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Reading Mazda RX7 86-88 Error Codes

FORWARD

This document describes how to read error codes reported by the Emission Control Unit (ECU). It is applicable only to Mazda's RX7 86-88 (both Turbo and N.A.). The procedure for other 2nd generation RX7 models (89-91) is completely different and is described in another document available on the Net. 

The document has been formatted as a long page so it can be easily printed. To navigate between the chapters you may use the links in the table of contents or jump directly to the error code list. 

TABLE OF CONTENTS

• Introduction
• The Diagnostic Connector 
• Monitoring the signals 
• Interpreting the signals 
• Document Maintenance

TABLE OF FIGURES AND TABLES

• Diagnostic Connector location 
• Diagnostic signals table 
• Diagnostic Connector signals 
• Tester Components
• LED based signal monitor 
• The complete tester
• Connecting the tester

INTRODUCTION

The ECU is the 'brain' of the RX7 2nd generation engine. It is located on the right hand side underneath the carpet next to the fire wall and is connected to various sensors and actuators that monitors and controls the operation of the engine and the emission system. Note that the ECU and the CPU (Central Processing Units) are two different and non related components. The ECU controls the functionality of the engine and the emission system while the CPU controls more benign functions such as interior lights and seat belt warning. 

When the ECU detects abnormal signals from a sensor it classifies the condition as one of several predefined 'error states' and handles the situation accordingly. In most error states, the ECU falls back to a predefined behavior that ignores the faulty sensor. Each error state is associated with an error code, a unique number between 1 and 15, that identifies it (in the this document we use the terms 'error states' and 'error code' interchangeably). The error codes are useful, together with the diagnostic flow charts of the Shop Manual, to troubleshoot problems in the engine and the emission system. 

Newer models of RX7 (i.e. 89+) and most other car brands of recent model year, provide a simple procedure of reading the error codes using a Check Engine lamp in the dashboard. The error code reporting is usually activated by short-circuiting two pins in a diagnostic connector and this causes the on-board computer to flash the Check Engine lamp in a sequence that uniquely identifies the error code. 

However, RX7 models 86-88, do not have a built in Check Engine lamp. Instead, the error codes are reported by two electrical signals in the Diagnostic Connector located under the hood near the front-left strut tower. The Shop Manual explains how to read error codes using Mazda's Digital Code Checker (DCC) which is an electronic device available from Mazda. (we know of at least one after-market DCC, available from Mazdatrix ) The DCC is connected to the diagnostic connector and ground, and it displays a two-digit error-code (or zero of no error exists) on a seven-segment LED display. The DCC is possibly the most convenient way of reading the error codes except that its cost ($1500+) makes it a non-practical solution for most RX7 owners. In this document we outline the construction and usage of a simpler DCC that be built in few minutes and costs less than $10. 

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THE DIAGNOSTIC CONNECTOR

The diagnostic connector is located near the left front strut tower (under the hood and between the battery and the left wheel). It is a green connector with stations for 6 pins of which only 4 are used. The following figure shows the diagnostic connector and its neighborhood. 

 

The the four pins of the diagnostic connector are:  

And their positions are as follows: 

 

The DCC1, DCC2, and GL signals are 'open collector' digital outputs of the ECU. The ABR signal is a switched +12V power of the Air Bypass Relay and is used as the +12V supply for the DCC. The DCC1 and DCC2 signals are used to read the error code. The signal GL is used to test/adjust of the Closed Loop System and is not used for the error-code reading procedure described here. The DCC1, DCC2, and GL signals are binary and can be in one of two states: LOW where the voltage is close to 0V and HIGH where the voltage is close to +12V. However, since these outputs are 'open-collector' they require a 'pull-up' to +12V to have high voltage in the HIGH state. This pull-up can be for example a 1K Ohm resistor connected between the signal and the ABR. However, if you are using the LED based monitoring circuit described later, you don't need to connect a pull-up resistor since the LED and its serial resistor provide the pull-up. 

NOTE: The ECU seems to protects its output by limiting to 30 ma the current its DCC1, DCC2 and GL output sinks. It is strongly recommended however not to rely on this fact and to use a load that will not require more than 20-30ma. The ECU is essential for proper operation of the engine and is pretty expensive so you cannot be too careful in protecting it. 

