ELS63 - Voltage Back-Feed Faults
Objective
Upon completion of this module, you will be able to:
● Create a voltage back-feed fault using the electrical test board, observe circuit behavior, and measure voltage drops.
● Create a voltage back-feed fault using the electrical test board, and troubleshoot the fault using DMM measurements.
● Troubleshoot a voltage back-feed fault on a vehicle by dividing the circuit using electrical schematics and a DMM.
Why This Module is Important
Dividing the circuit during electrical troubleshooting is the most efficient way to isolate many problems. Using this method will help you break a complex back- feed fault into more manageable pieces and eliminate the sections that do not have a problem. By using this technique to narrow your troubleshooting focus, your productivity and accuracy as a technician will increase.
Module Overview
In this module you will use the information you learned in the cognitive module ELC65 to answer questions and demonstrate skills needed to troubleshoot voltage back-feed problems by dividing a circuit.
Decision Point
If you feel you can demonstrate the skills as outlined in the Module Objectives, see your instructor for on-the-job skills validation. This may require hands-on demonstrations for your instructor.
Otherwise, proceed with the module.
STOP
Getting Started
1. See your instructor for your assigned vehicle.
2. Throughout the module, a box designates the required activities.
3. Gather all of the items listed in the What You Will Need section.
4. Work on one Skill Objective at a time.
5. Fill in ALL blanks as you complete each Skill Objective.
6. Move to the next Skill Objective after you have completed the current Objective.
7. Your DMM measurements and calculations may vary slightly (up to
0.1 volt) due to connection quality, etc. This is normal. ALL measured
readings in the Feedback are approximate values.
ELS63 - Voltage Back-Feed Faults
What You Will Need
● Electrical test board and these sub-mount components (stored inside the test board).
■ Switch - board A (1)
■ Fuse - board AA (1)
■ Turn signal bulb - board B1 (1)
■ Fan Motor - board HF (1)
● DMM and Users Manual. (If you brought your own DMM, check with your instructor before starting this module).
● Service Information for printing schematics and component location images for the on-car troubleshooting exercise.
● Instructor assigned Honda or Acura vehicle.
■ Side marker bulb - board C (2)
■ Hazard Switch - board U (1) (Use when requested only.)
■ Trouble Box - board RR (1) (If not inside the test board,
check inside the other boards.)
ELS63 - Voltage Back-Feed Faults
Skill Objective 1: Create a Circuit with Ground Fault Back-Feed and Measure Voltage Drop
An open in the body ground path can change the nature of a circuit. This open can create a fault called “back-feed”, which can be difficult to troubleshoot.
Back-feed means a circuit has lost its normal path to ground, and has found a new ground path using other circuits and loads. The circuit that has lost it’s path to ground is called the “Source Circuit,” and the circuit providing the new path is called the “Affected Circuit.” This objective provides an example of back-feed, and shows typical voltage drop values associated with the fault.
Using the electrical test board, build the circuit pictured to the right.
■ Use Switch Card A for the left side switch
■ Use Switch Card U for the right side switch
■ Use Light Card C for the left side and center bulbs
■ Use Light Card B1 for the right side bulb
■ Be sure to connect Switch U to the GREEN wire on Card B1
Confirm the circuit operation. Turn both switches ON, and confirm that all bulbs light.
1. Identify the circuit type
Series circuit or Parallel circuit Switch to ground or Load-to-ground 2. With both switches ON, measure the available voltage of the following test points:
Left Bulb Point 1 Point 2
Center Bulb Point 3 Point 4
Right Bulb Point 5 Point 6 Turn OFF both switches after you complete your measurements.
1
B D
C A
6 5 3
2 4
12 Volts
Switch A Switch U
Bulb C Bulb C Bulb B1
Ground 1 Ground 2
Fuse AA A
ELS63 - Voltage Back-Feed Faults
NOTE:
In this example circuit, the following statements apply:
■ The right bulb and center bulb would be part of the same circuit, and turn on at the same time.
■ On the vehicle, the right bulb and center bulb are not physically close and do not share the same ground. Example: Taillights and dash lights are part of the same circuit, and use a common switch, but do not share the same ground.
