Isuzu Impulse XS 1990 - 1990 ENGINE PERFORMANCE Self-Diagnostics - Impulse - Fuel Injection 1.6L

Isuzu Impulse XS 1990 - INTRODUCTION

If no faults were found while performing basic test procedures in the BASIC TESTING - EFI article, proceed with self-diagnostics in this article. If no fault codes or only pass codes are present, proceed to H - TESTING W/O CODE - EFI article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.).

Isuzu Impulse XS 1990 - SELF-DIAGNOSTIC SYSTEM HARD FAILURES

Hard failures cause CHECK ENGINE light to illuminate and remain on until problem is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, Electronic Control Module (ECM) will use a substitute value in its calculations to continue engine operation. In this condition, commonly known as limp-in mode, the vehicle runs but driveability will not be optimum.

Isuzu Impulse XS 1990 - INTERMITTENT FAILURES

Intermittent failures may cause CHECK ENGINE light to flicker or illuminate and go out after intermittent fault goes away. However, the corresponding trouble code will be retained in Electronic Control Module (ECM) memory. If related fault does not reoccur within a certain time frame, related trouble code will be erased from ECM memory. Intermittent failures may be caused by a sensor, connector or wiring related problems. See INTERMITTENTS in H - TESTING W/O CODE - EFI article.

Isuzu Impulse XS 1990 - RETRIEVING CODES

NOTE:

Codes can also be retrieved using a Scan tester.

1. The CHECK ENGINE light will come on when ignition is on and engine is not running. When engine is started, CHECK ENGINE light should go off. If light remains on while engine is running, a trouble code is present.
2. If light did not come on with key on and engine off, the CHECK ENGINE light circuit must be checked before continuing. See appropriate DIAGNOSTIC CIRCUIT CHECK in this article if CHECK ENGINE light failed to operate. Circuit checks are located with code charts.
3. To retrieve codes on Impulse models, install jumper wire between Orange/Black & Black/Blue wires of ALDL connector. The ALDL connector is a 3-wire White connector, located behind passenger's side kick panel.
4. On all models, turn ignition on and count number of flashes from CHECK ENGINE light. A Code 12 will be identified by a flash, pause and flash, flash. A long pause will exist and Code 12 will be repeated 3 times. See Fig. 1 . Each additional code will flash 3 times starting with lowest code to the highest and repeat 3 times.
5. If system is operating properly (with no codes), a Code 12 should exist with ignition on and engine off. This indicates the diagnostic system is capable of storing codes.
6. Each individual code will be repeated 3 times and then display the next code. See Fig. 1 . Once all codes are displayed, codes will be repeated. Remove jumper wire from ALDL connector after codes are retrieved.

Fig. 1: Isuzu Impulse XS 1990 - Component Locations - Typical Code Display

Isuzu Impulse XS 1990 - CLEARING TROUBLE CODES

CAUTION:

Ensure ignition is off when disconnecting or reconnecting power supply to ECM.

1. After repairs are performed, clear ECM memory of all stored trouble codes. To clear memory, turn ignition off.
2. On Impulse, remove ECM fuse (30A) from fuse block, located in left front corner of engine compartment, for at least 30 seconds. Replace fuse.
3. On Trooper, disconnect Green fusible link at fusible link box located in right front corner of engine compartment, for at least 30 seconds.

   NOTE:

   The Green fusible link is connected to a White wire.

4. On all models, trouble codes can also be cleared by disconnecting vehicle battery. However, other memory functions (clock, radio, etc.), will require resetting.

Isuzu Impulse XS 1990 ECM LOCATIONS

Model	Location
Impulse	(1) Behind Left Side Of Instrument Panel
(1) DO NOT confuse transaxle Electronic Control Unit (ECU), located on left side of instrument panel, with ECM located behind air duct. The ECM has a 24-pin and 32-pin connector. Transaxle ECU has a 16-pin and 20-pin connector.

