Sporadic non- starting C-280

[Myros]

My '94 C-280 straight 6 has let me down three times this week by refusing to fire up. Once for 2 hours, once for an hour, once for 5 mins.
The starter cranks away as normal, all the pre-starting noises are normal, the engine turns over as usual, very healthy, but no start. All warning lights/gauges etc normal. There is no smell of petrol either in the engine-bay or at the exhaust. The symptoms are similar to no fuel/no spark, but I haven't got as far as dismantling anything yet.
It makes no difference whether or not I have used the alarm or the key to lock and unlock the car. The longer let downs were from cold/overnight, the shorter one from warm, about 10 mins after the 2 hour non-start. The car has until now always fired first time, within about 1 second of cranking over, and runs smooth as a nut. Plugs, battery etc are nearly new and in apple-pie order.
None of the things I try, like alarming on/off, locking/unlocking seem to make a difference, the car starts when it wants to. I'm not over-cranking, just my usual amount and a little bit more, and if it doesn't start, I leave it for 5 mins and try again. Eventually, it has started, but with no rhyme or reason discernible to me.
Does anyone have any ideas or suggestions or advice?

 

[mjtray]

If you car is still fitted with its original Over Voltage Protection relay (OVP) there's a good chance that its this component that is causing the trouble.


The original relays had a design fault in them which caused the soldering to fail internally within the relay.

Replacement is a 10 minute job and costs about £45 from your friendly Benz dealer.

If its starting to break up it causes all sorts of starting, idling and stalling problems.

Good luck.

 

[Myros]

C-280 sporadic non-starter.

MJTray,
many thanks. I'll get on the blower to Mercedes and order the part now. Any idea where this (potentially) offending piece of kit lives? I've had a general look-see, and the only thing I recognize as relay-like is tall silver thing with a fuse in a transparent housing, sited near the computer ignition ECM section in the engine bay. Is that it?

Thanks again for the prompt assistance.
Everything on the car except brakes and tyres. plugs and battery is original, at 90k so-far trouble-free and very pleasant miles.

The problem occurred again last night, but only lasted 5 mins.

 

[mjtray]

I think you may hav found it..........as you look at the engine bay, its up by the ECU's on the right hand side of the engine bay, but infront of the ECU's themselves.

About 3 inches high with a fuse on the top under a clear cover.

I'll be interested to hear if this cures the problem.

Cheers

 

[Myros]

Ah, that must be it then

I'll have a go at it as soon as I can get the part from my ever-loving Merc dealer. I dropped in on them when I was first attacked by this problem last week, but it didn't really engage any recognition from them. They seemed to think it was something to do with the alarm or the door cut-out, despite those killing the starter dead when they operate.
I'll let you know how it works.

Cheers,
Myros

 

[Myros]

hopefully sorted

Just back from the dealers, where, almost as though it were a Quentin Tarantino script, the car wouldn't fire up outside after a 10 min wait. I swapped the relay for the new one, and bingo, first turn of the key, back in business. Many many thanks, I'll set off on my various trips a little more relaxed.

 

[mjtray]

Nice one