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Diagnosing Electrical Problems

Diagnosing Electrical Problems. Basic Procedures. Wiring Diagrams. The first step to locate an electrical problem is to understand the circuit. This includes: Does it have a fuse? Does it include a relay? Where is the ground connection? Where are the electrical connectors?

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Diagnosing Electrical Problems

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  1. Diagnosing Electrical Problems Basic Procedures

  2. Wiring Diagrams • The first step to locate an electrical problem is to understand the circuit. This includes: • Does it have a fuse? • Does it include a relay? • Where is the ground connection? • Where are the electrical connectors? • Where are the switches/relays? • Is it a simple series or a parallel circuit?

  3. Fuse Switch Connector Ground Connection An older transmission wiring diagram. Modern transmissions are more complex.

  4. Circuit Requirements • All circuits must have power. Most automotive circuits run on 12 volts. This is really about 12.5-13V (engine off) or 13.6-15.6V (engine running). • To operate, a circuit must be complete from the power source to ground. • The circuit must have the proper resistance so it will have the correct current (amperage) flow.

  5. Battery Quick Checks With the engine off, battery voltage must be at least 12.6 V. A lower reading indicates a discharged or failing battery. This battery checks good, 12.87 V.

  6. Battery Charge 12.66 V indicates a fully charged battery. 12.45 V indicates a charge level of about 75%. The 12.87 V in Slide 5 tells us the engine was running recently, and the alternator has boosted the voltage.

  7. Battery Load Procedure 1. Depress throttle to floor. 2. Crank engine for 30 seconds. 3. Read voltmeter while engine is cranking. The battery must be able to maintain at least 9.5 - 10 V under a cranking load for 30 seconds. Hold the throttle wide open to prevent starting. Lower than 9.5 V indicates a battery that is failing or undercharged. Note: If the engine starts, shut it off immediately, and disable the ignition or fuel injection to complete the test.

  8. Charging System When the engine is running, a good charging system will increase battery voltage about 0.5 to 2 V above the quick check, engine off, reading. Procedure 1. Start engine. 2. Increase engine speed to about 2,000 rpm. 3. Read voltmeter.

  9. Charging System Load A good alternator will power the headlights and A/C at idle speed. Battery voltage should remain the same, about 0.5 to 2 V above the static voltage measured in the battery quick check. Procedure 1. Start engine. 2. Allow engine to idle. 3. Turn on headlights and A/C. 4. Read voltmeter.

  10. The engine is running with the headlights and A/C on, and the battery voltage is 14.51 V. With the engine off, battery voltage was 12.87 V. 14.51 – 12.87 = 1.64 The battery and charging system check good.

  11. Circuit Source Voltage The source or available voltage usually can be easily measured at the fuse for that circuit. Attach the – (negative) voltmeter lead to a good, clean ground and touch the + (positive) lead to both of the bare portions of the fuse. Voltage at both sides indicates a good fuse.

  12. Current Flow A modified fuse provides an easy way to break into a circuit to measure the amperage. It is installed in place of the circuit fuse, and one lead of the ammeter is connected to each of the connections.

  13. Modified Fuse Old Fuse A pair of wires have been soldered onto a burned-out fuse to make connection with test meters.

  14. Resistance of Load If we know the voltage and current/amperage, we can determine the resistance by dividing V by C. The amount of resistance in a circuit should match the resistance specification for the major component. The example shows excessive circuit resistance, possibly caused by poor connections. Source Voltage: 14.5 V Current Flow: 0.23 A Computed Resistance: 14.5 ÷ .23 = 63  Specified Resistance: 20-40 

  15. Voltage Drop A faulty, high resistance connection causes a voltage drop during current flow. Voltage drops reduce current flow through a circuit. Unwanted voltage drops should not exceed 0.5 V. This circuit has a problem; meter B shows a 1 V drop.

  16. Voltage Drop Checks A voltmeter measures the voltage difference between two points. These points can be selected by you to check portions of a circuit. This meter shows an excessive voltage drop across the switch.

  17. Excess Current Flow Good Blown A burned fuse indicates that the current flow was too high. An excessive current flow indicates a shorted or partially grounded circuit.

  18. Resistance Check 5 Ohms Ohmmeter Solenoid An internal short reduces the resistance. This can be checked using an ohmmeter. If the resistance of the solenoid is less than the specifications (10-15 Ohms), the solenoid is shorted and should be replaced.

  19. Resistance Check 25 Ohms Ohmmeter Solenoid An open or high resistance increases the resistance. This can be checked using an ohmmeter. If the resistance of the solenoid is more than the specifications (10-15 Ohms), the solenoid has high resistance and should be replaced.

  20. Solenoid Quick Check - + Battery Solenoid Make sure you use the correct polarity and momentarily touch the jumper wires to the solenoid terminals. The solenoid plunger should quickly move and should “click”. If not, the solenoid is bad. This check is not recommended for PWM solenoids; they can burn out during your check.

  21. DTCs If certain portions of the circuit have voltages that are out-of-value, during self-diagnosis the control module will sense a problem. It will then turn on the malfunction indicator light, MIL and set a diagnostic trouble code, DTC. This code can be read using a scan tool or other method as determined by the vehicle manufacturer. If the MIL lights with the vehicle in operation, the problem can indicate either an electrical malfunction or improper operation. Too shift that takes too long or excessive sliggage can do this.

  22. Scan Tool Data Link Connector Scan Tool A scan tool connected to the data link connector (DLC) allows a technician to read transmission data, including DTCs, and check the operation of transmission systems.

  23. MIL On this vehicle, the OD cancel lamp (OD OFF) also serves as the malfunction indicator lamp (MIL). A transmission problem is indicated if this light is flashing. OD Cancel Lamp

  24. OBDII MIL Vehicles with On Board Diagnostics 2nd generation, OBDII, have powertrain DTCs. This vehicle uses the Check Engine Light for the MIL for both engine and transmission problems. Check Engine Light

  25. OBDII DTC Power train P07XX or P08XX Transmission A transmission DTC will appear as an alpha-numeric, 5 digit code beginning with the letter P. The P will be followed by a 0 or 1 and then by either a 7 or 8. The final two digits are the key to what the problem is. A listing is in the text, Appendix D.

  26. Using a DTC A DTC like P0722, Output Speed Sensor Circuit No Signal, indicates an electrical problem. There’s probably an open/break in that particular circuit. Other codes like P0731, Gear 1 Incorrect Ratio, indicate a mechanical problem. This can be caused by a slipping clutch or band or the wrong transmission for the vehicle.

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