Benz Won't Start

Anyone out there with a functional crystal ball??
It's a 1990 420 SEL with 188,000 miles. I've had it for 10 years and it's given me no problems. I've been anal regarding service and repairs. Now, it
has let me down!
I parked it in the garage Thursday afternoon. It wouldn't start Friday morning, but it cranked over just fine. It always catches and starts within a second, and I think it did run for about a second. I don't see any applicable fuses in the fuse box. I found what I think is the fuel pump relay, with a rectangular 10 amp fuse on the top (is this the fuel pump relay?), but the fuse is good.
I guess my question is: Is there any common cause, or one more likely than another? Trying to simplify it, I figure it's got to be either fuel or ignition. Duh! Any insight will be greatly appreciated. It's going to embarrass me if it has to be towed away to the shop.
A couple of questions: Should I hear a pump running when I turn on the ignition?? And. Does the Mercedes road service cover this type of problem, seeing that it's in my garage?
John
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Road service is first aid, not repair; they'll tell you to tow it to a shop.
The relay box with a top fuse is the over voltage protection relay; its a prime suspect here.
Yes you ought to hear the fuel pump run very briefly prior to cranking the engine, if not its relay is suspect.
The age of the car makes both of these relays very suspect for they do get tired over time. Common.
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In defence of MB, I've gotten some _very_ helpful advice from their 'road service'. Maybe I just got lucky and got a mechanic on the other end. In any case, as a novice 'wrencher', I've been 'walked' through more than one repair by them.
cheers, guenter
ps maybe it's different here in Canada

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Thanks TG. I'll deal with it when I get home from vacation.
A couple more questions: What does the overvoltage protection relay do if it fails or senses a condition? Who would you suggest for a reliable and reasonable MB parts source?
At this point I don't know whether to pay a towing bill or take a chance and replace a couple of relays.
John
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OVP (over voltage protection) relay limits its output voltage to around 16 or 17 volts (seen the value in manual but I can't remember exactly what it is) so other expensive, downstream electronics (like engine control or ABS brain which can run like $1k) will not be fried.
When OVP fails, engine runs rough or doesn't run at all. Sometimes ABS light will come on. From many past newsgroup postings here, it seems this relay tends to age fast. Pull it out and check its manufacture date. If it is over 10 years old, get a new one.
The online stores I have used: 1. PerformanceProducts.com. In California, great pictures and catalog 2. BuyMBParts.com, in Georgia. Usually the best price 3. BenzBin.com, in New Jersey. One of the early sites using WorldPac Choices 2 & 3 use WorldPac as their supplier. WorldPac (wholesale only) provides software to its distributors so it is very easy to set up a site to sell parts. Most online stores (probably dozens now) use WorldPac, though each vendor has its own price structure.
GermanStar.com is also a good site (which does not use WorldPac) but I have not bought from it yet.
Tiger (a great contributor in this group) mentions Mr-Auto-Parts.com. I have not used it either.
John Simpson wrote:

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someone with good ears) couldn't hear the fuel pump run when the ignition switch was turned on. That fixed the problem. But now I'm nervous about the overvoltage relay. It appears to be the original.
I'm really impressed with benzbin. I ordered the relay online Monday morning and it was delivered to me in Connecticut Tuesday afternoon. The price was good and the order status emails were timely
Thanks again for all the good advice.
John
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