Isuzu Impulse XS 1990 - TROUBLE CODE CHARTS

Isuzu Impulse XS 1990 TROUBLE CODE IDENTIFICATION

Code	System Affected	Probable Cause
12	System Normal	System Normal
13	O2 Sensor (Open)	Sensor or Circuit, ECM
14A (1)	Coolant Temp. Sensor	Sensor or Circuit, ECM
14B (2)	Coolant Temp. Sensor	Sensor or Circuit, ECM
21A	TPS Signal (High)	Sensor or Circuit, ECM
21B	TPS Signal (Low)	Sensor or Circuit, ECM
23A (2)	MAT Sensor	Sensor or Circuit, ECM
23B (1)	MAT Sensor	Sensor or Circuit, ECM
24	Vehicle Speed Sensor	Sensor or Circuit, ECM
33A	MAP Sensor (High)	Sensor or Circuit, ECM
33B	MAP Sensor (Low)	Sensor or Circuit, ECM
42	Elec. Spark Timing	Circuit, Ign. Module, ECM
44	O2 Sensor (Lean)	Injectors, Fuel Press., EGR, Exhaust Leaks, O2 Sensor or Ckt, MAP Sensor, ECM
45	O2 Sensor (Rich)	Injectors, Fuel Press., EGR, O2 Sensor or Ckt, MAP Sensor, TPS, Canister Purge, Ignition Shielding, ECM
51	ECM/PROM Failure	ECM or PROM Failure
(1) High temperature indicated by temperature sensor. (2) Low temperature indicated by temperature sensor.

Isuzu Impulse XS 1990 - DIAGNOSTIC CIRCUIT CHECK

The diagnostic circuit check will identify a problem created by an Electronic Engine Control System (EECS) malfunction. It is the starting point for diagnosis. This directs you to the next logical step in diagnosing a complaint.

After completing diagnostic circuit check and finding on-board diagnostics functioning properly with no trouble codes displayed, use appropriate SCAN DATA table for comparison. The typical values are an average of display values recorded from a properly operating vehicles and are intended to represent a normal system display.

NOTE:

Only parameters supplied by manufacturer are used in this article. If a Scan tester reads other parameters, values are not recommended by manufacturer for diagnostic use. A Scan tester displaying faulty data should not be used or a misdiagnosis will occur. Report problem to Scan tester manufacturer.

Isuzu Impulse XS 1990 SCAN DATA

SCAN Position	(1) Data Value
A/C Clutch	Off
A/C Request	No
Base Pulse Width (BPW)	.8-3.0 Milliseconds
Battery	12.5-14.5 Volts
Block Learn Cell	1
Block Learn Enable	Yes
Block Learn Memory (BLM)	100-150 Counts
Coolant Temperature	185-221?F (85-105?C)
Desired RPM	ECM Idle Command
EGR Solenoid	Off
Engine RPM	800-900 RPM
IAC	1-50 Counts
Idle Throttle	Yes
Integrator (INT)	100-150 Counts
MAP	(2) .5-2.0 Volts
MAT Temperature	(3) 50-194?F (10-90?C)
Open/Closed Loop	Closed Loop (Open w/Extended Idle)
Oxygen Sensor	(4) .1-.9 Millivolts
Park/Neutral Switch	Park/Neutral
Power Steering Switch	Normal
Shift Light (M/T)	Off
Spark Advance	Varies
Throttle Angle	0-1%
TPS	.4-.9 Volts
Vehicle Speed	0
(1) With engine at idle, normal operating temperature, Park or Neutral, system in closed loop and all accessories off. (2) Depending on vacuum and barometric pressure. (3) Depending on underhood temperature. (4) Voltage will continuously vary.

Fig. 2: Isuzu Impulse XS 1990 - Component Locations - Diagnostic Circuit Check Flow Chart

Isuzu Impulse XS 1990 - ECM TERMINAL IDENTIFICATION

The following conditions must be met before testing:

• Engine at normal operating temperature.
• Engine idling at closed loop.
• Scan tester not installed.
• Test terminal not grounded.
• Transaxle in Neutral or Park.

NOTE:

All voltages showing B+ indicate battery or charging system voltage. Voltages showing 0 are ground circuits and should indicate less than .1 volt.

Fig. 3: Isuzu Impulse XS 1990 - Component Locations - Pin Voltage Charts

Isuzu Impulse XS 1990 - CHART A-1: NO CHECK ENGINE LIGHT

There should always be a steady CHECK ENGINE light when ignition is on and engine stopped. Battery voltage is supplied to bulb through C7 fuse. The Electronic Control Module (ECM) turns the light on by providing a ground path through Violet wire.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Battery voltage at circuit No. 240 is protected by a fusible link, located in relay/fuse box under the hood, near the battery.
2. If voltage is present on circuit No. 240, check for faulty ECM ground or ECM.
3. Using a test light connected to 12 volts, probe each system ground circuits to ensure a good ground is present.

