Djet Weak Spark - PeachParts Mercedes-Benz Forum

knarf · #1 06-11-2013, 12:07 AM

Greetings Forum Members,

I've been a member for over 11 years but had to take a break to set up my garage for doing some serious body restoration work on my 30+ year old
107's.
I recently purchased a 1973 450SL to use as a driver or parts car depending on how much it will cost to get it in road worthy condition. Being a Northeast car It has rust issues enhanced by what I 'm discovering was a butchered amateur restoration.
I tuned the engine a few weeks ago and it was running strong. I did not put it on the road due to needed suspension work. Now that I have completed the suspension I cannot get the car to start.
The pump comes on, the ECU relay engages(clicks), engine turns over strong but the spark observed from the coil wire seems to be too weak to fire the plugs.
I swapped out a good coil from another car. Still only weak sparking when engaging the starter.
At one point during my trouble shooting, with the ignition in run position, the two resistors next to the coil got VERY hot. Anyone experienced with this issue? Bad ignition module? Please help.

Thanks,

Knarf

97 SL320 · #2 06-11-2013, 05:50 PM

Key on engine not running can load the coil making the resistors hot. Do you have a wiring diagram for the ignition system?

Are you checking the spark at the coil or at the plugs?

knarf · #3 06-11-2013, 07:01 PM

Thanks for replying 97 320sl,

I do have a wiring diagram for the car.
I was checking the spark directly from the coil.
It should be a strong snapping high voltage spark directly from the coil.
Not a weak clicking spark as was discovered.

97 SL320 · #4 06-11-2013, 09:17 PM

Are you sure this car has electronic ignition? ( I don't know these cars well enough to be sure. ) If not, dirty points / bad capacitor can cause this and, point cars don't have very strong spark even when working "well".

If there is a ballast resistor, it might have a cranking bypass that isn't working.

Also, early electronic ignitions were just point replacement systems so spark is still kind of weak.

knarf · #5 06-11-2013, 10:07 PM

There are 2 external ballast resistors in the ignition system.
It's the early D Jetronic system.
As I mentioned before the engine was running strong before
the suspension work and new water pump. I'm pretty sure the distributor was reinstalled correctly. I just came in from measuring voltages across the
ballasts and inspecting the points per a troubleshooting guide. Again, I have a steady on off spark to ground from the coil wire while cranking the engine. I have an intermittent spark from a pulled plug wire that I attached a spark plug to. The same steady spark that I see from the coil wire I don't see coming from the spark plug. Also after careful inspection I see that the installed rotor is not the correct part number. I'll install the correct rotor tomorrow and see what happens.

97 SL320 · #6 06-12-2013, 05:05 PM

D jet is fuel control only, the only interconnection to the distributor will be a tach signal and 2 trigger points at the base of the distributor used for injector trigger.

" Again, I have a steady on off spark to ground from the coil wire while cranking the engine. I have an intermittent spark from a pulled plug wire that I attached a spark plug to. The same steady spark that I see from the coil wire I don't see coming from the spark plug. "

Intermittent spark from the plug is normal, it will be 1/8 the frequency of what you see at the coil.

"Also after careful inspection I see that the installed rotor is not the correct part number. I'll install the correct rotor tomorrow and see what happens. "

Given the dist was out, turn the motor to cyl 1 TDC on the compression stroke and look at rotor position it should point to plug wire 1.

Do you have the old rotor? Compare dimensions and measure resistance from the center to tip as some have a resistor that may have opened. Also check the carbon button under the distributor cap, it may be damaged.

knarf · #7 06-18-2013, 10:59 PM

Hi Again 97 SL320,

Here's the update on my no start problem. It was the timing. I was off rotation TDC by 360 degrees and had the rotor pointing to #1 cylinder lined up with the wrong make on the distributor housing. After correcting my errors the car started right up. I removed the valve cover to ensure that
the #1 valve was closed on the compression stroke. Upon removing the cover I discovered engine sludge in the recesses of the cam bolts,
rust on one of the valve springs and almost black engine oil. This was an indication to me that the engine had not been run nor the oil changed in quite a while. I thought I would have to do a valve job due to the amount of blue smoke and white smoke blowing out of the tail pipe. I changed the oil, adding Sea Foam and Marvel mystery oil to loosen anything that might be stuck and move the sludge. The smoke from the tail pipe is markedly reduced and the engine runs much smoother. I'm hoping for a miracle that the oil that I found in the coolant when I changed the water pump also will be no more.