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Marty
Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53166
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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1976 280, that wasn't supposed to send yet, 1976 280 turn

Customer Question

1976 280
JA: What is the model of your '76 Mercedes?
Customer: that wasn't supposed to send yet
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: 1976 280 turn key to start rotates fine when key is released it very quickly sounds like it is going to start, turns over but not enough to start. I got it to start like this a couple of times.
JA: Can you turn the steering wheel at all? And what about the key? Is it worn or damaged?
Customer: is this a person somehow automated
JA: I'm the Mercedes Mechanic's Assistant. I work with them to help customers like you.
Customer: how are you responding so quickly?
JA: Anything else you want the mechanic to know before I connect you?
Customer: ignition switch turns fine
Submitted: 1 month ago.
Category: Mercedes
Expert:  Marty replied 1 month ago.

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.

Customer: replied 1 month ago.
the battery is fine what would cause low voltage to the coil
Expert:  Marty replied 1 month ago.

Bad ballast resistor, bad ignition switch, bad wiring or incorrect wiring.

Customer: replied 1 month ago.
how can i determine which it is? i have the ignition switch apart i that helps
Expert:  Marty replied 1 month ago.

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.

Customer: replied 1 month ago.
okay I can check that. There was no work done on the car at least a month before this started. Alternator was replaced a few months ago but not starter.
Customer: replied 1 month ago.
is there anything inside the switch I should check or just put it back together?
Expert:  Marty replied 1 month ago.

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.

Expert:  Marty replied 1 month ago.

Let me know if you need anything else. Please remember to rate my service in order for the website to credit me for helping you.

Customer: replied 1 month ago.
switch is back together sent a picture are these the resistors
Expert:  Marty replied 1 month ago.

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.

Customer: replied 1 month ago.
12 copper jumper across both-does the same thing, doesn't start, tries when key is let go.
Expert:  Marty replied 1 month ago.

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.

Expert:  Marty replied 1 month ago.

Excessive plug gap could also cause the problem.

Customer: replied 1 month ago.
battery voltage is fine as well as fuel pressure. how long are you available? what do you mean by excessive air gap
Expert:  Marty replied 1 month ago.

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.

Customer: replied 1 month ago.
Back story just in case it matters. I was driving the car and it died, when I tried to start it would turn over but would not fire up. Eventually it did start. Drove for about 5 minutes when it died again would not start after trying a short time. It was late so I left it. The next day when I tried to start it, it took a few tries but it did start. I drove it to my destination, stopped turned it off, then I turn it on and off 4 or 5 times never hesitated to fire up. A few days later I went back to it, same story, wouldn't start finally did drove it home - about 10 - 15 min. Since then I've tried a dozens of times but will not start. Any new ideas after this story? If I keep this page open will you be available tomorrow evening?
Expert:  Marty replied 1 month ago.

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.

Customer: replied 1 month ago.
I haven't and probably won't be able to get back to it tonight that's why I asked about tomorrow evening. I was off work today,
going back tomorrow. Maybe beck to it about 4:00 CT. VIN - 11406012121155. As for as the fuel system, the person I got it from replaced the original carburation system with a Weber. I have no idea how it was but it did run well.
Expert:  Marty replied 1 month ago.

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.

Customer: replied 1 month ago.
Okay, thanks.
Expert:  Marty replied 1 month ago.

You're quite welcome.

Customer: replied 1 month ago.
hello again, i check from the coil to the distributor, i got a spark, so I'm guessing it"s in the cap.
Expert:  Marty replied 1 month ago.

It's possible, you should remove the cap and inspect the center carbon brush.

Customer: replied 1 month ago.
I'm not sure, what do you think
Customer: replied 1 month ago.
Changed my mind. Put a screwdriver in the coil with the plug next to, only spark was when I let go of the key.
Expert:  Marty replied 1 month ago.

The cap looks fine but the points look bad. Try cleaning the points with a clean cloth and alcohol.

Customer: replied 1 month ago.
cleaned the points, not much came off, didn't change. What would prevent the coil from providing current to the distributor when the key is in the start position and only a spark when the key is let go to the run position?
Expert:  Marty replied 1 month ago.

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.

Expert:  Marty replied 24 days ago.

Just checking in. Did you ever get this squared away?