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  #1  
Old 01-13-2006, 04:46 PM
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300E Hard start - Feul pump relay?

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.

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  #2  
Old 01-15-2006, 02:45 PM
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Fuel pressure regulator

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.

Last edited by dave_rose69; 01-15-2006 at 02:52 PM.
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  #3  
Old 01-15-2006, 03:05 PM
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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.
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  #4  
Old 01-22-2006, 04:08 PM
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300E- relay has power

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.
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  #5  
Old 01-22-2006, 04:20 PM
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Sounds like the relay isn't working properly. I'd get a new one and see if it starts.
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  #6  
Old 01-22-2006, 05:05 PM
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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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  #7  
Old 01-22-2006, 09:47 PM
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1993 300E's are really E320's.

They have the same 3.2 liter M104 engine as in '94 - '95 E320's.
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2001 E430, Bourdeaux Red, Oyster interior.
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1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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  #8  
Old 01-25-2006, 11:13 PM
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no start

Check your Crankshaft position sensor, rotor adapter and Ign coil output. You need a bluish white color spark, not a redish orangy color one to afford good ignition. If your mechanic claims the FP relay is getting voltage to its coil and its still not picking, the coil is open.... The car should start if you manually close the FP relay contacts. If you still get no combustion when the pumps are energized, and you have at least 80 psi fuel pressure at the fuel distributor inlet, and the plugs come out wet, its an ignition problem.

Last edited by dave_rose69; 01-26-2006 at 08:55 PM.
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  #9  
Old 02-16-2006, 02:15 PM
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wiring harness?

Still no luck with the car. My mechanic was doing some research and he found some things on wiring harness problems. He checked mine and the insulation on the wires are crumbling and it needs to be replaced. I will replace this part but I'm wondering if this could be the answer to my starting problem or if it's a seperate issue all together. I'd like to take care of the starting problem before I move on to other issues with the car. Could anyone let me know or give thier opinion on if this could be the answer (or at least part of the answer) or am I looking at a completly different problem? Thanks.
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  #10  
Old 02-16-2006, 04:23 PM
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Indeed this might be the solution. While I do not own this type of car I have read quite a few posts on this forum that suggest the wiring harness is a common, almost inevitable failure on 93 to, I beleive, 95 models.
Good luck, Bruno
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  #11  
Old 02-16-2006, 04:29 PM
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Whether or not the wiring harness is causing the no-start is irrelevant. It needs to be fixed before anything else is done. IMHO, it's a waste of time to look past a found problem in search of another problem that may be related to the problem you just found.
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  #12  
Old 02-16-2006, 05:36 PM
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I agree - wiring harnesses are a known problem on 93-95 W124 cars. It's not an inexpensive part, but you can save a bundle on labour by doing it yourself. Just lay out the new harness alongside the old and as you unplug the old connectors, plug in the new one, no special tools required.
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  #13  
Old 03-08-2006, 06:52 PM
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Cam sensor?

I don't know what to do with my car. It now has a new engine wiring harness along with all the other new parts I've thrown at it over the past year. Thay include a crank shaft sensor, fuel pumps, fuel pump relay, mass air flow sensor, spark plugs and wires, computer(used) and battery. It still won't start. My mechanic said the computer is not turning on pin 86 of the fuel pump relay. The coil and injectors are getting power but thay don't cycle. Could the cam sensor cause this? As a side note, he told me that the manual says the fuel pumps should run for 1 second when the key is turned to the on position but mine doesnt. I don't know if that is relevant or not but I just thought I'd mention it. Thank You.
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  #14  
Old 03-08-2006, 07:19 PM
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So the computer is doing nothing. It isn't turning the pump on, it isn't turning the injectors on, and it isn't controlling the coils (both injectors and coils are controlled by the computer on the GROUND side, they are always powered when the key is on). Do you have a problem with throwing cheap parts at it? Half a page back I suggested a OVP which is the power relay that turns on the computer. I don't see that on the pile of wasted parts you have thrown at the thing. That doesn't mean its the problem but since testing is not part of your game plan, why not throw cheap parts at it too?

Does your tech participate in these crystal ball diagnostics? Do you have any idea how much money you would have saved by seeing a true professional in the first place? I understand that all this guessing is fun, but you sound a bit frustrated.
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  #15  
Old 03-08-2006, 08:13 PM
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No start

CHANGE YOUR IGNITION COIL AND OVP RELAY

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