I just got a 1983 380SL that I picked up in California and drove home to Missouri. I got the car and drove it the few blocks to my daughters home in Murietta. It ran very rough. Then it would not start. I traced this to the ignition coil, no sparks at the coil or plugs. Replaced this and drove home to MO. Took it to my Mechanic to get it ready for inspection and find the rough run and idle problem and again it would not start. Incidently, this was a 3 day trip and I stopped and started the car at least 20 times.
As it stands right now, it starts sometimes and sometimes not. Plugs and plug wires have been replaced and when it runs it runs smoothly but idle might vary from time to time from 500 to 1200 rpm.
Also, I noted on the drive home that the tach was very erratic. It would go along for awhile at 2700 at 70mph (this is a 4speed) and suddenly go to 3500 to 4000 but no rpm change had occured with the engine. Then just as suddenly it would retlurn to the 2700 mark. Right now with the engine running, if you tap the horn the tach goes bannanas. There is a MB service bulletin that addresses the tach and notes that the starting problem I am describing could be the tach. We have checked all pertinant voltages etc and now need to disconnect the tach to see what happens. I am assuming that these tachs have some kind of overrev protection and that this might be malfunctioning.
A tach is a $400+ item. Is it possible that we are overlooking something. I would really appreciate some input and suggestions from my MB friends. I read with interest BILL17s post regarding a similar problem. I havent looked at the interior capacitor, could this be the problem, hmmmmm! Thanks for any help, Lee


Thanks Lee

Doubt that the tach is a problem! HOWEVER the ignition ECU can cause that problem & I have replaced 100's of them on 1981-85 V-8's!

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Doc
Thanks so much for your speedy reply. Question: The unit you refer to as the "ECU", might it also be known as the Ignition Switching Unit or Ignition Control Unit? There is a grey metal "box" below the coil that has wires going to the distributer, the coil, the fuel pump relay by way of the diagnostic plug and the tach, and to ground. Is this what you are refering to. Thanks again: Lee
I like older MBs that I can afford, I'm 67
1979 450SL sold
1979 450SLC Totalled, no one hurt. Another reason for liking MBs
1980 450SLC 5.0 Euro This one is a long term project as money is available.
1988 300SE
1988 560SEC

Anyway, Now the car will start with a little help. Here's how: If I try to start it and it doesn't fire instantly, I turn the key off then turn it back to start and toot the horn at the same time. It starts immeadiatly!!!! (Truth) I guess the good news is that at least in the short term I'm less likely to be stuck. Has anybody heard of this one before???

the same problem. first i had to honk the horn, then i had to honk the horn and roll the back windows down at the same time, then i had to pull on my left ear ....just kidding.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Thanks M.B.Doc. The car quit starting horn or no horn. I replaced the ECU (ignition control unit) and the car has started and ran, no problem ever since. You were right and my thanks. Lee