1974 Mercedes Benz 450sl Car turns over but will not start

Tiny
BUCK63I
  • MEMBER
  • 1974 MERCEDES BENZ 450SL
  • V8
  • 2WD
  • AUTOMATIC
  • 117,000 MILES
I have been chasing a starting problem for several months. The car would turn over but would not start, as though there was no spark. My local Mercedes Club technician finally traced the problem to the transistorized ignition control module (the car would start and run fine when a condenser was used in place of the module). The module was replaced, along with replacing the new Bosch coil with an older available coil (the new Bosch was returned to the auto parts store because it was said to be bad). The car ran fine for a week, starting every day with no problem. After sitting without starting it for a couple of days, it would not start. It would act like it had spark, but not sufficiently to catch and run.

Eventually I replaced the coil with an available coil I had run the car with for four years. The car started immediately, ran for that day and the next day with several starts and stops. After sitting for two days, the car would not start - acting like there was no spark at all going to the cylinders. Last resort was to replace my old four-year running coil with another new Bosch coil, with no apparent success in spark to the cylinders.

I am wondering if I should go back to a points-and-condenser setup on this car and avoid the transistor setup, resistors, etc, or is there some kind of intermittent circuit problem that is causing this? I don't have enough money to continually chase a ghost of a problem, and AAA is getting tired of my free towing calls.
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Sunday, November 15th, 2009 AT 4:47 PM

5 Replies

Tiny
SERVICE WRITER
  • EXPERT
Get access to a spark tester and plug it in. Do you really have spark or not?

Post back and we will pick up from there.
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Wednesday, November 18th, 2009 AT 8:03 PM
Tiny
BUCK63I
  • MEMBER
The only spark tester I have is a timing light. However, I tried again last weekend to start the car, and got some response; a couple of cylinders would fire, but not enough to catch and run. I put a voltage tester across the coil and have voltage in both the start position and the run position, so the ignition switch can be ruled out. I tried 5 or 6 times to start, but with no success. The engine then gets loaded with gas and won't fire at all.

Any suggestions as to what to try next? A new coil and a new ignition control module (plus new points within the last 100 miles or so) makes me wonder if there's not something else wrong.
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Tuesday, November 24th, 2009 AT 3:53 PM
Tiny
SERVICE WRITER
  • EXPERT
After re-reading, I question the new module or wiring at the module. I might try a different brand than bosch. I have neve been a fan of bosch electronics for the most part. I would stay away from Autozone for electrical parts.

Something may be causing the module to go bad or a connection problem may be part of this. Weak plugs and or wires can take out a module as it is requiring more energy to provide spark. Check wiring going to the module as well for faults.
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Wednesday, November 25th, 2009 AT 4:42 AM
Tiny
BUCK63I
  • MEMBER
Finally got the car started yesterday, just by waiting on it to decide to cooperate, apparently. Your mention of weak wires is a possibility, I replaced the ignition wires 4-1/2 years ago. The ignition control module is brand new, as is the coil, points, etc. I now suspect moisture or weak wiring, and am investigating electronic ignition to get a hotter spark, if I can find a kit compatible with the 1974 SL. Thanks for your ideas, even though it didn't seem that many of them were applicable.
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Sunday, November 29th, 2009 AT 4:51 PM
Tiny
SERVICE WRITER
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Working on a 35 year old car with no test equipment does make it a little tough.
If you suspect that moisture is a possible cause, you can mist water onto the wires and watch how it reacts to it.

BTW, today we just replaced a bad coil that was just replaced recently, last week 2 starters that were bad out of the box. I would still be suspicous of that module.

You can wiggle test wiring going to the module while it is running to see if it stalls to help flush out a problem.
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Monday, November 30th, 2009 AT 6:05 PM

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