Isuzu Impulse XS 1990 - Diagnostic Aids

Engine Runs Okay - Check for a faulty light bulb, open in circuit No. 419, or defective C7 (10-amp) fuse. This will result in no oil or alternator lights, seat belt reminder, etc.

Engine Cranks, But Will Not Run - Check for open in battery fuse or fusible link, ECM ignition fuse, battery circuit No. 240 to ECM, ignition circuit No. 439 to ECM, or poor connection at ECM.

Fig. 4: Isuzu Impulse XS 1990 - Component Locations - Chart A-1: No Check Engine Light Circuit Diagram

Fig. 5: Isuzu Impulse XS 1990 - Component Locations - Chart A-1: No Check Engine Light Flow Chart

Isuzu Impulse XS 1990 - CHART A-2: NO ALDL DATA OR NO FLASH CODE 12 (CHECK ENGINE LIGHT ON STEADY)

There should always be a steady CHECK ENGINE light when ignition is on and engine off. Battery voltage is supplied to the bulb by the ECM, which activates the light by grounding Violet wire. With diagnostic terminal grounded, light should flash a Code 12, followed by any trouble codes stored in memory. A steady light could be a short to ground in the light control circuit No. 419 (Violet wire), or an open in diagnostic circuit No. 451 (Orange/Yellow wire).

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. If a problem exists with the ECM that causes a Scan tester not to read serial data, ECM should NOT flash a Code 12. If Code 12 does flash, ensure Scan tester is working properly on another vehicle. If Scan tester is functioning properly and circuit No. 461 (serial data) is okay, the ECM may be at fault.
2. If light goes off when ECM connector is disconnected, circuit No. 419 is NOT shorted to ground.
3. This tests for an open in circuit No. 451 (diagnostic enable).
4. At this point, CHECK ENGINE light wiring is okay. The problem is a faulty ECM. If Code 12 does not flash, replace ECM.

Fig. 6: Isuzu Impulse XS 1990 - Component Locations - Chart A-2: No ALDL Data or No Flash Code 12 Circuit Diagram

Fig. 7: Isuzu Impulse XS 1990 - Component Locations - Chart A-2: No ALDL Data or No Flash Code 12 Flow Chart

Isuzu Impulse XS 1990 - CODE 13: O2 SENSOR (HEATED SENSOR ONLY) OPEN/SHORTED CIRCUIT

The ECM supplies a .45 volt signal between terminals D6 and D7. When oxygen sensor reaches operating temperature, it varies this voltage from approximately .1-.9 volt (lean to rich exhaust). Oxygen sensor does not produce voltage when temperature is less than 600?F (360?C) and engine operates in open loop.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 13 will set if all the following conditions are met for 40 seconds:
   • Engine temperature is greater than 157?F (69?C).
   • Engine has been running for at least 50 seconds after start.
   • O2 signal voltage steady between 0-.347 volt or steady above .547 volt.
   • Throttle angle greater than 5 percent.

   If conditions for Code 13 exist, system will not go into closed loop.

2. This determines if sensor, wiring or ECM is the cause of Code 13.
3. Use only a high impedance (10-megohm) digital volt/ohmmeter. This tests continuity of circuits No. 412 and 450. If circuit No. 450 is open, ECM voltage on circuit No. 412 will be greater than .6 volt.

Isuzu Impulse XS 1990 - Diagnostic Aids

Normal Scan tester voltage varies between .1-1.0 volts while in closed loop. Code 13 sets in approximately one minute if voltage remains between 0 and .35 volt, but system will enter open loop in approximately 15 seconds. For intermittent inspection, see INTERMITTENTS in H - TESTING W/O CODE - EFI article.

Fig. 8: Isuzu Impulse XS 1990 - Component Locations - Code 13: O2 Sensor (Heated Only) Open/Shorted Circuit Diagram

Fig. 9: Isuzu Impulse XS 1990 - Component Locations - Code 13: O2 Sensor (Heated Only) Open/Shorted Flow Chart

Isuzu Impulse XS 1990 - CODE 14A: CTS CIRCUIT (HI TEMP INDICATED)

Coolant temperature sensor uses a thermistor to control signal voltage to the ECM. The ECM applies voltage on Gray/Black wire to the sensor. When engine is cold, thermistor resistance is high and ECM will see a high signal voltage.

