Electrical Myth La Resistance! [E209]
Thanks to our Partner, NAPA Autotech Training and Pico Technology
Matt Fanslow opens with “Resistance is futile” and tackles a common belief: “Resistance always makes current go down.” He walks through why that’s mostly—but not always—true, and shows how electric motors (especially starters) can draw more current when unwanted resistance slows them down by reducing counter-EMF. Along the way he ties Ohm’s Law to real diagnostics, shares a Rust Belt cable-smoker story, and closes with a heartfelt reminder about seeking help for the “stuff” we all carry.
Key Topics
- The “always/never” trap in electrical claims
- Ohm’s Law in real life: fixed voltage vs. changing conditions
- Why motors misbehave: counter-EMF as dynamic “resistance”
- Starter example: inrush current, RPM drop → current rise
- High-resistance cables that increase current (and make heat)
- Where the energy goes: heat in brushes/cables vs. mechanical work
- Instantaneous truth of Ohm’s Law: accurate at a moment in time, not across changing dynamics
- Practical tell-tales: slow crank + rising amps + hot/smoking cables
- Mental health note: removing stigma and getting professional help
Practical Takeaways
- Motors are dynamic loads. If RPM drops (binding, poor supply, worn pump), counter-EMF falls and current can increase even as “resistance in the circuit” rises.
- Heat = the clue. Elevated current with slow rotation often means energy’s being dumped as heat (cables glowing, insulation softening, brushes cooking).
- Measure what matters. Combine voltage drop, current measurement, and temperature/thermal observation under load to find where the power is going.
- Interpret Ohm’s Law correctly. It holds at an instant; across changing conditions, re-evaluate with the values at that moment.
Case Study Highlight
- Chevy Suburban (late ’80s/early ’90s): Slow crank, ~400 A draw when ~150 A expected; braided negative cable glows red under a 10–20 s crank. Root cause: high-resistance path + reduced counter-EMF → higher current and wasted power as heat.
Tools & Concepts Mentioned
- Current probe / ammeter
- Voltage drop testing
- Starter relative compression patterns
- Counter-EMF (a.k.a. back-EMF)
- Old-school VAT-style analyzer (Snap-on digital variant)
Quotes / Moments
- “It’s rare we can say always or never.”
- “Ohm’s Law isn’t broken—it’s instantaneous.”
- “If it isn’t turning it into work, it’s turning it into heat.”
Thanks to our Partner, NAPA Autotech Training
NAPA Autotech’s team of ASE Master Certified Instructors are conducting over 1,200 classes covering 28 automotive topics. To see a selection, go to napaautotech.com for more details.
Thanks to our Partner, Pico Technology
Are you chasing elusive automotive problems? Pico Technology empowers you to see what's really happening. Their PicoScope oscilloscopes transform your diagnostic capabilities. Pinpoint faults in sensors, wiring, and components with unmatched accuracy. Visit PicoAuto.com and revolutionize your diagnostics today!
Contact Information
- Email Matt: mattfanslowpodcast@gmail.com
- Diagnosing the Aftermarket A - Z YouTube Channel
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The Automotive Repair Podcast Network: https://automotiverepairpodcastnetwork.com/
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