Electrical Symbols and Schematics

Electrical/Electronic Symbols

In addition to the schematic symbols initially introduced, there are a few more that are used.

Electrical schematic diagram styles found in GM service manuals.

Service Manual SymbolsIn addition to the electrical/electronic symbols. Service manuals commonly use different symbols.

Typical Power and Ground Distribution.

This is an example of a power distribution schematic. It shows how voltage is applied from the positive battery terminal to the various circuits on the car.

Typical Power and Ground Distribution.

Battery voltage is applied through the B terminal of the starter solenoid, fusible link B, the red wire, and connector C100 to the CTSY fuse and tail fuse and the fuse block.These fuses are “HOT at all TIMES,” since battery voltage is always supplied to them.

Typical Power and Ground Distribution.

Battery voltage is also applied through fusible link C and the RED wire to the ignition switch.The ignition switch supplies voltage in RUN, BULB TEST or START through the PNK wire to the turn fuse and gauge fuse.Therefore, these fuses are “HOT in RUN, BULB TEST, or START.”

Ground DistributionThis is an example of the Ground Distribution.It shows which components share this ground.This information can be a timesaver when troubleshooting ground circuits.

Circuit Faults and Diagnosis

Circuit Faults The various failures that occur in a

circuit will, dictate what must be done to repair the problem. These failures can be categorized as follows.

Open

The open is a physical break in the path of current flow.In a series circuit, the circuit stops operating.In parallel circuits, an open in one branch will stop operation of that branch, but the other branches will continue to operate.The ohmmeter is useful in finding the open with continuity checks.

Short to Ground

The short to ground is where the circuit is grounded due to insulation breakage.The conductor touches a ground, causing the fuse or fusible link to blow.If there is no fuse, the circuit may burn, and even cause flames.If the short occurs after the load, circuit may be lost causing operation when it is not wanted.The test light is a good device in this case.

Short to Ground

Place the test light in place of the fuse.Disconnect circuit components in a systematic and logical manner.When the test light goes out, the part of the circuit with the condition will be found.

Short to Voltage

The short to voltage is a condition where a circuit, due to insulation breakage, causes the conductor to touch the voltage of another circuit.This will cause the circuit to operate improperly.This problem can cause odd things to occur, and can be difficult to find.To Locate this problem, a thorough examination, using steps one and two of “Troubleshooting Philosophy.”

Short to Voltage

Observe the symptoms to recognize associated circuits involved.Isolation by removing fuses will help isolate the circuit branches involved.Then voltage and resistance checks at strategic locations will isolate the problem.

High-Resistance Problems

A high-resistance problem is often the hardest to find.This is a condition where it’s important to use test meters.High resistance could be caused by loose, dirty, or corroded connectors.Current flow is lowered, which can cause dimmed or flickering lamps, or inoperative components.

Troubleshooting Philosophy

Step 1 Check the Malfunction Operate the faulty circuit to be sure to

understand what’s wrong. Don’t waste time fixing part of the

condition! Check for missing, cut or damaged wires,

defective, misaligned or malfunctioning components, ect.

Troubleshooting Philosophy

Step 2 Read the Electrical Diagran Study the diagram to understand how the

circuit should work. Read the detailed description. Start your troubleshooting At a readily

accessible point in the circuit. If it is functional at your “check point,”

continue checking “downstream.” If not, check “upstream.”

Troubleshooting Philosophy

Step 3 Find the Cause and Repair it Use the troubleshooting section for each

circuit problem. Use the wire repair procedures that follow.

Troubleshooting Philosophy

Step 4 Test the Repair Check soldered joint integrity visually and

physically (pull-test), connector pin and socket alignment.

Troubleshooting Equipment

Electrical troubleshooting requires the use of common electrical test equipment.

Test light

Use a test light to check for voltage.A test light is made of a 12-volt light bulb with a pair of leads attached.After grounding one lead, tough the other lead to various points along the circuit where voltage should be present.When the bulb goes ON, there is voltage at the point being tested.

Voltmeter/Digital Multimeter

A voltmeter can be used instead of a test light.While a test light shows whether or not voltage is present, a voltmeter indicates how much voltage is present.Voltages with a 10-megohm or higher impedance should be used.

Test for VoltageConnect one lead of a test light to a known good ground.Connect the other lead of the test light or voltmeter to a selected test point.

Test for VoltageIf the test light glows, there is voltage present.Voltage should be within one volt of battery voltage.

Test for Continuity

Disconnect the car battery.Connect one lead of a self-powered test light or ohmmeter to one end of the part of the circuit.

Test for Continuity

Connect the other lead to the other end of the circuit.If the self-powered test light glows, there is continuity.If using an ohmmeter, low or no resistance means good continuity.

Testing for Voltage Drop With a Voltmeter or DMM

Connect the positive lead of a voltmeter or DMM to the end of the wire that is closer to the battery.Connect the negative lead to the other end of the wire.

Testing for Voltage Drop With a Voltmeter or DMM

Operate the circuit.The voltmeter or DMM will show the difference in voltage between the two points.A difference of more than one volt indicates a problem.

Testing for short to Ground

Remove the blown fuse and disconnect the load.Connect a test light or ammeter/DMM across the fuse terminals.

Testing for short to Ground

Beginning near the fuse block, wiggle the harness from side to side. Continue this at convenient points while watching the test light or ammeter.When the test light glows, or the ammeter registers, there is a short to ground in the wiring near that point.

Self-Powered Test Light or Ohmmeter/DMM

Remove the blown fuse and disconnect the battery and load.Connect one lead of a self-powered test light or ohmmeter/DMM to the fuse terminal on the load-side.

Self-Powered Test Light or Ohmmeter/DMM

Beginning near the fuse block, wiggle the harness from side to side.Connect the other lead to a known good ground.Continue this at convenient points while watching the self-powered test light or ohmmeter.

The End

Circuit Diagnosis

Troubleshooting Philosophy

Step 1. Check the Malfunction

Operate the faulty circuit to understand what’s wrong.Don’t waste time fixing part of the condition!

Step 2. Read the Cause and Repair It

Study the diagram to understand how the circuit work.Read the detailed description.

Step 3. Find the Cause and Repair It

Use the troubleshooting section in the service manual for each circuit problem.