As engine warms, thermistor resistance is less and ECM will see a low signal voltage. When engine reaches normal operating temperature, signal voltage will be approximately 1.5-2.0 volts at ECM terminal No. A11. Coolant temperature is an input used by the ECM to control fuel delivery, Electronic Spark Control (EST) and cooling fan.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if code set because of a hard failure or an intermittent condition. Code 14A sets if signal voltage indicates a coolant temperature greater than 275?F (135?C) for 3 seconds.
2. This simulates conditions for Code 14B. If ECM sees open circuit (high voltage) and displays low coolant temperature, ECM and wiring are okay.

Isuzu Impulse XS 1990 - Diagnostic Aids

After engine is started, engine coolant temperature should rise to approximately 194?F (90?C) and stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature and Manifold Air Temperature (MAT) should read nearly the same with a Scan tester. When Code 14A is set, the ECM turns on the cooling fan. Code 14A results if circuit No. 410 is shorted to ground. For intermittent inspection, see INTERMITTENTS in H - TESTING W/O CODE - EFI article.

Fig. 10: Isuzu Impulse XS 1990 - Component Locations - Code 14A: CTS Circuit (Hi Temp Indicated) Circuit Diagram

Fig. 11: Isuzu Impulse XS 1990 - Component Locations - Code 14A: CTS Circuit (Hi Temp Indicated) Flow Chart

Isuzu Impulse XS 1990 - CODE 14B: CTS CIRCUIT (LOW TEMP INDICATED)

Coolant temperature sensor uses a thermistor to control signal voltage to the ECM. The ECM applies voltage on Gray/Black wire to the sensor. When engine is cold, thermistor resistance is high and ECM will see a high signal voltage. As engine warms, thermistor resistance is less and ECM will see a low signal voltage. When engine reaches normal operating temperature, signal voltage will be approximately 1.5-2.0 volts at ECM terminal No. A11. Coolant temperature is an input used by the ECM to control fuel delivery, Electronic Spark Control (EST), idle speed and cooling fan.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if code set because of a hard failure or an intermittent condition. Code 14B sets if signal voltage indicates a coolant temperature less than -22?F (-30?C) after engine has been running for 2 minutes.
2. This simulates conditions for Code 14A. If ECM sees grounded circuit (low voltage) and displays high coolant temperature, the ECM and wiring are okay.
3. This determines if wiring or ECM is faulty. If circuit No. 452 is open, a Code 33 may also be stored. Check for defective connection at Black engine harness connector.

Isuzu Impulse XS 1990 - Diagnostic Aids

After engine is started, engine coolant temperature should rise to approximately 194?F (90?C) and stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature and Manifold Air Temperature (MAT) should read nearly the same with Scan tester. Code 14B results if circuit No. 410 or 452 is shorted to ground. For intermittent inspection, see INTERMITTENTS in H - TESTING W/O CODE - EFI article.

Fig. 12: Isuzu Impulse XS 1990 - Component Locations - Code 14B: CTS Circuit (Lo Temp Indicated) Circuit Diagram

Fig. 13: Isuzu Impulse XS 1990 - Component Locations - Code 14B: CTS Circuit (Lo Temp Indicated) Flow Chart

Isuzu Impulse XS 1990 - CODE 21A: TPS CIRCUIT (SIGNAL VOLT HIGH)

Throttle Position Sensor (TPS) provides a voltage signal which changes relative to throttle opening. Signal voltage varies from less than .9 volt at idle to approximately 4.5 volts at wide open throttle. The TPS signal is used by ECM for fuel control and most ECM controlled outputs.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if Code 21A is from a hard failure or an intermittent condition. Code 21A will set if TPS reading is greater than 2.5 volts, engine speed is less than 1800 RPM, vacuum reading at MAP is less 9 in. Hg., or if all conditions are present for 2 seconds.
2. This step simulates conditions for a Code 21B. If ECM recognizes the change, the ECM and circuits No. 416 and 417 are okay.
3. This step isolates a faulty sensor, ECM or an open in circuit No. 452. If open is present in circuit No. 452, Code 23A may also be present.

