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2016 E350 wagon; 1994 E320 wagon
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the fuel pumps are only powered for about 1 second when the ignition is first switched on... they come on when the engine is cranking over as determined by the crank position sensor (CKP, L5). When the engine stops turning over, the pumps stop about 1 second later. On a HFM-SFI M104, this is all managed by the Motronic ECM.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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Relay replaced. All fuses in front test good. Where rose should I look?
When you say relay replaced, which one - fuel pump or OVP? On the M104 HFM systems a bad OVP will cause a no-start condition (including no fuel pump operation even if the fuel pump relay is good).

Also, has your engine wiring harness be updated? If not, you may have an ecco junk issue and the only cure for that is to replace the harness. There is tag on the harness by the battery - it should say Delphi and the date should be 1998 or later.
 
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Premium Member
2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
Joined
1,721 Posts
the fuel pumps are only powered for about 1 second when the ignition is first switched on... they come on when the engine is cranking over as determined by the crank position sensor (CKP, L5). When the engine stops turning over, the pumps stop about 1 second later. On a HFM-SFI M104, this is all managed by the Motronic ECM.
IIRC, there is a direct "starter in operation" connection from the starting solenoid to the ECU that causes the fuel pumps to run while the starter is in operation. Once the starter is released and the engine comes up to speed, the signal from the CPS then keeps them running.
 

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2001 C230, 1995 E320
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Discussion Starter · #5 ·
When you say relay replaced, which one - fuel pump or OVP? On the M104 HFM systems a bad OVP will cause a no-start condition (including no fuel pump operation even if the fuel pump relay is good).

Also, has your engine wiring harness be updated? If not, you may have an ecco junk issue and the only cure for that is to replace the harness. There is tag on the harness by the battery - it should say Delphi and the date should be 1998 or later.
the wiring harnesses has been replaced. Now I replaced the fuel pump relay. I had this problem some months ago and relay replacement fixed it. I did not think such a new relay would fail again so soon, but went ahead and replaced it. No luck, of course. Now the only symptom is cranking but no start.

is OVP easy to replace? If so I will give it a try befor ordering a tow to a shop. Thanks for responding.
 

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2016 E350 wagon; 1994 E320 wagon
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IIRC, there is a direct "starter in operation" connection from the starting solenoid to the ECU that causes the fuel pumps to run while the starter is in operation. Once the starter is released and the engine comes up to speed, the signal from the CPS then keeps them running.
hmm, I do see a violet/white wire from the starter control signal going to the ECM pin 21, this is the same signal that goes to the starter solenoid. I know on other cars that i've worked on it was just the crank position pulses that drove the fuel pump.
 

Premium Member
2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
Joined
1,721 Posts
the wiring harnesses has been replaced. Now I replaced the fuel pump relay. I had this problem some months ago and relay replacement fixed it. I did not think such a new relay would fail again so soon, but went ahead and replaced it. No luck, of course. Now the only symptom is cranking but no start.

is OVP easy to replace? If so I will give it a try befor ordering a tow to a shop. Thanks for responding.
The OVP is very easy to replace - it is right behind the battery and should be under a plastic shield. Check the fuse(s) on it first, but if they are good, replace the OVP before towing to a shop.
 

Premium Member
2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
Joined
1,721 Posts
hmm, I do see a violet/white wire from the starter control signal going to the ECM pin 21, this is the same signal that goes to the starter solenoid. I know on other cars that i've worked on it was just the crank position pulses that drove the fuel pump.
Most of the EFI systems I have worked with the innards of have three phases - ignition on pulse to prime, forced pump operation during cranking, and then the tach signal keeps the pumps alive.
 
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