How to Proceed With Troubleshooting

A large number of ECU controlled systems are used in the LEXUS SC4OO, SC3OO. In general, the ECU controlled system is considered to be a very intricate system requiring a high level of technical knowledge and expert skill to troubleshoot. However, the fact is that if you proceed to inspect the circuits one by one, troubleshooting of these systems is not complex. If you have adequate understanding of the system and a basic knowledge of electricity, accurate diagnosis and necessary repair can be performed to locate and fix the problem.





HOW TO PROCEED WITH TROUBLESHOOTING:
Carry out troubleshooting in accordance with the procedure outlined below. In the image above, only the basic procedure is shown. Details are provided in each section, showing the most effective methods for each circuit. Confirm the troubleshooting procedures first for the relevant circuit before beginning troubleshooting of that circuit.

[1]

Ask the customer about the conditions and the environment when the problem occurred.

[2] [3]

Confirm the symptoms and the problem conditions, and check the diagnostic trouble codes. (When the problem symptoms do not appear during confirmation, use the symptom simulation method described later on.)

[4] [5] [6]

Check the results obtained in Step 2 then confirm the inspection procedure for the system or the part which should be checked using the diagnostic trouble code chart or the matrix chart of problem symptoms.

[7]

Check and repair the affected system or art in accordance with the instructions in Step 6

[8]

After completing repairs, confirm that the problem has been eliminated. (If the problem is not reproduced, perform the confirmation test under the same conditions and in the same environment as when it occurred for the first time.)

[1] CUSTOMER PROBLEM ANALYSIS:
In troubleshooting, the problem Symptoms must be confirmed accurately and all preconceptions must be cleared away in order to give an accurate judgment. To ascertain just what the problem symptoms are, it is extremely important to ask the customer about the problem and the conditions at the time it occurred.





Important Points in the Problem Analysis:
The above 5 items are important points in the problem analysis. Past problems which are thought to be unrelated and the repair history, etc. may also help in some cases, so as much information as possible should be gathered and its relationship with the problem symptoms should be correctly ascertained for reference in troubleshooting.





(Sample) Engine control system check sheet:

[2] SYMPTOM CONFIRMATION AND DIAGNOSTIC TROUBLE CODE CHECK:
The diagnostic system in the LEXUS SC4OO, SC3OO fulfills various functions. The first function is the Diagnostic Trouble Code Check in which a malfunction in the signal circuits to the ECU is stored in code in the ECU memory at the time of occurrence, to be output by the technician during troubleshooting. Another function is the Input Signal Check which checks if the signals from various switches are sent to the ECU correctly.
The air conditioning system has an Actuator Check function whereby the ECU automatically operates the actuators of the damper and blower motor, etc. to check the operation. By using these check functions, the problem areas can be narrowed down quickly and troubleshooting can be performed effectively. Diagnostic functions are incorporated in the following systems in the LEXUS SC4OO, SC3OO.





In diagnostic trouble code check, it is very important to determine whether the problem indicated by the diagnostic trouble code is still occurring or occurred in the past but returned to normal at present. In addition, it must be checked in the problem symptom check whether the malfunction indicated by the diagnostic trouble code is directly related to the problem symptom or not. For this reason, the diagnostic trouble codes should be checked before and after the symptom confirmation to determine the current conditions, as shown in the table below. If this is not done, it may, depending on the case, result in unnecessary troubleshooting for normally operating systems, thus making it more difficult to locate the problem, or in repairs not pertinent to the problem. Therefore, always follow the procedure in correct order and perform the diagnostic trouble code check.





DIAGNOSTIC TROUBLE CODE CHECK PROCEDURE:
Taking into account the above points, a flow chart showing how to proceed with troubleshooting using the diagnostic trouble code check is shown below.





This flow chart shows how to utilize the diagnostic trouble code check effectively, then by carefully checking the results, indicates how to proceed either to diagnostic trouble code troubleshooting or to troubleshooting of problem symptoms.