Isuzu Impulse XS 1990 - Diagnostic Aids

With throttle closed, TPS voltage should be less than 1.25 volts. TPS voltage should steadily increase as throttle is opened. Code 21A will result if circuit No. 452 is open or circuit No. 417 is shorted to voltage. For intermittent inspection, see INTERMITTENTS in the H - TESTING W/O CODE - EFI article.

Fig. 14: Isuzu Impulse XS 1990 - Component Locations - Code 21A: TPS Circuit (Signal Volt Hi) Circuit Diagram

Fig. 15: Isuzu Impulse XS 1990 - Component Locations - Code 21A: TPS Circuit (Signal Volt Hi) Flow Chart

Isuzu Impulse XS 1990 - CODE 21B: TPS CIRCUIT (SIGNAL VOLT LOW)

Throttle Position Sensor (TPS) provides a voltage signal which changes relative to throttle opening. Signal voltage varies from less than .9 volt at idle to approximately 4.5 volts at wide open throttle. The TPS signal is used by ECM for fuel control and most ECM controlled outputs.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if Code 21B is from a hard failure or an intermittent condition. Code 21B will set if engine is running and TPS reading is less than .2 volt (200 mV).
2. This step simulates conditions for a Code 21A. If Code 21A is set, or Scan tester displays over 4 volts, ECM and wiring are okay.
3. Scan tester may not display 12 volts, but ECM recognizes the voltage as over 4 volts, indicating that circuit No. 417 and ECM are okay.
4. If circuit No. 416 is open or shorted to ground, a Code 33B may also be stored.

Isuzu Impulse XS 1990 - Diagnostic Aids

With throttle closed, TPS voltage should be less than 1.25 volts. TPS voltage should steadily increase as throttle is opened. Code 21B will result if circuit No. 416 or 417 are open or grounded. For intermittent inspection, see INTERMITTENTS in H - TESTING W/O CODES - EFI article.

Fig. 16: Isuzu Impulse XS 1990 - Component Locations - Code 21B: TPS Circuit (Signal Volt Lo) Circuit Diagram

Fig. 17: Isuzu Impulse XS 1990 - Component Locations - Code 21B: TPS Circuit (Signal Volt Lo) Flow Chart

Isuzu Impulse XS 1990 - CODE 23A: MAT SENSOR CKT (LO TEMP INDICATED)

The MAT sensor uses a thermistor to control signal voltage to the ECM. The ECM applies 4-6 volts on circuit No. 472 to MAT sensor. When manifold air is cold, sensor resistance is high and ECM will see a high voltage. As air temperature increases, sensor resistance decreases and the voltage drops.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if Code 23A is result of a hard failure or an intermittent condition. Code 23A will set if engine is running longer than 2 minutes and signal voltage indicates a MAT temperature below -22?F (-30?C).
2. This step simulates conditions for a Code 23B. If Scan tester displays a high temperature, ECM and wiring are okay.
3. This step checks continuity of circuit No. 452 and 472. If circuit No. 452 is open, a Code 21A may be present.

Isuzu Impulse XS 1990 - Diagnostic Aids

If engine is allowed to cool overnight, coolant temperature and Manifold Air Temperature (MAT) should read nearly the same with Scan tester. A Code 23A results if an open is present in circuit No. 452 or 472. For intermittent inspection, refer to INTERMITTENTS in the H - TESTING W/O CODE - EFI article.

Fig. 18: Isuzu Impulse XS 1990 - Component Locations - Code 23A: MAT Sensor (Lo Temp Indicated) Circuit Diagram

Fig. 19: Isuzu Impulse XS 1990 - Component Locations - Code 23A: MAT Sensor Circuit (Lo Temp Indicated) Flow Chart

Isuzu Impulse XS 1990 - CODE 23B: MAT SENSOR CKT (HI TEMP INDICATED)

The MAT sensor uses a thermistor to control signal voltage to the ECM. The ECM applies 4-6 volts on circuit No. 472 to MAT sensor. When manifold air is cold, sensor resistance is high and ECM will see a high voltage. As air temperature increases, sensor resistance decreases and the voltage drops.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if Code 23B is the result of a hard failure or an intermittent condition. Code 23B will set if MAT temperature is greater than 334?F (135?C) for more than 2 seconds.

