Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1992 Mercedes Benz 300SDFACTORY OEM SERVICE PROCEDURE
Engine Controls - System/Component Tests - 3.5L Diesel: Key-Shutoff
- Connect Vacuum Tester (201 589 13 21 00) with "Y" fitting to vacuum shutoff unit located on injection pump. With engine idling, vacuum tester should indicate zero mbar. If vacuum is as specified, go to next step. If vacuum is not as specified, go to step 3).
- Turn engine off. Vacuum tester should indicate greater than 300 mbar for 5-6 seconds. If vacuum is as specified, go to step 4). If vacuum is not as specified, go to next step.
- Repair or replace key shutoff relay, key shutoff switchover valve, and vacuum shutoff unit. Check vacuum line connections and fuse No. 24. Repair as necessary.
- Start engine and idle. Connect a voltmeter in parallel to 2-pin key shutoff switchover valve electrical connector. Turn engine off. Voltage should be greater than 12 volts for 5-6 seconds. If voltage is as specified, go to step 6). If voltage is not as specified, go to next step.
- Check fuse No. 24 and replace as necessary. Repair or replace key shutoff relay as necessary.
- Connect Vacuum Tester (201 589 13 21 00) with "Y" fitting to key shutoff switchover valve vacuum supply line (Brown). With engine idling, vacuum tester should indicate greater than 500 mbar. If vacuum is as specified, system is functioning properly. If vacuum is not as specified, repair or replace vacuum pump or vacuum line as necessary.
Factory Service Tools
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.