Hi, I've been having a problem with my '93 300E for a couple of years now. 95% of the time it starts up perfectly. The other 5% is a mystery, there is no rhyme or reason to it. It will crank over but not start right away. It has happened after a 3 hour trip, a 5 minute trip or when its been sitting for hours. When it happens, sometimes it takes 3 or 4 cranks before it starts, sometimes its 20 minutes of trying it before it will start. On 2 occasions it did'nt start at all. I had it trailered to my mechanic's and both times it started right up and I drove it off the trailer. It also hesitates when accelerating. I'm hoping the two problems are related. The Mercedes dealership changed the feul pump relay, feul pumps, feul filter, spark plugs and wires. My mechanic changed the crank sensor, oxygen sensor and a (used) engine computer. Nothing has changed. He says the feul pump relay is getting power but the computer is not telling the relay to turn the feul pumps on. The wires between the computer and relay are fine. What else could it be? Sorry this was long and thank you for your help.

You mechanic seems to be on top, but you have one of those super aggravating intermittant problems. Unless some security device is intermittantly causing an ignition interuption, I would do this with your mechanics help. Install a T fitting on the output side of your fuel filter and hook up a good quality fuel guage to it. Run the line into the trunk and make sure nothing is going to chaff the hose during normal riding conditions. When the car fails to start, or stalls, go see what the fuel pressure reading is during cranking. Naturally, if the fuel pressure is off spec during these failures, and then when you eventually see the pressure rise and the car starts again, you need a fuel delivery overhaul. Fuel Tank cleaning or draining, a new pickup screen, main filter, fuel accumulator, and fuel pressure regulator. An ignition failure can be seen by immediately connecting a spare spark plug with its side electrode cut off, and remove the most closest high tension lead from engine to plug the test plug into. Let the plug body lay accross some large metalic area of the engine, and keep the sunlight out of the area to see the plug firing. Dont ever hold the lead in your hand!!!! The reason we cut of the side electrode is too see how powerful the spark is. If you see no spark, and considering all else that was changed, you might have a intermittant connection to your ign coil, or a weak coil. Clean up all connectors on your EZL module too. (if you have one) I would put my money on a fuel sys problem though, the MB ign sys is very reliable.....
PS: The fuel pump relay is probably incorperated into the MAS Module. The module can be opened to visually observe the relay energizing, but this requires skill to avoid accidental shorts or damage. Some mechanics clean the relay and bend the moving contact to get it working well again. Its the MAS module that needs an exchange to see if its a fuel pump power delivery problem. Check grounds on the fuel pumps.

A couple of relatively easy ways to check if it's the relay:

1. When it doesn't start, check to see if the fuel pump is energized when you turn the key to the "run" position. The pump should buzz lightly for about a second, then shut off. If it doesn't, it's not getting power.

2. If the fuel pump isn't being energized, remove the relay and bridge (I think, you may want to search the forums to check) pins 6 and 7 with a jumper and see if that turns on the pump. If so, then the relay (at least) is faulty. Note that this pressurizes the fuel system, in case you are doing any other work.

In my case, I had similar symptoms - randomly would crank, but not start - and the relay turned out to be the culprit.

Thanks for your replies. The car is at my mechanics and now the car won't start at all. He has jumped the relay pins and the fuel pump runs. The relay is getting power when the key is on but its not clicking. What else should we look for? Thank You.

Sounds like the relay isn't working properly. I'd get a new one and see if it starts.

Does the car start when the fuel pump works? I would say that you have a HFM car with a 93. Have you checked the OVP relay. If you have a KE car you won't have a OVP you will have a MAS. THe OVP powers the controller which powers the FP relay, I think.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Seen this movie. In my case when I jumpered pins 1 & 2 in the MAS, the fuel pump ran and car started, indicating the fuel pump relay incorporated into the MAS was bad. A new MAS got me going again. I'm not trying to make a point about the MAS itself, just pointing out that it contains the fuel pump relay in some models.

I see yours is a 93 300E. Out of curiosity, I looked up your model at this site parts data base and saw that neither the 2.8l or 3.2l 93 300E have a MAS. I'm assuming this is not an over-sight. If it's not, that's good for you because you must then have an independent fuel pump relay which costs WAY less than a MAS unit.

I didn't read everything prior to this. Has your fuel pump relay been replaced?

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"