Tips
TESTING PREPARATIONProper operation of the fuel, ignition and emission systems should be verified before beginning extensive testing of the COMPUTERIZED ENGINE CONTROLS system.
It is essential that the vehicle is at normal operating temperatures, except when attempting to diagnose a cold driveability problem, or when directed otherwise by a testing procedure. The catalytic converter and the oxygen sensor will not function as designed if this is not achieved and inaccurate test results may occur.
ON-BOARD DIAGNOSIS
Before using this section you should have performed the ON-BOARD DIAGNOSTIC procedures that apply to the model on which you are working. This will verify that the CIS-E control unit and the "CHECK ENGINE" light are operational.
Testing Schematic:
USING SCHEMATIC SYMBOLS
In this example the Mercedes Benz Socket Box is being used to conduct a voltage test on the CIS-E system. "N3" in the upper section of the box refers to the CIS-E control unit; the symbol in the middle of the box refers to the Mercedes Benz Socket Box; the voltage symbol at the bottom of the box refers to the connection between socket "5" and "14" of the socket box. Refer to DIAGRAMS / ELECTRICAL for the appropriate electrical diagrams.
NOTE: If the Mercedes Benz Socket Box is not available carefully backprobe control unit harness connectors. Refer to DIAGRAMS / ELECTRICAL for specific electrical diagrams.
TEST EQUIPMENT
The use of incorrect test equipment may result in damage to the CIS-E system components or the test equipment used. If the Mercedes Benz socket box is not available careful backprobing of the CIS-E control unit harness connectors will be necessary. It may also be necessary to partially disassemble connectors for accessibility; care must be taken to prevent damage to the wiring and individual pin connectors.
Isolating electrical problems requires the use of a voltmeter, an ohmmeter, and sometimes an ammeter. A voltmeter measures voltage at selected points in a circuit. An ohmmeter measures a circuit's resistance to current flow. An ammeter measures the current flow through a circuit.
When using an ohmmeter, disconnect battery power to the circuit being tested. Battery voltage will cause the ohmmeter to give false resistance readings.
CAUTION: DO NOT use an ohmmeter on solid state or computer related circuits. The voltage that an ohmmeter applies to a circuit could damage these components.
Fused Jumper Wire:
A jumper wire can be made up from an in-line fuse holder and a set of test leads. The fuse holder should contain a five amp fuse. Use the fused jumper wire for bypassing open circuits. Do NOT use a jumper across high load circuits, such as electric motors.
SYSTEMATIC ELECTRICAL TROUBLESHOOTING
1. Operate the problem circuit to check the accuracy of the complaint. Note the symptoms of the inoperative circuit.
2. Refer to the schematic diagrams in CHASSIS ELECTRICAL DIAGRAMS. Determine how the circuit is supposed to work by tracing the current path(s) from the power source through the circuit components to ground. Based on the symptoms noted in step 1 and an understanding of the circuit operation, identify one or more possible causes of the problem.
3. Make circuit tests to prove or disprove the preliminary diagnosis made in step 2. A logical, simple procedure is the key to efficient troubleshooting. Test for the most likely cause of failure first. Try to make tests at points that are easily accessible.
4. Once the specific problem is identified, make the repair using the proper tools and safe procedures.
5. Operate the circuit to check for satisfactory circuit operation. Good repair practice calls for rechecking all circuits that have been worked on.
TROUBLE SHOOTING TESTS
Voltage Test:
VOLTAGE TEST
This test measures voltage in a circuit. By taking measurements at several points (terminals or connectors) along the circuit, you can isolate the problem.
To take a voltage measurement, connect the negative lead of the voltmeter to the battery's negative terminal or any other known good ground. Connect the positive lead of the voltmeter to the point you want to test. The voltmeter will measure the voltage present in the circuit.
Voltage Drop Test:
VOLTAGE DROP TEST
Wires, connectors, and switches are designed to conduct current with a minimum loss of voltage. A voltage drop of more than one volt indicates a problem.
To test for voltage drop, connect the voltmeter leads to connectors at opposite ends of the circuit's suspected problem area. The positive lead should be connected to the circuit connector closest to the power source. The voltmeter will show the voltage drop between the two points. Any switches in the circuit should be ON during this test.
Continuity Test:
CONTINUITY TEST
Before performing a continuity test, disconnect the car's battery. Zero the ohmmeter by holder the test leads together and setting the meter to zero. Isolate the circuit to be tested. Connect the ohmmeter leads to opposite ends of the circuit's suspected problem area. The ohmmeter will show the resistance across the circuit. Continuity can also be tested with a self-powered test light.
CAUTION: DO NOT use an ohmmeter or self-powered test light on solid state or computer related circuits. The voltage that an ohmmeter/test light applies to a circuit could damage these components.
Short Test Using Voltmeter:
SHORT TEST USING A VOLTMETER
To locate a wiring short to ground, remove the blown fuse and disconnect the load. Connect the voltmeter leads to the fuse terminals. The positive lead should be connected to the terminal closest to the power source.
Starting near the power source, move the wire harness back and forth while watching the voltmeter. If the voltmeter shows a reading while moving a particular section of the harness, the harness has a short to ground.
Short Test Using Ohmmeter:
SHORT TEST USING AN OHMMETER
Disconnect the battery. Zero the ohmmeter by holding the test leads together and setting the meter to zero. Remove the blown fuse and disconnect the load. Connect one lead of the ohmmeter to the outlet side of the fuse terminal and the other lead to ground.
Starting near the outlet side of the fuse, move the wire harness back and forth while watching the ohmmeter. If the ohmmeter shows a low or no resistance reading while moving a particular section of the harness, the harness has a short to ground.
Current Test Using Ammeter:
CURRENT TEST USING AN AMMETER
To measure the current in a circuit, connect the ammeter leads to the connectors or terminals in SERIES with the circuit. The ammeter will show the current through the circuit.