Isuzu Impulse XS 1990 - Diagnostic Aids

If engine is allowed to cool overnight, coolant temperature and Manifold Air Temperature (MAT) should read nearly the same with Scan tester. A Code 23B results if circuit No. 452 or 472 becomes open or shorted to ground. For intermittent inspection, see INTERMITTENTS in the H - TESTING W/O CODE - EFI article.

Fig. 20: Isuzu Impulse XS 1990 - Component Locations - Code 23B: MAT Sensor (Hi Temp Indicated) Circuit Diagram

Fig. 21: Isuzu Impulse XS 1990 - Component Locations - Code 23B: MAT Sensor Circuit (Hi Temp Indicated) Flow Chart

Isuzu Impulse XS 1990 - CODE 24: VSS CIRCUIT

The ECM provides battery voltage to the Vehicle Speed Sensor (VSS), located behind instrument panel. The VSS alternately grounds circuit No. 347, which toggles the voltage high and low. This pulsed signal is used by the ECM to determine vehicle speed.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 24 sets if vehicle speed equals 0 MPH when:
   • Engine speed is between 1500-4400 RPM.
   • TPS is at zero percent (closed throttle).
   • MAP reads low voltage with high vacuum (low load condition).
   • Transaxle not in Park or Neutral.
   • Speed sensor indicates speeds less than 5 MPH.
   • All conditions are met for 3 seconds.

   These conditions are met during a road load deceleration. Disregard Code 24 that sets when drive wheels are not operating.

Isuzu Impulse XS 1990 - Diagnostic Aids

Scan tester should indicate a vehicle speed whenever the drive wheels are turning faster than 3 MPH. Check circuit No. 437 for proper connections. Ensure wire connector is clean and tight and harness is routed correctly. A problem in circuit No. 437 will not affect the VSS input or Scan tester readings. For intermittent inspection, see INTERMITTENTS in H - TESTING W/O CODE - EFI article.

Fig. 22: Isuzu Impulse XS 1990 - Component Locations - Code 24B: VSS Circuit Circuit Diagram

Fig. 23: Isuzu Impulse XS 1990 - Component Locations - Code 24B: VSS Circuit Flow Chart

Isuzu Impulse XS 1990 - CODE 33A: MAP SENSOR (SIGNAL VOLT HI)

The MAP sensor responds to changes in manifold vacuum. The ECM receives a signal voltage from MAP sensor varying from .5-2.0 volts at idle closed throttle) to 4-5 volts at wide open throttle. If MAP sensor fails, ECM will substitute a fixed MAP value and use the TPS input to control fuel delivery.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if Code 33A is the result of a hard failure or an intermittent condition. Code 33A will set if:
   • MAP signal indicates low vacuum.
   • No Code 21A or 21B is set.
   • TPS is less than 5 percent.
   • Conditions are present for more than 5 seconds.
2. This step simulates conditions for a Code 33B. If ECM recognizes the change, the ECM and circuits No.416 and 432 are okay. If circuit No. 452 is open, Code 21A may be stored.

Isuzu Impulse XS 1990 - Diagnostic Aids

With ignition on and engine off, manifold pressure is equal to atmospheric pressure and signal voltage will be high. This information is used by ECM as an indication of vehicle altitude and is referred to as BARO. To check sensor, compare BARO reading with a known good vehicle with the same sensor. Readings should be the same within .4 volt.

Code 33A results if circuit No. 452 is open, or if circuit No. 432 is shorted to voltage or circuit No. 416. For intermittent inspection, see INTERMITTENTS in H - TESTING W/O CODE - EFI article.

Fig. 24: Isuzu Impulse XS 1990 - Component Locations - Code 33A: MAP Sensor (Signal Volt Hi) Circuit Diagram

Fig. 25: Isuzu Impulse XS 1990 - Component Locations - Code 33A: MAP Sensor (Signal Volt Hi) Flow Chart

Isuzu Impulse XS 1990 - CODE 33B: MAP SENSOR (SIGNAL VOLT LO)

The MAP sensor responds to changes in manifold pressure (vacuum). The ECM receives a signal voltage from MAP sensor varying from .5-2.0 volts at idle (closed throttle) to 4-5 volts at wide open throttle. If MAP sensor fails, ECM will substitute a fixed MAP value and use TPS input to control fuel delivery.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This determines if Code 33B is result of a hard failure or an intermittent condition. Code 33B will set if:
   • Engine RPM is greater than 1200 RPM.
   • No Code 21A or 21B is set.
   • TPS is greater than 15 percent.
   • MAP signal voltage is low.
2. Connecting harness terminals "B" to "C", will determine if MAP sensor is defective or if problem exist in ECM or wiring.
3. Scan tester may not display 12 volts. Important thing is that ECM recognizes voltage as more than 4 volts, indicating that ECM and circuit No. 432 are okay.

