Do you remember the first time voltage drop kicked your butt?

Bruce from Spring Hill Technician Posted   Latest  

Michael Christopherson's post got me thinking about this. I'm not sure if it was the first time but it's the first time that I remember. It was a Nissan pickup no crank. I remember unplugging the starter solenoid and checking for power with a DVOM. Hooked it up and turned the key to crank, meter showed 12 volts so I sold a starter. Of course it didn't fix it. So I replaced the relay and it started. That was about 18 years ago. 

Unfortunately, that didn't set me on the path to figure all this stuff out. I didn't go to school for auto repair and my "mentor", my Dad, always said there was no money in that part of the business. It wasn't until late 2016 that I started really trying to learn electrical and diag. Since then I have taken several ATG and SMP classes, Super Saturday last year, Vision and ATE this year. Going back to Super Saturday this year. Wish I had started down this road a long time ago. 


Dustin from Lakeland



Yes I do, it was an older gm, 3.8....bad body ground that took me way to long to find. 

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Scott from Claremont



About ~30yrs ago I remember an early 80’s GM towed in for a no crank. I started with an under hood inspection look and ‘feel’ and this is where I learned! I’m sure that there are many here that remember the small flat braided copper ground strap that ran from the firewall to the intake or the back of a cyl head. Well, as I was probing (pre labscope days) for some reason I reached in to touch that strap and my finger instantly received a very cool signature... at that point I knew there was too much current flowing through that circuit.

It hurt like a bitch but I think got paid .7 to change out the defective negative cable. Back then there was no testing/diagnostic fees...

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Geoff from Lahaina



Yup. Wiper motor on an 80's Chevy pickup. When the reman didn't work, I just ordered another one. When that one didn't work I gave it some more thought. I found that the fuse was dirty. Didn't take long, but I felt stupid for the rest of the day.

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John from Beaver



1977 Ford Maverick Grabber. It would shut down on the road at random times. When it quit it lost spark. The typical routine back then was to connect a test light from ground to the coil negative and watch the ignition command. Those Duraspark systems turned the coil on, and only turned it off to fire the spark plug. The lamp would go out when spark was lost and the first few times I experienced the failure I didn't know that what I was losing was the power to the coil. I had to move the test light to the other side of the coil to find that out. The next problem was why was I losing power? It turned out to be an abraded wiring harness beneath the engine that went between the coil and the starter solenoid that bypassed the resistor wire during cranking. When it went to ground the resultant voltage drop across the resistor wire took all of the power away from the coil .

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Walter from Sarasota



In all actuality voltage drop is what keeps guy's like us in demand! I had this one last week, 96 VW Golf running rough with check engine light. The codes P0501, P1237,P0125,P0300 and P0301. It already had No.1 injector and tune up components done to it so I hooked up the scope on injector's 1 & 4 and current on injector fuse, Immediately I knew I had a connection or ECM issue. I inspected the harness and I did fine corrosion at connector T28a cleaned it and no change. The rest of the harness looked good engine side and decided to go to the ECM which sets under the cowl, There I found what looked like minor rodent damage but the wire that grey wire was my injector control for no.1. I tugged on the wire and came in two with little effort. Only three strands of copper were intact. Found several other damaged wires and ended up fixing a few other issue. This thing would run fine at idle but driving and under load would misfire, In the scope pic the thing that caught me eye right off was a the voltage diff when the injector was grounded by the ecm, almost 2.4 volts and a current difference of about 150 milliamps,

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Walter from Sarasota



The scope capture of 1 & 4 was not included in post at this time. 

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Martin from Burnaby



Absolutely! I recall the details with full clarity like it was just yesterday. I always use this particular example and openly share my "butt kicking" experience with my students, when discussing the value of performing voltage drop testing.