[3] SYMPTOM SIMULATION:
The most difficult case in troubleshooting is when there are no problem symptoms occurring. In such cases, a thorough customer problem analysis must be carried out, then simulate the same or similar conditions and environment in which the problem occurred in the customer's vehicle. No matter how much experience a technician has, or how skilled he may be, if he proceeds to troubleshoot without confirming the problem symptoms he will tend to overlook something important in the repair operation and make a wrong guess somewhere, which will only lead to a standstill. For example, for a problem which only occurs when the engine is cold, or for a problem which occurs due to vibration caused by the road during driving, etc., the problem can never be determined so long as the symptoms are confirmed with the engine hot condition or the vehicle at a standstill. Since vibration, heat or water penetration (moisture) are likely causes for problems which are difficult to reproduce, the symptom simulation tests introduced here are effective measures in that the external causes are applied to the vehicle in a stopped condition.














Important Points in the Symptom Simulation Test:
In the symptom simulation test, the problem symptoms should of course be confirmed, but the problem area or parts must also be found out. To do this, narrow down the possible problem circuits according to the symptoms before starting this test and connect a tester beforehand. After that, carry out the symptom simulation test, judging whether the circuit being tested is defective or normal and also confirming the problem symptoms at the same time. Refer to the matrix chart of problem symptoms for each system to narrow down the possible causes of the symptom.








[4] DIAGNOSTIC TROUBLE CODE CHART:
The inspection procedure is shown in the table above. This table permits efficient and accurate troubleshooting using the diagnostic trouble codes displayed in the diagnostic trouble code check. Proceed with troubleshooting in accordance with the inspection procedure given in the diagnostic chart corresponding to the diagnostic trouble codes displayed. The engine diagnostic trouble code chart is shown below as an example.





[5] MATRIX CHART OF PROBLEM SYMPTOMS:
The suspect circuits or parts for each problem symptom are shown in the table above. Use this table to troubleshoot the problem when a "Normal" code is displayed in the diagnostic trouble code check but the problem is still occurring. Numbers in the table indicate the inspection order in which the circuits or parts should be checked.

HINT: When the problem is not detected by the diagnostic system even though the problem symptom is present, it is considered that the problem is occurring outside the detection range of the diagnostic system, or that the problem is occurring in a system other than the diagnostic system.

[6] CIRCUIT INSPECTION:
- Refer to the appropriate diagnostic trouble code.








Location of Connectors in Engine Compartment:














HOW TO USE THE DIAGNOSTIC CHART AND INSPECTION PROCEDURE:
1. For troubleshooting, diagnostic trouble code charts or problem symptom charts are provided for each circuit with detailed inspection procedures on the following pages.
2. When all the component parts, wire harnesses and connectors of each circuit except the ECU are found to be normal in troubleshooting, then it is determined that the problem is in the ECU. Accordingly, if diagnosis is performed without the problem symptoms occurring, the instruction will be to check and replace the ECU, even if the problem is not in the ECU. So, always confirm that the problem symptoms are occurring, or proceed with inspection while using the symptom simulation method.
3. The instructions "Check wire harness and connector" and "Check and replace ECU" which appear in the inspection procedure, are common and applicable to all diagnostic trouble codes. Follow the procedure outlined below whenever these instructions appear.

Check Wire Harness and Connector:
The problem in the wire harness or connector is an open circuit or a short circuit.

OPEN CIRCUIT:
This could be due to a disconnected wire harness, faulty contact in the connector, a connector terminal pulled out, etc.

HINT:
1. It is rarely the case that a wire is broken in the middle of it. Most cases occur at the connector. In particular, carefully check the connectors of sensors and actuators.
2. Faulty contact could be due to rusting of the connector terminals, to foreign materials entering terminals or a drop in the contact pressure between the male and female terminals of the connector. Simply disconnecting and reconnecting the connectors once changes the condition of the connection and may result in a return to normal operation. Therefore, in troubleshooting, if no abnormality is found in the wire harness and connector check, but the problem disappears after the check, then the cause is considered to be in the wire harness or connectors.

SHORT CIRCUIT:
This could be due to a short circuit between the wire harness and the body ground or to a short inside the switch, etc.

HINT:
- When there is a short between the wire harness and body ground, check thoroughly whether the wire harness is caught in the body or is clamped properly.

















