You need to scan it with a MB impulse tester, once you have the codes, all thats left is part replacement, go to an MB shop and have it scanned, or buy a scan tool that can do it!
A/C-HEATER SYSTEM - AUTOMATIC -1990 Mercedes-Benz 300E
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NOTE: Before proceeding with test, check No. 7 fuse.
DIAGNOSTIC/IMPULSE READOUT TESTING
1. Connect Impulse Counter (013) to terminal No. 7 of diagnostic connector located in engine compartment. Turn ignition switch on. U BATT indicator light (LED) on impulse counter must be displayed. If LED does not light up, test voltage between terminal No. 1 (ground) of test connector and positive battery terminal. Voltage should be 11-14 volts. 2. Test for voltage between terminals No. 1 and 7 of test connector, voltage should be 6-12 volts. If voltage is not 6-12 volts, check wiring circuit. 3. Start impulse testing procedure. Press start button for no less than 2, but not more than 4 seconds. Observe and write down impulse readout code(s). 4. Press start button again for no less than 2, but no more than 4 seconds. If there are no further system malfunctions, the first indicated code will reappear. If trouble codes are present, repair as indicated. See IMPULSE DISPLAY CODES.
Thursday, August 28th, 2008 AT 3:24 PM