Isuzu Impulse XS 1990 - Diagnostic Aids

With ignition on and engine off, manifold pressure is equal to atmospheric pressure and signal voltage will be high. This information is used by ECM as an indication of altitude and is referred to as BARO. To check sensor, compare BARO reading with a known good vehicle with the same sensor. Readings should be the same within .4 volt.

Code 33B will result if circuit No. 452 is open, or if circuit No. 432 is shorted to voltage or circuit No. 416. For intermittents, see INTERMITTENTS in H - TESTING W/O CODE - EFI article.

Fig. 26: Isuzu Impulse XS 1990 - Component Locations - Code 33B: MAP Sensor (Signal Volt Lo) Circuit Diagram

Fig. 27: Isuzu Impulse XS 1990 - Component Locations - Code 33B: MAP Sensor (Signal Volt Lo) Flow Chart

Isuzu Impulse XS 1990 - CODE 42: EST (ELECTRONIC SPARK TIMING)

Ignition module sends reference signal (circuit No. 430) to ECM when engine is cranking. When engine speed is less than 600 RPM, the ECM applies 5 volts to by-pass line (circuit No. 424) to switch the timing to be controlled by ECM.

When system is running on ignition module (no voltage on by-pass line), ignition module grounds the Electronic Spark Timing (EST) signal. The ECM expects to see no voltage on the EST line. If ECM sees a voltage signal, Code 42 will set and system will not go into EST mode.

When RPM for EST is reached (approximately 400 RPM), and by-pass voltage is applied, EST should no longer be grounded in the ignition module so EST voltage will vary. If by-pass line is open or grounded, ignition module will not switch to EST, so EST voltage will be low and Code 42 will be set.

If EST line is grounded, the ignition module will switch to EST, but because line is grounded there will be no EST signal and a Code 42 will be set.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 42 indicates ECM has detected an open or short to ground in EST or by-pass circuit. This confirms Code 42 and fault causing code is present.
2. This checks for normal EST ground path through ignition module. An EST circuit No. 423 shorted to ground will also read less than 500 ohms.
3. As test light voltage touches circuit No. 424, the module should switch, causing ohmmeter to overrange if ohmmeter is adjusted to the 1000-2000 ohms range. Selecting the 10,000-20,000 ohms range will indicate greater than 5000 ohms. The important thing is that module is switched.
4. If module did not switch, this checks for shorted EST circuit No. 423, open in circuit No. 424, faulty ignition module connection, or faulty module.
5. This step confirms that Code 42 is a faulty ECM and not an intermittent in circuits No. 423 and 424.

Fig. 28: Isuzu Impulse XS 1990 - Component Locations - Code 42: EST (Electronic Spark Timing) Circuit Diagram

Fig. 29: Isuzu Impulse XS 1990 - Component Locations - Code 42: EST (Electronic Spark Timing) Flow Chart

Isuzu Impulse XS 1990 - CODE 44: O2 SENSOR LEAN FUEL MIXTURE (HEATED SENSOR ONLY)

The ECM supplies a voltage of approximately .45 volt between ECM terminals D6 and D7 (if measurement is taken with a 10-megohm DVOM, reading may be as low as .32 volt). The O2 sensor varies voltage within a range of approximately one volt if exhaust is rich and down to about .10 volt if exhaust is lean. When sensor temperature is less than 600?F (316?C), the sensor acts like an open circuit and will not produce voltage. An open sensor circuit or cold sensor will cause an open loop condition.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 44 will set under following conditions:
   • Coolant temperature is greater than 158?F (70?C).
   • Engine running for more than 50 seconds.
   • Integrator count is not at 128.
   • No Code 21A, 21B, 33A, or 33B is set.
   • Oxygen sensor voltage is below .278 mV for 99 seconds.
   • System is in closed loop.
   • TPS is greater than 3 percent.

Isuzu Impulse XS 1990 - Diagnostic Aids

Using Scan tester, observe block learn value at different RPM. Scan tester also displays block cells, so block learn values can be checked in each of the cells, to determine when Code 44 set. If conditions for Code 44 exist, block learn values will be approximately 150.

