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  #1  
Old 11-02-2007, 04:22 AM
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What is a Ignition Control Unit (Swithgear)?

I have a 1973 450SL with ignition problems.

At first when I would crank the car it would cut out when the engine got warm. Now I have to spray starting fluid to start the engine, but it want start, even though it trys.

What is a Ignition Control Unit?

Will the car crank if it is bad?

How can I test the unit to see if it is bad?

Thanks for any help!!
mjolley
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  #2  
Old 11-02-2007, 08:49 AM
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Quote:
Originally Posted by mjolley View Post
I have a 1973 450SL with ignition problems.

At first when I would crank the car it would cut out when the engine got warm. Now I have to spray starting fluid to start the engine, but it want start, even though it trys.

What is a Ignition Control Unit?

Will the car crank if it is bad?
Your car has the Bosch D Jetronic system, it was used from 72-76. Under the dash on the passenger side there's a "computer" for the engine and in the engine compartment on the driver side, toward the front of the car, there's another silver box (probably dusty brown after all these years). The one in the engine compartment controls spark to the plugs. The one under the dash controls fuel mixture. The one in the engine compartment is fairly expensive, the one under the dash is horribly expensive. Your car will crank with either or both of these bad. If the one in the engine compartment is bad you won't get any spark, if the one under the dash is bad you won't get any gas.

DO NOT crank the engine with the center lead on the coil disconnected. It'll fry your ECU - the horribly expensive one.
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  #3  
Old 11-02-2007, 11:37 AM
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Quote:
Originally Posted by ScottinSoCal View Post
Your car has the Bosch D Jetronic system, it was used from 72-76. Under the dash on the passenger side there's a "computer" for the engine and in the engine compartment on the driver side, toward the front of the car, there's another silver box (probably dusty brown after all these years). The one in the engine compartment controls spark to the plugs. The one under the dash controls fuel mixture. The one in the engine compartment is fairly expensive, the one under the dash is horribly expensive. ....
On the early cars (mine is a '72), there is no "silver box" in the engine compartment!

BUT, there is a dirt grimy box located under the car in front of the driver's side front wheel, behind the bumper. It's fully exposed to the elements - Find it by looking under car in location mentioned! Or follow a cable that connects to the two resistors near the coil and runs toward front of car, exiting through a grommet hole.

This has decided me to give it a good clean! Relies on fins for cooling.
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85 300D,72 350SL, 98 E320, Outback 2.5
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Old 11-02-2007, 02:25 PM
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Location: Houston, Texas
Posts: 211
The car will crank if either one of those boxes are misbehaving.
It just won't stay running.
Have you checked crank sensor? Before I sprung for a new ICU I would check all wires, plugs, etc.
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Old 11-02-2007, 07:32 PM
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Originally Posted by mjolley View Post
I have a 1973 450SL with ignition problems.

At first when I would crank the car it would cut out when the engine got warm. Now I have to spray starting fluid to start the engine, but it want start, even though it trys.

mjolley
Have you checked the points? make sure there is a good sized gap - some say about the thickness of a business card, maybe a little more.

While you have the distributor cap off, make sure the contact points in the cap are clean as well as on the the rotor.

Make sure the plug and coil leads are properly seated.

Is fuel pump good? Shop manual gives method of checking that you have proper pressure in the injector loop.

If you have fuel & spark she should run!
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85 300D,72 350SL, 98 E320, Outback 2.5
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  #6  
Old 11-02-2007, 08:32 PM
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Test spark by cranking with the wire off the distributor and resting on the valve cover. If you see it sparking while cranking your ignition system is working. If not, something is amiss. This is the first thing to check. I doubt the switchgear is bad but, if it is, you can either:
A) Post a topic asking for one, plenty of people have spares (like me)
B) Replace your ignition system with a modern one (Petronix or Crane).

If it sparks but you're not running, check to see if your injectors are firing and you have fuel pressure at the rails.

If it's either, we can help you diagnose it with ease. Just ask!
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