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I have been reading this forum since before if got my 2008 E550 4Matic 4 years ago that now has 169000 miles. Thanks to all that share a wealth of knowledge on here. Searching this site and others as well as the internet and youtube found no solutions to not starting that apply.
First start of the day takes 5 to 10+ attempts but after started everything is fine and it will restart. Voltage on dash is 12.7 at rest and 11.4 when cranking. Icarsoft MB II shows secondary air system malfunction but no check engine light and I do not think it is the cause.. There is no pressure at fuel rail when problem is present. Fuse is good and swapped horn relay to test but didn't work. Jumping the relay terminals does work. Checked things under back seat and found no problems. People talk of hearing the pump run which would be a problem in itself because the pump is quiet and the MB engineers saw to it that you should not hear it.
My question is where does the control circuit for the relay come from? ECU I suspect. I have a new relay on the way but dont have much hope that will work since I have tried 2. Can it be driven with the relay jumped? I know it can not be parked in that condition.
You may be able to drive with the relay jumped but the trouble begins if you are involved in an accident and your fuel pump will not shut off. Could get interesting quickly.
While Plutoe's answer is technically correct, there may be a more helpful member who can post up the wiring diagram for the pump and relay circuit. Fix it, don't jump it.
You may be right Plutoe. I have WIS but no longer have a computer sufficiently obsolete to run it. What I really need is a Star system. Driving with relay jumped sets codes as the car does not like it one bit. Main suspects are ECU or rear SAM but which? No signs of water in trunk but that does not prove anything.
Ground point W6 is located in the trunk on left side wheel well. Pin 17 up top is on the Motor Electronics - ME (I did not dig deeper to see which connector). Connector X36/3 is the one on the fuel tank. Connector 1, pins 3, 7, and 4 are on the rear SAM.
You can try hooking up a multimeter to the relay socket on the SAM (monitor the trigger input for the solenoid). Monitor while you have someone turn the key fob to position 2; at that time you should see power for approximately 1 second (intended to pressurize the fuel rail) then nothing. Once key is turned to the start position and the ME "sees" the CPS signal, you should see power continuously so long as the engine is running. You can do the same at connector X36/3.
Last edited by bbirdwell; Jul 11, 2021 at 05:14 PM.
Thank you for the information. Looks like the motor and two level sensors below the fuel tank connector with wires to SAM connector.. Relays not shown. Correct? Probed relay and had 8.8v which stayed on long enough for me to insert relay and start the car. It is probably my imagination but connecting the scanner may coax a start.
Relay control circuit has battery voltage on hot leg but the ground leg has 60 ohms resistance thus the only 9 volts measured across both legs. My understanding is the ECU provides ground to start the pump. Checked grounds in rear fender and reseated ECU connectors. All other rear SAM items function properly. As much as i would rather replace the SAM than the ECU signs point to the ECU. Can anyone confirm or refute? Any additional diagnostic suggestions?
This is my only transportation and need to resolve.
This was solved by replacing the fuel pump circuit in the ECU. If you have a no start condition make a visit to the fuel rail high on your troubleshooting list. Wanted to post this since many threads are left hanging with no resolution.
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