• Ensure oxygen sensor pigtail is not contacting exhaust manifold.
• Check for ground in wire between connector and oxygen sensor.
• Water delivered to fuel injectors can cause a lean mixture. This can set a Code 44.
• Fuel system will be lean if pressure is too low. Monitor fuel pressure while driving at various road speeds and/or loads to confirm a problem (if necessary).
• If exhaust leak is present, outside air can be pulled into the exhaust and past the sensor. Vacuum or crankcase leaks can also cause a lean condition.
• If heater portion of oxygen sensor fails, this may set a Code 44 or 45. See SYSTEM/COMPONENT TESTS - EFI article.

For intermittent inspection procedures, refer to INTERMITTENTS in the H - TESTING W/O CODE - EFI article.

Fig. 30: Isuzu Impulse XS 1990 - Component Locations - Code 44: O2 Sensor Lean Mix (Heated Only) Circuit Diagram

Fig. 31: Isuzu Impulse XS 1990 - Component Locations - Code 44: O2 Sensor Lean Mix (Heated Only) Flow Chart

Isuzu Impulse XS 1990 - CODE 45: O2 SENSOR RICH FUEL MIXTURE (HEATED SENSOR ONLY)

The ECM supplies a voltage of approximately .45 volt between ECM terminals D6 and D7 (if measurement is taken with a 10-megohm DVOM, reading may be as low as .32 volt). The O2 sensor varies voltage within a range of approximately one volt if exhaust is rich and less than .10 volt if exhaust is lean. When sensor temperature is less than 600?F (316?C), the sensor acts like an open circuit and will not produce voltage. An open sensor circuit or cold sensor will cause an open loop condition.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 45 will set when sensor voltage signal is greater than .75 volt under following conditions:
   • Coolant temperature is greater than 158?F (70?C).
   • Integrator count is not at 128.
   • No Code 21A, 21B, 33A, or 33B is set.
   • TPS is greater than 3 percent.
   • System is in closed loop.

Isuzu Impulse XS 1990 - Diagnostic Aids

Code 45 is most likely caused by one of the following:

• Fuel system will go rich if pressure is too high. The ECM can compensate for some increase. If pressure is excessive, a Code 45 may be set.
• If an open occurs in circuit No. 450, HEI-induced electrical noise may result, causing simulated reference pulses to be picked up by ECM. Additional pulses result in a higher than actual engine speed signal. The ECM will increase injector pulse width to match fuel requirements of increased RPM signal. Tachometer will show higher than actual RPM, which can help in diagnosing this problem.
• Check charcoal canister for fuel saturation. If full of fuel, check canister control and hoses.
• An output causing ECM to sense lower than normal vacuum can cause system to go rich. Disconnecting MAP sensor allows ECM to set a fixed value for the sensor. Substitute a different MAP sensor if rich condition is gone when sensor is disconnected.
• An intermittent TPS output causes system to go rich due to a false engine acceleration indication.
• A contaminated oxygen sensor can cause a false signal. The ECM will reduce fuel delivery causing a driveability problem. Inspect oxygen sensor for silicone contamination from fuel or improper use of RTV sealant.
• A sticking EGR valve creates a rich condition, accompanied by rough idle. Check EGR valve operation.

For intermittent inspection procedures, see INTERMITTENTS in the H - TESTING W/O CODE - EFI article.

Fig. 32: Isuzu Impulse XS 1990 - Component Locations - Code 45: O2 Sensor Rich Mix (Heated Only) Circuit Diagram

Fig. 33: Isuzu Impulse XS 1990 - Component Locations - Code 45: O2 Sensor Rich Mix (Heated Only) Flow Chart

Isuzu Impulse XS 1990 - CODE 51: ECM FAILURE

Check all ECM connections. If connections are okay, clear memory and recheck system. See CLEARING TROUBLE CODES in this article. If Code 51 reappears, replace ECM. Clear codes and confirm closed loop operation. Ensure CHECK ENGINE light is off.

Isuzu Impulse XS 1990 - SUMMARY

If no hard fault codes (or only pass codes) are present, driveability symptoms exist or intermittent codes exist, proceed to the H - TESTING W/O CODE - EFI article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.) or intermittent diagnosis procedures.