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.
1990 Mercedes Benz 560SELFACTORY OEM SERVICE PROCEDURE
Fuel Pump Delivery Volume
- Remove fuel pump relay from relay board (under hood on fuse panel). Check fuel pump current draws. Draw of 6 amps indicates fuel pump is good. Replace fuel pump if draw is more than 7 amps. Disconnect return line at diaphragm pressure regulator.
- Attach a hose to fitting on regulator. Place other end of hose in measuring glass or beaker. Bridge relay board terminals No. 7 and 8 with jumper wire and run fuel pump for 40 seconds. See FUEL PUMP PERFORMANCE tables.
- If delivery volume is low, ensure pump is receiving 11.5 volts. Check if dirty strainer in feed connection of fuel distributor is causing restriction. Check for pinched fuel lines.
- Pinch by-pass line-to-fuel inlet hose at fuel accumulator and repeat delivery volume test. If volume is correct, replace fuel accumulator. If volume is low, replace fuel filter and repeat test. Replace fuel pump if volume remains low. Reconnect fuel return line and install fuel pump relay.
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.