■ The left bulb and center bulb are in two different circuits, but are physically close, and share the same ground. Example: Taillights and brake lights do not use the same control switch, but are close together and can share the same ground.
Disconnect Ground 1, but be sure the left side bulb and center bulb remain connected between test points 2 and 4.
Turn on Switch U.
Turn on Switch A.
3. Has circuit operation changed?
4. If you answered yes, describe the change:
5. If you answered yes, explain what caused the change:
6. Measure the available voltage at the points listed here:
Left Bulb Point 1 Point 2
Center Bulb Point 3 Point 4
Right Bulb Point 5 Point 6
1
B D
C A
6 5 3
2 4
12 Volts
Switch A Switch U
Bulb C Bulb C Bulb B1
Ground 1 Ground 2
Fuse AA A
ELS63 - Voltage Back-Feed Faults
Turn OFF Switch U (Leave ON Switch A) 7. Has circuit operation changed?
8. With Switch A ON, measure the available voltage at the following test points:
Switch A Point A Point B
Switch U Point C Point D
Left Bulb Point 1 Point 2
Center Bulb Point 3 Point 4
Right Bulb Point 5 Point 6
9. Based on your available voltage measurements, use a highlighter to trace current flow through the schematic on this page.
10. Which switch provides power to the source circuit?
11. Which switch provides power to the affected circuit?
Turn OFF Switch A (Both Switches are OFF)
12. Based on your measurements at points 1 and 2, calculate the voltage drop of bulb 1:
13. Based on your measurements at points 3 and 4, calculate the voltage drop of bulb 2:
14. Based on your measurements at points 5 and 6, calculate the voltage drop of bulb 3:
1
B D
C A
6 5 3
4 2
12 Volts
Switch A Switch U
Bulb 1 Bulb 2 Bulb 3
Ground 1 Ground 2
ELS63 - Voltage Back-Feed Faults
Turn Switch A back ON (Keep Ground 1 disconnected)
15. Measure the voltage drop of each bulb, and confirm your calculations:
Left Bulb: Center Bulb: Right Bulb:
16. Do all three bulbs drop an equal amount of voltage?
17. If you answered no, explain why:
18. If you answered yes, explain what caused the change:
Turn OFF both switches (keep ground 1 disconnected).
Set your DMM to measure resistance, and turn on the audible alert.
Measure continuity between Test Point 2 and Ground 2.
19. Did the Continuity Alert Sound?
20. Would this indicate that there is continuity (Path to Ground)?
21. What is the resistance reading?
22. Does the continuity test provide you with useful data?
23. Would you use a continuity test for this type of fault?
Teaching Point
Continuity tests establish a path to ground, but do not indicate if the path can carry enough current to operate circuit components. In contrast, a voltage drop test shows that a portion of the voltage drops across the load, indicating there is a resistance in the circuit after the load.
1
B D
C A
6 5 3
4 2
12 Volts
Switch A Switch U
Bulb C Bulb C Bulb B1
Ground 1 Ground 2
Fuse AA A
ELS63 - Voltage Back-Feed Faults
Skill Objective 2 Troubleshooting Voltage Back-feed
An open in the body ground path can change the nature of a circuit. This open can create a fault called “back-feed” which can be difficult to understand.
Back-feed occurs when a circuit finds a new path to ground using other circuits and loads. This objective provides an example of back-feed, and shows typical voltage drop values associated with the fault.
Using the electrical test board, build the circuit pictured to the right.
● Use Switch Card A for the left side switch
● Use Switch Card U for the right side switch
● Use Light Card C for both bulbs
● Use Fan Card HF for Fan HF
Confirm the circuit operation - Turn both switches ON, confirm that both bulbs are lit and the fan spins.
1. With both switches ON, measure the available voltage at the following test points:
FAN HF Point 1 Point 2
Left Bulb Point 3 Point 4
Right Bulb Point 5 Point 6
Turn both switches OFF after you complete your measurements.
Disconnect Ground 1, but be sure test points 2 and 4 remain connected.
Turn on Switch U.
