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Section 11

ECM Diagnosis

Learning Objectives:

1.Diagnose ECM specific Diagnostic Trouble Codes.

2 Troubleshooting the diagnostic circuit.

3.Reprogramming the ECU.

Engine Control Systems II - Course 874

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Section 11

ECM Diagnosis

Overview

The ECM is rarely the cause of a problem. If the ECM needed to be

replaced, it was often because of a change in calibration. Now, ECMs

can be reprogrammed and replacement under these conditions is no

longer necessary.

Still, nearly all troubleshooting procedures involve checking all systems

and subsystems connected to the ECM. Additionally, with multiplexing,

there are more ECUs connected to the ECM that need to be checked.

Presently, there are some DTCs that are directly associated with the

ECM.

P0605: The Internal Control Module Read Only Memory Error, is

displayed when the ECM has detected a problem in the read only

memory area. At the time of this publication, when this DTC is

displayed the ECM must be replaced.

P1600: ECM BATT malfunction is stored when there is no power to the

BATT terminal of the ECM. Battery voltage is supplied to the BATT

terminal even when the ignition switch is OFF. The ECM uses this

power for memory and adaptive memory values such as A/F Ratio

control. If DTC P1600 is displayed, the ECM does not store another

DTC. The BATT circuit must be checked and confirmed normal before

replacing the ECM.

P1633: ECM Malfunction (ETCS Circuit), is stored when the ECM

detects an internal problem with the electronic throttle control circuit.

At the time of this publication, when this DTC is displayed the ECM

must be replaced.

Trouble-

shooting the

Diagnostic

Circuit

On rare occasions, you will find that the Malfunction Indicator Lamp or

the Diagnostic Tester fails to operate properly. The following information

is designed to help you determine a diagnostic course of action when

you encounter this situation.

Malfunction

Indicator Lamp

Check

Before connecting your Diagnostic Tester it is important to confirm that

the Malfunction Indicator Lamp (MIL) is functioning normally. A bulb

check of the MIL is performed when the ignition is switched ON. If the

MIL does not illuminate, it indicates that a problem exists in the MIL

sub-system. This condition must be corrected before any further

diagnostic work can be performed. See the diagnostic procedures for

inoperative MIL in the repair manual.

11-1

Engine Control Systems II - Course 874

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Section 11

Once the engine is started the MIL should turn off. If the MIL remains

on after the engine is running, the diagnostic system has detected a

problem in the engine control system.

Failure of

Diagnostic Tester

to Establish

Communications

with ECM

After MIL operation is confirmed, connect the Diagnostic Tester to the

appropriate DLC and program for the on-board diagnostic system

supported by the vehicle. Once the tester is programmed for the correct

vehicle and diagnostic system, you should be able to display an ALL

DATA LIST.

If any of the following messages are received you must correct this

condition before proceeding with troubleshooting:

• NO COMMUNICATION WITH VEHICLE

• UNABLE TO CONNECT TO VEHICLE

• OBD II COMMUNICATIONS TIME OUT

There are several possibilities when the Diagnostic Tester fails to

communicate with the vehicle. The problem could simply be the way

you programmed the tester, or connecting to the wrong DLC. Once you

have confirmed correct programming and proper lead connection, you

will need to establish whether the problem is in the tester or the

diagnostic circuit

To isolate a tester problem from a vehicle problem, simply try the tester

on another vehicle. If the tester communicates normally with another

vehicle, it is probably O.K. and the vehicle diagnostic system must be

inspected.

11-2

TOYOTA Technical Training

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ECM Diagnosis

DLC3 (OBD II) Connector

The vehicle’s ECM uses the ISO 9141-2

communication protocol. The terminal

arrangement of DLC3 complies with SAE

J1962 and matches the ISO 9141-2

format.

DLC3

Terminal No.

Connection/Voltage or Resistance

Condition

7

Bus Line/Pulse generation

During Transmission

4

Chassis Ground

Body Ground/1

or less

Always

5

Signal Ground

Body Ground/1

or less

Always

16

Battery Positive

Body Ground/9 – 14V

Always

Fig. 11-1

TL874f1101

11-3

Engine Control Systems II - Course 874

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Section 11

DLC3 Circuit

Fig. 11-2

TL874f1102

Inspection of

the OBD II

Diagnostic

Circuit

The OBD II diagnostic circuit is shown in the schematic. DTC functions,

serial data, active test functions, and all OBD II functions are

communicated across the SDL (Serial Data Link) circuit. The Diagnostic

Tester initiates communication by sending a data request signal to the

ECM on the SDL circuit. The ECM will respond by sending a Variable

Pulse Width signal back to the tester.

11-4

TOYOTA Technical Training

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