Its embarassing! I replaced my starter motor on my 85 300D because three months ago I turned the key and heard no response. 30 minutes later I came out of the house and the car started on the first turn of the ignition switch and worked perfectly for six months...until it happened again..

I figured the solenoid was dying on my starter and went to a reputable junkyard in Seattle that had a starter motor that looked like new on its shelf. I'm pretty sure it was, also..

Anway my wife had the car out shoppinng today and she said the same thing happened again! Luckily when I put the new starter in I had also wired up an emergency wire to the solenoid that could be used to hit screw post #1 in that 3 pin box. I figured I could also use it for service to turn the engine over when I was doing my valves....

Now my question is, what could it be? I have a new battery, new cables, new clamps... I am wondering if its the ignition switch or a bad wire to the starter solenoid....

Ignition switches are known sources for this kind of problem. There's a thread on here somewhere describing how to clean the contacts. Mine does it a about once every 4 months. I carry a short piece of wire to jump terminals 1 and 3. Then it's good to go for another 4 months or so.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

also I am going to tear down the whole system and replace the Neutral Safety switch

Here you go:

Starter delay in cold weather.........???

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I think this problem is not uncommon on especially older cars. Usually the turn on voltage for the solenoid becomes substandard over the years. Resistance points in the circuit accumulate with time.
I have one 240d that the previous owner changed starters several times for this problem. His eventual solution was to wire a ford solenoid into the circuit so he could deliver a nice clean twelve volts to the mercedes solenoid. The ford unit will activate comforatably at lower voltages and current than the mercedes one.
Another was an old jetta I had. The trigger voltage was just too low for consistant turn on of the starter. If you have a wiring diagram and ohmeter you just might be able to locate where the resistance in that circuit is.

i'm going to be using this as my daily next month

Also, check thoroughly the engine-to-chassis ground strap. I had a similar scenario when the exhaust clamp the strap was attached to became loose.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I've had the same modification done to my 300D (by previous owner). Aftermarket solenoid bolts right up on the firewall to the right of the battery. The pics of this are at the end of this thread Replaced alternator, battery not charging, ???

I wonder if this is a common issue for w123's?

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Kevin

1978 300D
1979 240D