Mercedes-Benz 300D 1991 - G - TESTS W/CODES 1991 ENGINE PERFORMANCE Self-Diagnostics Diesel

Mercedes-Benz 300D 1991 - INTRODUCTION

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

NOTE:

All voltage tests should be performed with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless specifically stated different in testing procedures.

Mercedes-Benz 300D 1991 - SELF-DIAGNOSTIC SYSTEM HARD FAILURES

Hard failures cause CHECK ENGINE light to illuminate and remain on until problem is repaired. If CHECK ENGINE 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, EDS control unit will use a substitute value in its calculations to continue engine operation. In this condition, the vehicle runs but driveability will not be optimum.

Mercedes-Benz 300D 1991 - INTERMITTENT FAILURES

Intermittent failures may cause CHECK ENGINE light to flicker or illuminate and then go out after the intermittent fault goes away. However, the corresponding trouble code will be retained in EDS control unit memory. If related fault does not reoccur within a certain time frame, related trouble code will be erased from EDS control unit memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in the H - TEST W/O CODES article in this section.

Mercedes-Benz 300D 1991 - ENTERING SELF-DIAGNOSTICS IMPULSE COUNTER READOUT

Code retrieval is accomplished through use of Impulse Counter (124 589 19 21 00) connected to diagnostic connector. Impulse counter leads connect to diagnostic connector pins No. 1 and 4 and Red wire connects to battery voltage. Connect tester according to diagram. See Fig. 1 . U-BAT LED on impulse counter display will illuminate if impulse counter is properly connected. If display does not illuminate, check impulse counter fuse, and measure voltage at diagnostic connector pin socket No. 1. Voltage should be 11-14 volts.

Fig. 1: Mercedes-Benz 300D 1991 - Component Locations - Connecting Testers (300D, 350SD & 350SDL)

Mercedes-Benz 300D 1991 - RETRIEVING CODES IMPULSE COUNTER READOUT

Enter self-diagnostics. See ENTERING SELF-DIAGNOSTICS. Start engine, and run at idle. Ensure drive selector is in Park or Neutral. Press START button on impulse counter for 2-4 seconds maximum. (Pressing push button longer than 2-4 seconds will erase code in memory). Impulse counter will now indicate how many code(s) are stored in memory by digital readout display. Record readout displays. Press START button on impulse counter for 2-4 seconds maximum. If no codes are stored in memory, "1" will display. If codes are stored, first code number will be displayed. Record this number, and press START button again for next code number to display. Repeat sequence until No. 1 displays. Run engine at 900 RPM for 5 seconds minimum, and press START button for 2-4 seconds with engine still at 900 RPM. This sequence will check EGR system code. If no code is displayed, EGR system is okay.

Mercedes-Benz 300D 1991 - TROUBLE CODE DEFINITION

Mercedes-Benz 300D 1991 MALFUNCTION DIAGNOSTIC CODES - DIESEL

Code No.	Malfunction
1 (1)	No System Malfunctions
2	Fuel Rack Position Sensor (L7)
3	Airflow Sensor (B2/1)
4	Atmospheric Pressure Sensor Within EDS Control Unit (N39)
5 (2)	EGR Valve Vacuum Transducer (Y31/1) Or Malfunction In EGR Control Circuit
6	EDS Control Unit (N39), Internal Voltage Supply
7	Starter Ring Gear Speed Sensor (L3)
8	Coolant Temperature Sensor (EDS) (B11/4)
9	Intake Air Temperature Sensor (B2/1a)
10	Not Used
11 (3)	Idle Speed Control Actuator Or EGR Valve Vacuum Transducer
12	Not Used
13	EDS Control Unit (N39) Defective (Internal Malfunction Memory)
14	Air Pressure Sensor (EDS) (B5/1) Defective
15	Intake Manifold Air Pressure Control Valve Vacuum Transducer (Y31/2) Or Wastegate Vacuum Transducer (Y31/3) Or Malfunction In Intake Manifold Air Pressure Control Circuit
(1) If no malfunction codes are present but driveability complaints exist, perform FUNCTION TESTING FOR ELR, EGR & P2 CONTROL - DIESEL. (2) This code is not stored in memory. If suspected, perform test step 5.0/5. Run engine over 900 RPM for minimum of 5 seconds. Note if code displays and voltmeter reads less than 3 volts. (3) This code is displayed only if circuit is shorted.

