1988 Mercedes Benz 300CE (124 Chassis) Connecting Test Equipment and Reading Diagnostic Trouble Codes
Testing with the impulse counter should be carried out for rapid fault diagnosis. The
impulse counter output shows only existing faults, since the ACC system does not have
memory capabilities. If one or more faults are indicated, repair as necessary and repeat
the test. Be sure that fuse #7 is good, the battery voltage is between 11-14 volts and the
temperature selector wheel is set to 22°C.
1. Connect the impulse counter to the on-board diagnostic connector.
Black lead of the impulse counter to socket 1
Yellow lead of the impulse counter to socket 7
Red lead of the impulse counter to battery positive terminal.
2. Turn the ignition to the “ON” position.
The LED “U Batt” will be illuminated. If the LED “U BATT” does not illuminate,
check for voltage between socket 1 of the on-board diagnostic connector and
the positive terminal of the battery. If the voltage is not between 11-14 volts,
check for an open circuit or loose ground connection.
Test for voltage between socket 7 and socket 1 of the diagnostic connector.
The voltage should be 6 – 12 volts.
3. Depress the start button for 2 – 4 seconds.
Fault codes will be indicated by the number in the impulse counter display
window. Take note of the impulse display number.
Faults are displayed in ascending order.
The number 1 indicates that no faults are stored in memory.
During the impulse readout, The LED indicator in the fresh air/recirculation
switch will blink.
4. Repeat step 3 until all faults are indicated. After all fault codes have been
displayed, pushing the start button again for 2 – 4 seconds will repeat the fault code