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General Electrical Troubleshooting


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Finding Electrical Faults


Knowing just a few electrical basics can make the difference between identifying the exact cause of a problem, and blindly guessing , replacing parts and going down dead ends. It will save you hours of time, frustration, and money.




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Standard auto relays.



How relays work


Article: Understanding relays - What relays do, and how to use them in a circuit:


Internal schematics, and which pins do what. Look for the numbers on the bottom of your relay.


Diagnosing failed relays:


You can also purchase these bypass switches. They fit in the same slot as a standard relay, but they allow you manually switch the circuit on or off. This is very useful for diagnosing problems where they are controlled by computer, or when access to wires is difficult. Some even allow you to measure current with a clamp meter.

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Scotty's Electrical System Repair playlist on Youtube


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Charging System

If you are experiencing problems with the battery or generator (alternator), always test them BOTH. The battery and the generator work together to make the charging system. When one is weak, it will start to drag down the other.


It is not enough to simply measure the voltage from a generator. Sometimes, it needs be able to produce about 100 Amps of current to properly charge your batter and meet the power needs of the rest of the vehicle. This can only be tested by "loading" it.

Auto part stores often offer battery and alternator load testing as a courtesy.

You can perform a rudimentary check yourself which is not as thorough, but can be helpful to reveal issues. With the engine running, turn on everything that consumes the most power: All lighting (high beams, fog, brake, reverse, dome), blower on maximum, seat and mirror heaters, window grid heater, window motors, wipers, etc. Measure your system voltage (this can be done anywhere: on the battery, a fuse box, using an OBD tool, or even indicators that go in the cigar lighter socket). It should maintain 13-15V.


Typical Generator Output

(generator pulley ratio is in the neighborhood of 3:1. That means 500 engine RPM = 1,500 generator RPM)

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How to Read Electrical Schematics
Figure 1 - Example brake switch schematic from a service manual


Electrical diagrams provide invaluable information for troubleshooting electrical problems.

The most obvious fact is that they tell you what is supposed to be connected to what, which helps you know where to look for problems. That's not all...

  • Wires are labelled by color for easy identification.
  • Power distribution is shown so you can trace faults all the way back to the source, and identify which fuses are involved with the accessories you have.
  • Circuits are numbered, so you can cross-reference with other circuit diagrams, and connector views. (see "zoning" Figure 4)
  • Connectors are drawn and labelled, so you can find them on your car. They provide convenient locations for performing electrical tests. Each connector has a code which provides more information. 'G' for ground point. 'S' for splice. 'C' for in-line harness connector etc. The number indicates where the connector can be found. Each terminal on a connector is labelled (e.g. A, B, C etc.) so you can easily find it when you have it in your hand. Often they will even show the gauge the wire must be.
  • harness routing ...


Figure 2 - Basic circuit symbols


Figure 4 - Example of a zoned numbering system



Figure 5 - Circuit example (excerpt)


<numbered connectors>



Figure 6 - End view and pin description for connector in Figure 5




Usually manuals that provide diagrams will also provide harness routing views (Figure 7)



Figure 7 - Engine harness showing location of a ground (label corresponds to schematic).



Reading Toyota diagrams



Video Summary

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The Tools

Digital Multimeter, test light, OBD devices ...


types probes (backprobing, piercing etc.)

audible circuit tracers

Where to getschematics and service info

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These videos are good examples of how a mechanic troubleshoots an electrical problem , step by step. You can use it as sort of a tutorial.




Video: " Mechanic Parts Cannon FAIL Cost Customer A LOT! Let's Try This Again "

Mechanic "The Car Care Nut" walks you through the diagnosis of a valve that is not operating correctly, demonstrating the use of a digital multimeter. The previous shop did not perform any troubleshooting and instead guessed, replaced unnecessary parts, and ultimately charged the customer money without fixing the issue. A very common story.




Video: " I Thought I'd Seen It All And Then This Chevy Got Dragged In"

In this video, mechanic Eric (South Main Auto Repair LLC) takes you through the steps of troubleshooting a pickup that won't start. He demonstrates using an electrical schematic, diagnostic tools, and measuring techniques to efficiently zero in on the exact location of an electrical fault. Once again, the previous shop did not bother to do this, and were not able to fix the customer's truck.


Diagnose diagnose diagnose, ....

Here is what can happen when you throw parts at a problem.

This would have taken 30 seconds to diagnose with a meter.

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Circuit basics

complete circuit




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How to find what is draining your battery


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Basic troubleshooting

isolating, deduction



Don't Guess. (edit)



Is this wire broken? (or fuse, relay, etc.)

How to test the electrical connection between two points.



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What to look for when searching for faults


Some common electrical failures




Figure 1 - Photo of a relay with fretted terminals.



Figure 2 - Photo of a connector terminal with fretting


Figures 3,4 - Close-ups of fretting


Fretting happens where two surfaces meet. Micro-vibrations abrade the outer protective plating, causing corrosion of the underlying layer. This weakens the metal, accelerating further erosion. This process continues until good electrical contact is broken

How do reduce fretting?

  • Minimize movement by stabilizing the two parts to each other. That means fastening them properly to a solid substrate. Connectors with broken latches should be replaced, and good cable management should be used.
  • Application of dielectric grease to seal and reduce corrosion





Copper has a lot of free electrons, which is why we use it to conduct electricity. Unfortunately this same property makes it a reactive metal. Exposure to contamination (moisture, pollution, battery vapors, salt, and other chemicals) cause it corrode. This is partly why we put it in a jacket of plastic , put it in sealed enclosures, and sometimes plate it with other protective metals like tin.

Corroded copper turns black or green.


Figure 5 - Clean new copper wire, and wire that has corroded


Corrosion is undesirable for a few reasons:

The major reason is simply because the copper is being lost. The corrosion flakes off, or is dissolved by water, exposing more copper, and the cycle continues. Heat from a hot engine bay and the electrical current accelerates the chemical reaction. Eventually the wire is broken down into nothing.

The compounds that form when copper corrodes are not conductive enough, and the wire loses its ability to carry electrical current. Corroded wires have more resistance which makes them start to behave less like a wire, and more like a toaster. Corroded sections can heat up, scorch and even melt connectors, increasing risk of shorts and fire.

Surface corrosion can also affect things like connector terminals, battery post clamps, bulb sockets, ground points etc. It leads to poor electrical contact, which leads to the previously mentioned problems.

On an automobile, all of these can result in engine and electrical malfunctions and failures.

How can we prevent corrosion?

  • reducing exposure to weather and contamination. Keeping parts inside a dry enclosure, using weather sealed connectors, using good insulation, and using dielectric grease to seal out moisture
  • replacing wire that has begun to corrode or that has damaged insulation
  • A common source of failures is corroded fuse boxes. They have lids for a reason. If you lose it, replace it.
  • Remove sources of corrosive substances. Overcharged or compromised batteries can vent sulphuric acid vapors which attack surrounding metals. Neutralize and clean any spills of caustic substances (battery acid, brake fluid, diesel exhaust fluid, etc.)

Figure 6 - A badly corroded ground terminal.


Figure 7 - A melted and scorched headlight connector.




be careful where you drill