1
B D
A C
6 5 3
4 2
12 Volts
Switch A Switch U
Bulb C Bulb C
Ground 1 Ground 2 Fuse AA A
Fan HF
ELS63 - Voltage Back-Feed Faults
Turn on Switch A
2. Has circuit operation changed?
3. With the switches ON, measure the available voltage at the following test points:
FAN HF Point 1 Point 2
Left Bulb Point 3 Point 4
Right Bulb Point 5 Point 6
FAN HF Point 1 Point 2
Left Bulb Point 3 Point 4
Right Bulb Point 5 Point 6 Turn OFF Switch U (Leave ON Switch A)
4. With the switch A ON, measure the available voltage at the following test points:
Switch A (ON) Point A
Point B
Switch U (OFF) Point C
Point D
5. Based on your available voltage measurements, use a highlighter to trace current flow through the schematic on this page.
6. Which switch provides power to the source circuit?
7. Which switch provides power to the affected circuit?
1
B D
A C
6 5 3
4 2
12 Volts
Switch A Switch U
Bulb C Bulb C
Ground 1 Ground 2 Fuse AA A
Fan HF
ELS63 - Voltage Back-Feed Faults
Using the measurements made in step 4, answer the following questions.
8. Based on your measurements at points 1 and 2, calculate the voltage drop for FAN HF:
9. Based on your measurements at points 3 and 4, calculate the voltage drop for left bulb:
10. Based on your measurements at points 5 and 6, calculate the voltage drop for right bulb:
11. Measure the voltage drop of each component, and record the readings below:
Fan HF: Left Bulb: Right Bulb:
12. Do your measurements match your calculations?
13. Do all three loads drop an equal amount of voltage?
13a. If you answered no, explain why:
14. Based on your voltage drop results, what can you say about the bulb resistances?
12 Volts
Ground 1 Ground 2 Switch A Switch U
Fan HF Bulb 2 Bulb 3
ELS63 - Voltage Back-Feed Faults
Skill Objective 3: Divide a Ground Path On a Circuit With a Back-Feed Fault
Identify your assigned Training Center Vehicle and confirm the consumer complaint with your instructor.
Note your assigned vehicle here:
Customer Complaint:
Record your customer complaint here (provided by your instructor)
The customer complaint references two circuits, (Example- Brake light circuit and tail light circuit).
Verify the complaint and record your observations below.
1. Which circuit causes unwanted activity (when turned on)?
2. Which circuit contains the unwanted activity?
Teaching Point: Back-Feed problems involve two circuits:
1) We call the circuit that powers the unwanted activity the “Source Circuit”
2) We call the circuit that contains the unwanted activity the “Affected Circuit”
Teaching Point:
Affected circuits MAY operate normally with the source circuit turned off (because the fault is on the source circuit side).
When troubleshooting a Back-Feed problem, focus your troubleshooting on the source circuit.
Turn on the affected circuit (Do NOT turn on the source circuit).
3. Does it behave normally?
ELS63 - Voltage Back-Feed Faults
Use Service Information to find and print the source and affected circuit schematics.
4. Highlight the current path for each circuit using an orange highlighter to indicate source voltage (fused or switched voltage), a yellow highlighter to circle the loads, and a green highlighter for ground paths.
Do the circuit schematics show components between the source and affected circuit that share a ground?
5. If you answered “Yes,” list the components here:
Power up the source circuit.
6. Measure the available voltage at the points identified in Question 5:
Voltage available before load 1: Voltage available after load 1:
Voltage available before load 2 (if present): Voltage available after load 2 (if present):
Voltage available before load 3 (if present): Voltage available after load 3 (if present):
7. Based on these measurements, what fault is present?
8. What is your next troubleshooting step?
9. Where will you take your next measurement?
10. Continue troubleshooting. Where is the fault?
ELS63 - Voltage Back-Feed Faults
Instructor Evaluation
Before having your instructor sign your module, be prepared to:
■ Show your instructor how to diagnose a voltage back-feed problem by using a DMM and dividing the circuit.
■ Review your completed module worksheet with your instructor.
Evaluation
If you feel you are ready to demonstrate the skills in this module and receive credit, see your instructor.
Instructor’s initials: _____________
STOP
