Help please: 300 CE won't start

My 1990 24 valve 300 CE has this problem:
With no prior starting or running problems, it suddenly decided not to start today. It fired up as usual for only a second or two, then died.
The fuel pumps prime up, dash lights come on and the car cranks strong but it will not start. All fuses are intact, as far as I can tell. Since my car uses the MAS triple relay, and the fuel pumps pressurize on ignition, I don't think it or they are the problem. Also the accumulator, pumps and filter were replaced with factory new units several months ago. I even checked the fuses on the over-voltage protection relay, and they are intact. As I mentioned, this situation occurred suddenly. I've owned the car a long time, and it has been running well.
One thing, however, is that it occasionally dies when coasting (no gas applied), as when getting off the freeway or coming to a long stop. On these occasions the car will occasionally die suddenly, then upon turning the key and starting, will fire up again immediately. No one has been able to diagnose this, but it has never interfered with starting or running. (The idle speed control valve, ignition wires, plugs cap and rotor are fairly new as well.)
It was suggested I test the ignition to see if there is spark, but I'm not sure how to do that yet. I know it involves a 12 v. lamp and some leads. But if anyone has an idea of what else to check, I'd appreciate it. The various ignition modules are quite expensive, and most such items cannot be returned once purchased. Yes, I can have the car towed to the Mercedes dealer or and independent garage, but I was hoping to at least do what I can first.
Thanks for any advice.
C.A.
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One thing, however, is that it occasionally dies when coasting (no gas applied), as when getting off the freeway or coming to a long stop. On these occasions the car will occasionally die suddenly, then upon turning the key and starting, will fire up again immediately.
My suggestion is the fuel pump relay - even though it seems to run the pump prior to cranking to motor. Your description above is the key to an intermittent fuel pump relay IMHO.
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"-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote:

Thanks for the advice. I assumed since the pumps seem to pressurize when I turn the key, that the MAS relay is working. Perhaps I am wrong about this. Too bad I can't test it with a new relay. I don't think the dealer lets you buy one and then take it back. But I'll check further.
C.A.
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By noting your engine dies when coasting tells me two possibilities... fuel pump relay and throttle position sensor. If you never changed your fuel pump relay, it is time to do so... long overdue.
Throttle position sensor is also another issue with these 6 cylinders... will cause erratic shutdown.
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Thanks for the advice all.
With regards to a possible faulty fuel pump relay, my model uses the MAS relay, which is apparently three relays in one. if I turn the key and hear the pumps in the rear of the car pressurize for a second or two, I assume the relay is functional. Is this correct? And if so, wouldn't this rule out the pump relay as a possible cause?
Also, I'm not sure where the throttle position sensor is. It's not listed in the repair manual I have (Haynes) I'm sure the dealer can tell me, however.
Thanks,
C.A.
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No... not true on MAS or fuel pump relay. The main problem is when the relay get too hot, it releases the contact the the fuel pump shuts off. Other time, the contacts has too much oxidation that the fuel pump cannot get full voltage.
I noticed you said you replaced fuel pump... that tells me your old one draws too much power at the end that pretty much burned out your MAS or fuel pump relay.
You can get rebuilt one for less.
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