300CE Engine cuts off and won't start again

I've had my 91 300CE for about a month now, and it's been great.. until two days ago. I had been driving it around town no problem and then parked it
for a few minutes. Got back, started it back up, and as I was backing out the parking space, the engine just cuts off. No explanation, no warning.. it's just gone. I tried starting it back up, but the starter(?) just kept going around and round. I tried jumping the battery (which I quickly realized was low), but it did the same thing. Still I thought it could be the battery, so I replaced it but got the same result. It has coolant, oil, gas, everything. The only other obvious possibility is an aftermarket car alarm which a previous owner installed, and I have no idea how to work it. Any ideas?
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Did it ever start after that when cool? Do you have spark? See of the distrubuter rotor is turning while cranking. Do you smell strong fuel at throttle body? The standard newsgroup response to this is replace over voltage relay? (not sure) if intermittant and answer to questions is yes.
Kent
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The car never started back up, even after it cooled. I don't smell or sense anything unusual about the vehicle, but I don't know a lot about these cars. I'm starting to think that the alarm system somehow got goofed up and is causing all this. One thing I looked into was a little silver switch underneath the dash. Does anyone know anything about this?
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sense
cars.
My car had an inoperative alarm. I looked under the dash on drivers side and found alarm box. I unplugged it from wiring harness and threw away. I don't think alarm is problem. Alarm should disable ignition to kill car. Do you have spark when engine is cranked?
-Kent
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There are a couple of relays that may be implicated. The electric fuel pump is switched ON and OFF by the "fuel pump relay". If it gets tired and fails to stay ON the engine's fuel supply is cut off and it will stop and may not restart. The second relay causes odd behavior but not such a refusal to start; it's called the "over voltage protection relay" and has a fuse on its top. Either could be the cause, the fuel pump the more likely of the two.
It could also be that the reason this car was for sale was this intermittent stalling problem. If you suspect that may be so, I suggest you have a good M-B independent shop look into the stalling for it won't go away so it must be fixed. Some cars of this vintage had bad wiring harnesses that shorted out - the ecologically correct insulation dried out from the engine's heat and crumbled allowing the wires to short. Any shop can determine if this is your car's problem, unfortunately it's an expensive repair but then it's done, not to reoccur.
Then there's the mystery alarm. However, I'd discount the alarm because the engine was running when it stalled; it's not like it wouldn't start initially.
Whatever course you choose to deal with the problem, always ask the cost BEFORE agreeing to a repair for this may well be a "let's try this and if that doesn't work we'll try the other approach" situation. Some shops will install an new electrical component and reinstall the original if the original is proven to be OK. Ask if the shop will do that so you don't needlessly buy a lot of parts.
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