Automotive Engine Electronics

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b How to dLgnose and repair the automotive Compufer Control System.. t



Automotive Engine ElectronicsHow to Diagnose and Repair the Automotive Computer Control System

by Dr. Robert C. McElroy

Second Revised Edition

Accuracy Publishing, Homestead, Florida

photo by Rick Bernskoetter

Bob McElroy at the wheel of his Hilborn Fuel Injected Corvette at an SCCA autocross.Note video camera attached to windshield.

Published by: Accuracy Publishing Co. Post Office Box 514 Homestead, FL 33035-0514 Corvette Cover Artwork Compliments of: Micrografx Corporation

1511IBob McElroy Library of Congress Cataloging in Publication Data McElroy, Robert C. Automotive Engine Electronics Understand, Diagnose & Repair: Fuel, Ignition & Computer Control Systems 1. Automotive Electronics--Computers, sensors, etc. 2. Electricity and Electronics--Computers, sensors, etc 3. Diagnosis--Computers, sensors, etc. 4. Mathematics--Computers, sensors, etc I. Title ISBN 0-929603-37-0 S o f t c o v e r

Alrights reserved. No part of this book may be used or relproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system without prior written permission from the author except in the case of brief quotations embodied in critical reviews and articles. Copyright 1987,1988 by Robert C. McElroy, Ph.D. First Printing 1987 Second Printing 1988, revised Third Printing 1988, revised Printed in the United States of America

SECTION I -- Sensors & Systems

SECTION II -- Driveability Diagnosis


PAGE3 4 5 6 8 9 11 16 18 20 21 22 24 25 26 29 31 33 35 38 40 43 44 46 47 48 49 50 53 54 55



Table of Contents Objective Foreward Introduction About the Author Background of Today's Automobile Electrical & Electronic Terms OHM'S Law Series Circuits Parallel Circuits Series Parallel Circuits Diodes Despikeing and Clamping Diodes Transistors Point Style Ignition Systems HE1 -- High Energy Ignition EST -- Electronic Spark Timing Distributorless Ignition Systems -- CI CCC -- Computer Command Control EFI -- Electronic Fuel Injection PFI -- Port Fuel Injection Transition -- fuel to electronic systems Throttle Position Sensor -- TPS Temperature Sensors -- CTS & MAT Manifold Absolute Pressure -- MAP Electronic Spark Control -- ESC Mass Air Flow Sensor -- MAF Eshaust Gas Recirculation -- EGR Idle Air Control -- IAC Vehicle Speed Sensor -- VSS Evaporative Emission Control -- EEC

Driveability Diagnosis Philosophy Systems Approach to Diagnosis Electronic Control Module -- ECM ALCL Terminal Circuits Scanner Use Integrator & Block Learn Scanner Error Diagnostic Procedures Digital Multimeter -- DMM Jumper Wires & Connectors Computer Harness Adaptive Tester Diagnostics & ECM Voltage Checks Continuity Tests CHAT Basic Test Procedures CHAT Advanced Testing Procedures Advanced Circuit Testing ECM Codes Open & Closed Loop Fuel Injector Balance Test


Which can be read with CHAT.Note: CHAT will interface BCM and GMP4 design ECMs. BCM and GMP4 circuits have not been included in this publication. CCC and EFI systems incorporating "edgeboard"connectors were not included.

CHAT is covered by U.S. Patent No. 4,690,475 and is available from Diagnostic Products Co. P.O. Box 1136, Homestead, Fl. 33090Diagnostic Products the Electronics Subsidiary of Spectra Investments Ltd. Inc.

Objectives of this manual:Provide a comprehensive general explanation for electronically 1. spark controlled and fuel injected engines manufactured by GM. 2. Provide specific information for the use of ALCL "scan" tools. Provide specific information for the use of "CHAT' the Computer Harness Adaptive Tester. Provide specific information for the use of CHAT to perform voltage and continuity testing for all wires and circuits interfacing the ECM.


This manual has been prepared for:1.2.