For the purpose of reading the error codes, monitoring the signals means observing the HIGH/LOW states of the DCC1 and DCC2 signals. When the ignition switch is turned on, the two signals are LOW for about 3 seconds and then they change to HIGH (provided a proper pull-up is connected). If an error code exists, the ECU generate bursts of LOW pulses on DCC1 and/or DCC2 that uniquely identify the error code (this is described in details later). The duration of the pulses are 400 ms or higher and therefore they can be monitored using a simple binary indicator and a human eye, assuming that the indicator has fast enough response. To monitor these pulses, we use a simple tester with two LED indicators, one for DCC1 and one for DCC2. 

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MONITORING THE SIGNALS

The error codes signals can be monitored in various ways, including an analog voltmeter, or an oscilloscope. In this section we describe a simple way of monitoring the signals that provides a good balance between simplicity, convenience, and low price. This is done using tester that is based on two LED's (Light Emitting Diodes). A regular (or 'raw') LED  is a solid state device that has electrical characteristics of a diode and emits light when current flows through it. LED's comes in many colors including red, green, yellow, and even infra-red (invisible). The red LED's are the most bright ones and very common and therefor we recommend to use red LED assemblies. To be used in most application, a serial resistor is required to limit the current through the LED. (Note: the outputs of the ECU seem to have a current limiting of 30 ma but we don't recommend to take advantage of this). Radio-Shack carries several models of LED's assemblies with built-in resistors which makes the construction of the tester even simpler. The LED assemblies we used and will refer to in the rest of this document are Radio-Shack 'RED LED LAMP ASSEMBLY, Cat. No. 276-011a. They look like red lamp indicators and have two flexible leads about 5" long. The yellow one is the cathode (i.e. negative) and the red is the anode (positive). They are rated for 12VDC but their current (at 12V) is not specified (it is probably in the range of 10-20ma). They seem to be protected from reversed 12VDC but again, we don't have the specification of maximal safe reverse voltage. 

To build the tester you will need the following parts (or equivalents) available from Radio: 

• 2 units of Red LED Lamp Assembly, Cat. No. 276-011a. These are red LED's that looks like regular 12V lamp indicators and can be mounted in a 21/64 (8.3mm) diameter hold.
• 2 units of Male Quick Disconnects (10), Cat. No. 64-3038a. Each of these bags contain 10 assorted spades, among them 4 non-isolated and 2 isolated 0.25" spades. We highly recommend to use isolated spades to prevent short circuit between the spades when connecting them to the Diagnostic Connector. This is why you need two units to have the three 0.25" spades we need for the tester.
 
 

These components are pretty standard and compatible ones are available from various sources. If you can't get them from Radio-Shack, try any automotive store for the spades and an electronic supplier for the LED assemblies (Digikey might be a good candidate, and their full catalog is available on-line). If you can't get a LED assembly with built-in current limiting resistor, you can build them them from scratch as described in the diagram below. The resistor can be of any value from 330 to 1000 ohm. We recommend to use a 510 ohm resistor that will result in current of about 20 ma through the LED. The diode is used to protect the LED from reverse voltage in case the tester is connected improperly. Any diode such as 1N4001 with a breakdown voltage of 25V or more and maximal current of 100 ma or more will protect the LED just fine. 

 

The construction of the tester is very simple, just follow these two easy steps: 

1. Connect together the two anodes to a spade (use a crimping tool if you have one) . This will be ABR lead. 
2. Connect each of the cathodes to a spade. These leads will be DCC1 and DCC2 respectively. 
 


The following picture shows how the built tester looks like. If you like, you can install the two LED assembly in a small plastic box (also available from Radio Shack): 

 

To check the tester, connect the ABR lead to the (+) post of the battery and connect the DCC1 lead to ground. Make sure that the DCC1 LED lit. Repeat the step for the DCC2 lead. If the test was OK, you are ready to test it using the ECU itself. First turn the ignition switch OFF. Then insert the three spades of the tester to the ABR, DCC1 and DCC2 pins of the diagnostic connector. Make sure you connected the right spades to the right pins. Turn the ignition switch ON (you don't have to actually start the engine) and watch the two LED's. They should be on for about 3 seconds and then will go off. If any error code exists, one or two of the LED's will periodically flash. The next section contains a detailed description of how to interpret the flashes and determine what the error codes are. 

 

Note: When monitoring the signals with a LED as described above, the LED is lit when the signal is in LOW state and is off when the signal is HIGH. 

