coming

[@wyattoneable]

I appreciate the lesson Chad.

[@70corwin]

If you want the cursor to stay put when you zoom out I think if you put that dash curser over the curser it will stay put?

[@arthurfricchione8119]

Thanks for sharing your videos and your knowledge. I will try and catch up with some posts this week. It will give me a good opportunity to use my single channel Uscope, my two channel Pico and my four channel Ditex automotive scope. Keeping my fingers crossed as this is my first actual hookup. I have two Hondas a 3.5 07 Odyssey and a 99 3.0 EXL Accord. Both very low mileage. Will also be doing timing belt and valve adjustment checks just waiting for new seals from Honda to arrive. This is on the Odyssey. The Accord I’ve already accomplished those tasks. Thanks for sharing 😊 Hondas I will need to pull the fuel pump fuse or relay unless you have a better idea. Thanks

[@ThePracticalMechanic]

I don't think Honda supports the clear flood mode so disabling the fuel will be the best option. Fuse 4 under the dash should power the fuel pump on the van. You will need to crank it for a while to run it out of fuel before performing the RC test.

[@arthurfricchione8119]

@@ThePracticalMechanic thanks for the info. I will definitely try it. That was the route I was thinking of going. Appreciate the reply. 😊

[@bartscave]

Is the scope “ too fast “ to condemn cables for the 1volt drop? The initial spike is a fail but the average drop isn’t too bad.

[@ThePracticalMechanic]

Perhaps the scope is too fast to use the initial spike. What Daryll noticed was the way that the voltage drop increases over time as well. This indicates part of the cable is heating up rapidly and resistance is increasing as well. After the initial spike I was at 360mv and at the end of the cranking period I was over 400mv. I forgot to save the image but used the thermal imager on this truck afterwards and the negative cable end and next three inches of cable reached 170F and the positive reached 150F. The new ground cable was installed and the voltage drop spike dropped in half and then leveled out to less than 200mv. The positive cable was not replaced at this time. I think part of the problem is manufacturers cheapening up materials and running smaller than required cables. They work fine for one or two startups but quickly exceed the recommend work load. We will be working the amp clamp into this test in the future and I’ll address where the reading should be taken. Thanks for your input Bart as I sometimes overlook key points to a test.

[@hypersomrizz]

👍👍

[@Johnkb2059]

I think it'll be simpler just to use a Fluke meter with the min-max feature

[@ThePracticalMechanic]

This test can absolutely be performed with a DVOM but we can't get the full picture. When this test is performed with a labscope we can check relative compression, battery health check and voltage drop. In this series we are building scope skills that will become valuable in the future tests being performed. It can also give us a pretty picture to print out for our customers or to email to extended warranty companies for proof of work performed.

[@andreifilip1123]

👍

[@georgebonney90]

No current flow,No voltage drop

[@ThePracticalMechanic]

Correct. I should have mentioned that in the video.