Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
Hi, my name is***** you are describing is low voltage to the ignition system. If it doesn't start while you are turning the key but makes an attempt to start when you release the key you should check the voltage to the coil. Low voltage will act this way. When you release the key you get a little voltage spike and it tries to fire off. If the car runs okay once you get it started I think you will find a low voltage problem.
Bad ballast resistor, bad ignition switch, bad wiring or incorrect wiring.
You should first be doing voltage checks at the coil with a voltmeter. If you have low voltage to the coil you need to work your way back towards the switch and see where you are losing your voltage. Power to the coil goes through two ballast resistors. You should have 12V into the first resistor from the switch. When you crank the motor the voltage bypasses the first resistor. The second resistor gets 12V from the starter when cranking for extra voltage. If someone didn't wire the starter correctly you may not be getting the extra voltage when cranking.
Nothing in the switch that you can check while it's apart. You need to reconnect it in and turn the key on and measure the voltage to the 0.4 ohm ballast resistor. It should be pretty close to battery voltage.
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Yes those are the resistors. You should have 12V into the first resistor (0.4 ohm) and you should have 12V cranking into the second resistor (0.6 ohm). You can put a jumper wire across the terminals on each resistor then see if it will start.
Ok, check the voltage going to the resistors and make sure you have battery voltage. Incorrect ignition timing could also cause this problem. A bad distributor cap would also do it. Anywhere you have excessive air gap it could cause weak spark. You should also make sure that you have fuel pressure when cranking.
Excessive plug gap could also cause the problem.
Too much gap for the spark to jump. You could check the spark out of the coil when cranking. You should have a good strong blue spark that can jump 1/4" or more. If the center button of the distributor cap is worn out it would give you too much air gap between the cap and the rotor. If the timing was incorrect you would have too much air gap between the rotor and the distributor cap terminals. I will be around until about 8 PM EST but I will be tending to trick or treaters.
I wonder if your ignition control module has failed. Have you checked for spark from the coil?
Have you checked for spark to the spark plugs?
Have you tried starting it by spraying a little ether into the intake while cranking the motor?
Can you post your vin number? I want to see exactly which ignition and fuel system you have.
I will be around most of the day tomorrow.
So it's a carbureted model. You can trickle a little fuel into the throat of the carburetor and see if it tries to start. It still sounds like more of an ignition problem to me. Make sure the spark plugs are good and are gapped at about .028". Check spark at the coil and plugs. Inspect the center button of the distributor cap.
I'll be around tomorrow in the evening. Let me know how you make out.
You're quite welcome.
It's possible, you should remove the cap and inspect the center carbon brush.
The cap looks fine but the points look bad. Try cleaning the points with a clean cloth and alcohol.
The ignition points initiate the spark to the coil. I'd install a new set of points and make sure they are gapped properly or use a dwell meter to set the dwell. Low ignition system voltage when cranking is what would typically cause this problem.
Just checking in. Did you ever get this squared away?