Automotive relay wiring doesn’t have to be confusing. In this video, I show you exactly how automotive relays work and what terminals 30, 85, 86, 87, and 87a actually do in real circuits.
If you’ve ever looked at a relay diagram and felt stuck, this will fix that.
In this lesson you’ll learn:
• What a relay really does inside
• How the control side (85 & 86) works
• How the switch side (30, 87, 87a) carries load current
• How to wire a relay correctly
• Common mistakes that blow fuses or cause no-start issues
• How relays show up in real vehicle circuits
This is foundational electrical knowledge every technician and DIYer should understand.

Bill Teaches Automotive Technology (BTAT):
More automotive training, electrical diagnostic resources, and free technician tools:
https://billteachesauto.com

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TOOLS USED FOR DIAGNOSING ELECTRICAL ISSUES:
Test Light (simple and reliable circuit testing)
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Digital Multimeter for voltage drop and circuit testing
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Automotive Jumper Wire Kit
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Power Probe (quick power and ground testing)
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Professional Scan Tool
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ELECTRICAL DIAGNOSTIC TOOLS I RECOMMEND:
Back Probe Kit (test connectors without damage)
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Terminal Removal Tool Kit
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Heat Shrink Connector Kit
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Fuse Assortment Kit
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Wire Stripper / Crimper
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Battery Tester
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OBD2 Scanner (budget scan tool for DIY diagnostics)
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MY DEVOTIONAL FOR TECHNICIANS:
Toolbox Truths: A 365-Day Devotional Guide for Automotive Technicians & Tradesmen
Written specifically for mechanics and serious tradesmen.
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Affiliate Disclosure
As an Amazon Associate I earn from qualifying purchases.

Chapters:
0:00 Introduction
0:45 What is a relay?
2:10 Terminal numbering (30/85/86/87/87a)
4:00 Wiring example
6:30 Common mistakes
8:00 Practical applications