1. CONTINUITY CHECK (OPEN CIRCUIT CHECK):
- Disconnect the connectors at both ECU and sensor sides.
- Measure the resistance between the applicable terminals of the connectors.

Resistance: 1 Ohm orless

HINT:
- Measure the resistance while lightly shaking the wire harness vertically and horizontally.
- When tester probes are inserted into a connector, insert the probes from the back. For waterproof connectors in which the probes cannot be inserted from the back, be careful not to bend the terminals when inserting the tester probes.

2. RESISTANCE CHECK (SHORT CIRCUIT CHECK):
- Disconnect the connectors at both ends.
- Measure the resistance between the applicable terminals of the connectors and body ground. Be sure to carry out this check on the connectors on both ends.

Resistance: 1 m Ohm or higher

HINT: Measure the resistance while lightly shaking the wire harness vertically and horizontally.

3. VISUAL CHECK AND CONTACT PRESSURE CHECK:
- Disconnect the connectors at both ends.
- Check for rust or foreign material, etc. on the terminals of the connectors.
- Check crimped portions for looseness or damage and check if the terminals are secured in the lock position.

HINT: The terminals should not come out when pulled lightly.

- Prepare a test male terminal and insert it in the female terminal, then pull it out.

HINT: When the test terminal is pulled out more easily than others, there may be poor contact in that section.











Actual examples of the inspection method for open circuit and short circuit are explained below.
1. OPEN CIRCUIT CHECK:
- For the open circuit in the wire harness in Fig. 1, perform "(a) Continuity Check" or '(b) Voltage Check" to locate the section.
- Continuity Check:
- Disconnect connectors A and C and measure the resistance between them.
In the case of Fig. 2,
Between terminal 1 of connector A and terminal 1 of connector C -> No continuity (open).
Between terminal 2 of connector A and terminal 2 of connector C -> Continuity.
Therefore, it is found out that there is an open circuit between terminal 1 of connector A and terminal 1 of connector (C).
- Disconnect connector B and measure the resistance between connectors A and B, B and C.
In the case of Fig. 3,
Between terminal 1 of connector A and terminal 1 of connector B -> Continuity.
Between terminal 1 of connector B and terminal 1 of connector C -> No Continuity (open).
Therefore, it is found out that there is an open circuit between terminal 1 of connector B and terminal 1 of connector C.








- Voltage Check:
In a circuit in which voltage is applied (to the ECU connector terminal), an open circuit can be checked for by conducting a voltage check.
- As shown in Fig. 4, with each connector still connected, measure the voltage between body ground and terminal 1 of connector A at the ECU 5V output terminal, terminal 1 of connector B, and terminal 1 of connector C, in that order.
If the results are:
5 V: Between Terminal 1 of connector A and Body Ground.
5 V: Between Terminal 1 of connector B and Body Ground.
O V: Between Terminal 1 of connector C and Body Ground.
Then it is found out that there is an open circuit in the wire harness between terminal 1 of B and terminal 1 of C.
2. SHORT CIRCUIT CHECK
If the wire harness is ground shorted as in Fig. 5, locate the section by conducting a "continuity check with ground".








- Continuity Check with Ground:
- Disconnect connectors A and C and measure the resistance between terminals 1 and 2 of connector and body ground.
In the case of Fig. 6,
Between terminal 1 of connector A and body ground -> Continuity.
Between terminal 2 of connector A and body ground -> No continuity (open).
Therefore, it is found out that there is a short circuit between terminal 1 of connector A and terminal 1 of connector C.
- Disconnect connector B and measure the resistance between terminal 1 of connector A and body ground, and terminal 1 of connector B and body ground.
Between terminal 1 of connector A and body ground -> No continuity (open).
Between terminal 1 of connector B and body ground -> Continuity.
Therefore, it is found out that there is a short circuit between terminal 1 of connector B and terminal 1 of connector C.








Check and Replace ECU:
First check the ECU ground circuit. If it is faulty, repair it. If it is normal, the ECU could be faulty, so replace the ECU with a known good one and check if the symptoms appear.
- Measure the resistance between the ECU ground terminal and the body ground.

Resistance: 1 Ohm or less

- Disconnect the ECU connector, check the ground terminals on the ECU side and the wire harness side for bend and check the contact pressure.