Note: Since the ECU does not require manual activation of the error-code reporting, the monitoring of the DCC1 and DCC2 signals can be done permanently. This will give you immediate indication when an error state exists, and will be similar to the Check Lamp warning in cars of recent models. A simple way to achieve that is to install the two LED assemblies in the dashboard and to connect them permanently to the DCC1 and DCC2 signals. If everything is OK, the LED's will stay off. When an error state exists, they will start to flash, indicating the error code. 

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INTERPRETING THE SIGNALS

With the LED tester monitoring DCC1 and DCC2, turn the key switch to ON. The two LED's will go on for about 3 seconds and then will go off. If there are no more LED flashes, this indicates that there are no immediate failures. 

The codes are displayed with a sort of Morse code. There are short (1/2 second) flashes of light which correspond to a count of 1 and long (2 seconds) flashes which correspond to a 5. These pulses are counted until the long pause (2 seconds) that indicates the end of the code. 

DCC1 indicates the one's digit of the error code and DCC2 indicates the ten's digit. DCC1 will flash either short or long pulses. Because of the limited number of two-digit codes (12 and 15) DCC2 will only indicate a short pulse which counts as 10. There is a long pause after which time the code will be redisplayed. 

A code displayed doesn't necessarily mean that the sensor described is faulty. It could also be caused by a bad connector, loose wiring or a defective ECU. The possible error codes are summarizes in the list below. 'S' indicates a short pulse (about 1/2 sec), 'L' indicates a long pulse (about 1 sec), 'P' indicates the pause (about 2 sec) between the cycles, and '...' indicates repetition. 

 
 
Code 01 - Crank angle sensor 
       DCC1: S P .... (1 short light... long pause ... repeat), 
       DCC2: does not light. 
       Fail-safe mode: There is no fail-safe mode for this sensor. 

Code 02 - Air flow meter. 
       DCC1: SS P ... (2 short lights... long pause ... repeat) 
       DCC2: does not light 
       Fail-safe mode: Maintains basic signal at preset level. 

Code 03 - Water thermo sensor. 
       DCC1: SSS P .... (3 short lights... long pause ... repeat) 
       DCC2: does not light 
       Fail-safe mode: Maintains a constant 80 degree C command. 
       Comments: this is the two-wire sensor on the back of the thermostat housing 

Code 04 - Intake air temp sensor.  
       DCC1: SSSS P .... (4 short lights... long pause ... repeat) 
       DCC2: does not light 
       Fail-safe mode: Maintains constant 20 degree C command. 
       Comments: This sensor is built into the air flow meter. 

Code 05 - Oxygen (O2) sensor.  
       DCC1: L P.... (1 long light... long pause ... repeat) 
       DCC2: does not light 
       Fail-safe mode: The ECU stops feedback correction (open loop operation) 
       Comments: This sensor is located on the down-pipe / pre-cat. 

Code 06 - Throttle sensor.  
       DCC1: LS P ... (1 long light ... 1 short light ... long pause ... repeat) 
       DCC2: does not light 
       Fail-safe mode: The ECU assumes 100% throttle position. 
       Comments: This is the TPS and it is located underneath the intercooler. 

Code 07 - Boost/Pressure sensor.  
       DCC1: LSS P ... (1 long light ... 2 short lights ... long pause ... repeat) 
       DCC2: does not light 
       Fail-safe mode: Maintains constant command: 96 mm Hg (boost sensor), 26.3 kPa (pressure sensor) 
       Comments: This is located on the right shock tower. 

Code 09 - Atmospheric Pressure sensor.  
       DCC1: LSSSS P ... (1 long light ... 4 short lights ... long pause ... repeat) 
       DCC2: does not light 
       Fail-safe mode: Maintains constant sea-level command (boost sensor) 
       Comments: This is located next to the ECU. 

Code 12 - Trailing side coil failure. 
       DCC1: SS P ... (2 short lights ... long pause ... repeat) 
       DCC2: S P ...   (1 short light ... long pause ... repeat) 
       Fail-safe mode: Stops operation of trailing side ignition 
       Comments: indicates a failure within the trailing side ignition system. 

Code 15 - Intake air temperature sensor.  
       DCC1: L P ... (1 long light ... long pause ... repeat) 
       DCC2: S P ... (1 short light ... long pause ... repeat) 
       Fail-safe mode: Maintains constant 20C (68F) command. 
       Comments: This is located on the intake air pipe just prior to the throttle body. 

Note: From our observations, it seems that if more than one error code exists, the ECU reports only the one of lower numeric value.