no spark driving me crazy!

[johnk]

After a three year restoration in my brothers barn I am more than ready to get my 280 back to my garage to finish it up but I can't get any spark to get the engine started. For some background, I did disassembled some portions of the distributor in order to paint it before engine assembly. Initially I was able to start it with a lot of bangs and pops, but didn't go through the tune up process because I didn't have all the pulleys needed for installing AC. When I went to start the tune-up process several weeks later I couldn't get it to start. It would kick in and die out right away, and now it gets no spark to kick in at all. I did discover that I had the coil leads backwards, but since replaced the points and the coil.

 - When I hold the coil wire a half inch above the distributor and run a wire from the positive battery terminal to the positive coil terminal I get no spark to the distributor.
- I installed a new coil, rotor points and distributor insulator for the ground terminal. The coil is not a Bosch if that matters
- I do not get continuity from the points to the ground terminal when the points are open and the negative coil lead is disconnected
- with the ignition on and the points closed I get 7.4 volts between ground and the positive coil terminal, and 12.5 volts when the points are open. This difference is opposite of what the tech manual says they should be
- with the points open the voltage between ground and coil+ fluctuates wildly for about 20 seconds and holds steady at 12.4 volts for as long as I hold it (at least several minutes)
- I completely removed the condensers from the coil and distributor with no difference
- get a good spark at the coil if I run a wire from ground to the ground terminal of the coil

I'm at a loss here and appreciate any help. I had a friend who is an experianced mechanic look at it for me and his only find was to make sure the coil/resistor terminal on the distributor was not grounding to the distributor body. Unfortunately my problem didn't go away  once I installed a new insulator.

Thanks gents

John

[prefervintage]

Hi John...I went crazy one time when my 1970 280SE started having all kinds of ignition problems, until I found out that MB had used a whacky early for of electronic ignition where the points were used not to toggle the coil directly, but to activate a power transistor to ground...I removed it and installed a Pertronix ignition and it has never run better. Because you  may not want to go that route, or have time to order parts, can you just get some alligator clip wires (cheap packs are at Harbor Freight) try running +12 to the coil and the other side momentarily to ground, with the high tension output wire laying about 1/4" from any ground surface and see if you get a spark? Then move on to trying the same thing through your distributor and the point, maybe rocking it so the lobes open and close the points? I usually run some fine emery cloth over the contacts of the points to make sure they are clean and make contact. If that is good, then try connecting the coil positive to your ignition circuit...if it doesn't work, you'll need to verify that the ballast resistor is good, obvious stuff like the fuse, relays and such...don't run straight from the battery long without a ballast resistor or it may draw too much current and cook the coil. (A Pertronix coil can be run without a ballast however.) A good thing to have is a portable ignition aid...this is a self-contained high voltage source you can just hook up to a battery and connect the output lead to your distributor and crank your engine to start. I keep one in every older car I have as an emergency backup...you can find them from time to time on ebay...you'll have to back off your timing, and without an advance system your car won't run great, but you can start and get home, and of course, run it to troubleshoot. I've attached a photo of one. Good luck with your car!

[johnk]

Thanks prefer vintage.

Perhaps that is why I am not seeing spark at the coil. The section in the tech manual about checking the distributor set up says to connect the coil+ to the battery + which is what I did. The electrical part is my weakness but it does seem that I should be connecting the ground to the positive terminal to complete the circuit and ignite the coil.  I will start with that tomorrow. 

[johnk]

I tried touching wire from ground to coil + and still no spark at the coil.

I also reconfirmed that I get 7 volts between coil + and ground when the point gap is closed and 12 when it’s open. Weird.

It has to be something really simple that I am missing.

[MikeSimon]

It has to be something really simple that I am missing.

Do ypur contact breakers open?

[Benz Dr.]

If you still have all of the original ignition equipment in your car the coil terminals work differently than other systems. On earlier systems, 12 volts goes to the positive side of the coil and the negative side goes to the distributor. On the later system, the positive terminal goes to the ECU and negative goes to ground. Maybe someone else can explain this better but there is definitely a difference in how the two systems work.

[johnk]

Yes the points do open and close if I understand mike’s question

Mine must be the earlier system as the negative terminal on the coil is connected to the distributor. That is what I messed up at first.

Thanks gents

[Benz Dr.]

If your car was built in 1970 it should have an ECU unless it's a Euro model. Best figure out what system you have first.

[johnk]

Early 1968 euro model

[johnk]

With 051 distributor

[Benz Dr.]

OK, that narrows it down. The positive terminal on the coil should go to 12 volts and the negative side goes to your distributor. I've seen where the screw going through the distributor body was touching the housing and shorting out. If you open and close the points with a screwdriver, that should make the coil fire if the points are closed before your test.  If you have voltage on the moving side of the points, then you could have dirt or a film on the point surface because the points have to make good contact or they won't work. 

[stickandrudderman]

There's a really easy way to envisage how the ignition system works:
A voltage is applied to the coil at the "+" terminal and the other terminal goes to earth via a switch. (The switch is the contact breakers).
A magnetic field is set up inside the coil when this circuit is made. When the circuit is broken (by a switch on the earth side opening) the magnetic field collapses causing a high voltage to be generated by the coil.
So, to check if a coil is working you simply lift the king lead out a cm or so, confirm 12V at the "+" terminal (ignition on or just a jumper from the battery "+") use a jumper cable to apply and then release an earth (-) connection at the other terminal. You should see a spark at the coil.
If you do then you know that your coil is working and you have a switching problem (the points or thereabouts). If you don't and you've confirmed you've got 12V at the "+" terminal, then your coil is defective.

[johnk]

Thanks stick. I thinks at this point it is my coil. I am getting some
Spark now but it’s very week. I bought a new one at the auto parts store but I’m not sure it’s the right fit for the car. Is a generic coil.

[johnk]

So given I have an early 280sl with the 051 distributor am I correct that I need the red Bosch Non- transistorized coil with the 1.8 ballast resistor?  All I can find are The res transistorized coils so far.

[mrfatboy]

I think it might be this one but double check.

https://www.autohausaz.com/pn/00013

[johnk]

Thanks!  When I went to their site and searched on coils it only brought up the transistorized version.

I will order now along with the 1.8 ohm ballast resistor.

Fingers crossed!

[Benz Dr.]

So given I have an early 280sl with the 051 distributor am I correct that I need the red Bosch Non- transistorized coil with the 1.8 ballast resistor?  All I can find are The res transistorized coils so far.

That's what I use. The hotter 26 KV coil ( twice the voltage of the original black one ) also needs twice the ballast resistor rating at 1.8 ohms.