Professional Technicians in the field of automotive repair. Students of Automotive Technology. Automotive Enthusiasts who wish to know more about contemporary automotive engine electronics.


This manual has been prepared with a "systems" approach:1.2.

Diagnosis and repair must be performed in a step by step method, to ensure that you "isolate" the problem, and not simply replace what seems to be wrong. Automotive systems are broken down so that you can see how they are intended to work in a conceptual method. When you understand the general concept you will be able to apply your understanding to systems and components which differ from those included in this manual.

This manual is not intended to replace applicable service manuals pertaining to the service of any automobile. This manual is intended to complement service manuals specificallyprepared by the vehicle manufacturer.

In order to understand the complex nature of today's automobile we must be able to speak and understand the language used to describe these vehicles. You may be reluctant to want to learn anything, but you have made it this far by purchasing this book. A book which you can use and learn new technology from. A book written to help you each and every day on the job. Actually, when you start to dig into this material it will be fun and interesting. The trick is to just take your time and master these basic terms and concepts. One thing that you really need to do is understand how these things work, at least on a simple level. This book will approach everything on a simple level and use this basic principle of understanding to build upon. Some of the things which we will look at in the beginning will seem too simple to really work with. However, I have seen too many repairs done incorrectly. When you really analyze what you are doing the bottom line really must be to get the car fixed. Hopefully, this will happen the first time or else you will have a recheck to perform. Rechecks mean that you will not be making money on the next car waiting for you. Therefore, the fewer rechecks the more money you ought to make. That seems simple enough, doesn't it?

Always try and do the job right the first time even if it takes a few extra minutes. One good example would be a water pump. If it leaks then you will probably have to do the whole job over and I don't know of anyone who wants to redo a water pump. The same sort of thinking applies to this new technology. If the car still has a driveability problem then it is going to be a comeback and rechecks eat your earnings.


This book has been prepared with the objective of presenting basic concepts which apply to the modern automobile which uses a small computer or ECM to control fuel delivery to the engine. In order to understand how this system functions, you will be presented the basic concepts relating to how air and fuel management is accomplished. If you know and understand the relationships between the sensor inputs and computer outputs you will be able to diagnose problems which occur with the modern automobile. No one book can be expected to provide all information which can apply to all situations. However, the information provided will be of value when you must diagnose and repair these vehicles.

Your use of a definite method of analysis will increase your productivity and reduce the number of comebacks and your number of rechecks. This increase in productivity will make you a better technician which should also correspond to an increase in your paycheck. In order that you understand how this system works we will first investigate the ignition system. Understanding of today's ignition systems is 100%essential. Proper fuel delivery to the engine is based upon this input signal and without it the vehicle will not run due to loss of both spark and fuel delivery.

As you gain experience with computer equipped cars and trucks you will find that they do perform and react in a predictable way. There are certain things which you must know and understand. Some things must work or the vehicle simply will not run. These facts will be covered so that you will be able to NO WRENCHES ON quickly and accurately determine if THE RED TERMINAL these necessary inputs are present. Use of a definite strategy will mean that your diagnosis is done quickly and in a logical Always remove the ground cable first. If you procedure.

accidentally hit some sheet metal you will not risk blowing up the battery.

I believe that many technicians do not have a good understanding of how ignition systems operate. Therefore, we will study a conventional point-type ignition first. If you know and understand how this system works you will not have any difficulty with electronic systems. More cars are being designed each year without distributors. As you progress through your study of this book, you will clearly come to understand why a distributor is not needed. As you learn how these systems operate and how other inputs can provide the same necessary signal you will be able to figure out and troubleshoot systems which you have not personally worked on yet.As you study this manual and compare its information to the cars which you work with daily you will become a better technician. Additional education at your local technical school or community college will also help you to be more professional in your duties. If you are not already certified by ASE, you are encouraged to participate. ASE certification is a highly desirable goal and those technicians who wear the ASE patch take pride in their accomplishment. You make your living as a technician. Vehicles which you work on daily are highly complex and the m