In 1975, I worked in an Esso Service Station in Vancouver at the time and got "married" to a no start diagnosis for three days that kicked my butt good and proper. Of course, back then I'd been in the field for a whole seven years and knew "everything" like so many others with seven years experience! Expertise be darned, at seven years in, no matter what we believe our skill set is, we're still "wet behind the ears", because there is plenty that we do not known and have not yet experienced.

So, don't expect to know everything in such a short time line or even after fifty years or more. We never stop learning and practicing our skills. We might have the latest and greatest tools such as PicoScopes and the like at hand, but once in a while, be prepared to get your butt handed to you "on a plate" by a simple fault. Just ask someone who has been humbled!

While the topic of discussion here is voltage drop, there are many other aspects of our work where there are no high tech diagnostic tools involved and we can still manage to get tripped up on occasion! Mark my words!

So, to the culprit. A white 1971 Mazda 1200 two door was towed to the shop for a no start condition. It's funny how we can clearly recall the tiniest details of those vehicles that gave us grief isn't it?!!

Anyway, while the vehicle cranked plenty fast enough and had a good cranking cadence, there was no spark from the ignition system during cranking, while there was plenty of fuel available at the carburetor. The car has been used daily and had run fine until being parked over night outside the owner's home.

While we were working on the car, the other mechanic cranked the engine and the battery quickly started to lose power, unable to crank at sufficient RPM. I suspected the typical scenario of the owner having cranked the engine extensively, before calling for the tow truck and that the battery needed charging and testing.

After the battery was charged, it was tested and found to be fine, so diagnosis continued with some visual inspections and testing of the ignition primary circuit. The wiring checked out fine with an Ohmmeter and surprisingly the engine started the next time that it was cranked. Diagnosis was inconclusive and the waiting owner really needed the vehicle, taking it as soon as it was running.

The very next morning, the car was already parked out front of the service bay doors when I arrived at work. I'm sure that many of us have experienced that "uneasy tightening feeling" in the stomach when you suspect that you missed something in a diagnosis!

Same conditions and scenario as on the first visit and again, the vehicle started only after charging the battery for a few minutes, even though it did seem to have sufficient cranking capacity. Like the day before, the impatient owner needed to get to work, so took the car promising to return and drop it off later in the day.

He noted that on both days after the first cold soak starts in our shop, that all subsequent starts until an overnight cold soak had been uneventful. Hmmm.

Day three arrived and again the car had sat outside the bay door and would crank but not run, with the conditions being exactly the same as on the first two mornings. There was no spark from the ignition system unless a battery charger was first connected for a few minutes.

So, I began a more thorough diagnosis and when I touched the ignition primary lead to the ignition coil B+ terminal at the ring terminal, the wire fell off the terminal! Further inspection revealed that there had been only one strand of the conductor hanging on for dear life inside the insulation! Repairing the wire by stripping it back and installing a new terminal was the fix as confirmed by the now happy customer, on the following morning cold soak start.

I've never been shy about discussing errors that I have made in the past fifty years in this trade and as careful as I have been during that time, I found that as soon as I got a little "cocky" about my abilities, I could expect to soon be humbled as a reminder that I'm not perfect. I also know well that if others claim never to have had at least one notable FUBAR experience, then they are either lying, too embarrassed to share, or have never actually done anything!

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Scott from Tulsa



Do I remember the first one I got fooled by? And how... the first 'serious' (expensive) voltage drop problem that kicked my butt was on an early 80's GM cruise control. The activation signal at the ECM seemed fine (using my new neon test light I got off the tool truck) and I didn't really see much of a discoloration (the typical dull brightness of a weak signal) from the test light bulb. When I disconnected the ECM the test light would get a bit brighter, but ever so slightly. The long and short of it... I broke my own rules of always testing the same problem with two entirely different methods and crashed dived into replacing an ECM. Totally wrong... same exact problem existed. The actual problem..... charred contacts in the brake light switch knocking the signal down. Lesson learned. Oh, and my brand new neon test light. ... it found a new home. Fool me once - never twice.

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