Mercedes-Benz 300D 1991 - TESTING PREPARATION

To test EDS system after malfunction codes are retrieved, use Multi-meter and Socket Box Tester (124 589 00 21 00) connected by test cables to EDS control unit. See Fig. 1 . Connect multimeter leads to socket box tester pin sockets No. 11 and 3. Remove EDS control unit connector. Connect socket box tester connectors to EDS control unit and EDS control unit harness connector according to the diagram. See Fig. 1 .

Mercedes-Benz 300D 1991 - CLEARING CODES

Disconnecting battery will not erase codes. Each code stored in EDS control unit must be erased individually. After impulse counter readout codes are displayed, repeat sequence to display malfunction code. When code displays, wait 2 seconds and then press START button for at least 6-10 seconds. LED will blink once and/or impulse counter readout will display "=1", indicating code is erased. Continue this sequence until all codes are erased. Disconnect test equipment.

Mercedes-Benz 300D 1991 - SENSOR SPECIFICATIONS

Mercedes-Benz 300D 1991 COOLANT TEMP SENSOR & INTAKE AIR TEMP SENSOR SPECIFICATIONS

?F (?C)	Ohms	Volts at Socket
68 (20)	22,500-27,500	3.47-4.24
86 (30)	15,300-18,700	3.12-3.82
104 (40)	10,600-13,000	2.75-3.36
122 (50)	750-916	2.37-2.89
140 (60)	540-660	2.00-2.44
158 (70)	396-484	1.67-2.04
176 (80)	294-360	1.35-1.65
194 (90)	219-267	1.10-1.34
212 (100)	167-204	.89-1.09

Mercedes-Benz 300D 1991 - TEST STEP CHARTS

Fig. 2: Mercedes-Benz 300D 1991 - Component Locations - Test Step Charts (1 of 7)

Fig. 3: Mercedes-Benz 300D 1991 - Component Locations - Test Step Charts (2 of 7)

Fig. 4: Mercedes-Benz 300D 1991 - Component Locations - Test Step Charts (3 of 7)

Fig. 5: Mercedes-Benz 300D 1991 - Component Locations - Test Step Charts (4 of 7)

Fig. 6: Mercedes-Benz 300D 1991 - Component Locations - Test Step Charts (5 of 7)

Fig. 7: Mercedes-Benz 300D 1991 - Component Locations - Test Step Charts (6 of 7)

Fig. 8: Mercedes-Benz 300D 1991 - Component Locations - Test Step Charts (7 of 7)

Mercedes-Benz 300D 1991 - CONNECTOR ABBREVIATION DEFINITIONS

Mercedes-Benz 300D 1991 CONNECTOR ABBREVIATIONS DEFINITIONS - DIESEL

Connector	Definition
B 2/1	Airflow Sensor
B 2/1a	Intake Air Temperature Sensor
B 5/1	Air Pressure Sensor
B 11/4	Coolant Temperature Sensor (EDS)
F 1	Fuse Box
G 1	Battery
K 1/1	Overvoltage Protection Relay
L 3	Speed Sensor
L 3x1	Connector For Speed Sensor
L 7	Fuel Rack Position Sensor
N 6	A/C Compressor Control Unit
N 39	EDS Control Unit
S 2/2	Glow/Starter Switch
S 30/1	Kickdown Switch
W 10	Ground Connection
X 11/4	Diagnostic Connector
X 26	Engine Harness-To-Interior Connector
Y 22	Electromagnetic Actuator (ELR)
Y 30	Engine Overload Protection Switchover Valve
Y 31/1	EGR Vacuum Valve Transducer
Y 31/2	Intake Manifold Air Press. Control Valve Vacuum Transducer
Y 31/3	Wastegate Vacuum Transducer
100	Intake Manifold Air Press. Control Valve Vacuum Actuator

Mercedes-Benz 300D 1991 - SUMMARY

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