W116, no spark
 

Once again the K-Jet has me confused.

Coil seems ok ( ohm test ), new cable to the distributer, no spark.

So there's two resistors ( the largish off white ones ) and a silver box that might be causing a problem.

I turn the key on, and register 12V on both sides of the resistors, and on both sides of the coil. Correct ?

The injection system type is not pertinent to the ignition problem.
When the ignition primary circuit is powered, but not connected to ground through the amplifier (silver box) 12V will be observed at all points in the circuit. Only when current is flowing, due to either a constant ground (engine not running), or a cyclical ground (engine running), will voltages lower than 12V be observed at various points in the circuit.

Try this test: remove the coil secondary lead from the distributor and position the end of it about 1/2 inch from the cylinder head or block (a ground). Crank the engine; sparks? If no, begin by checking the ignition trigger points in the distributor for opening and closing, and for a continuous circuit from the points to the amplifier plug (green coaxial cable). If there are sparks from the coil wire, but the engine does not run, begin by examining the distributor cap and rotor.

Thanks Frank, no spark from the ( new ) lead coming from the coil ( spark plug to ground test ) was the first step in my troubleshooting the not running engine.

Managed to get a chance to quickly test the voltages while someone turned over the engine today, the standard 12V dropped to under 10V at both sides of the coil and at the resistors, but I'm assuming that was just the battery being drained by the start motor, the battery itself was reading just over 10V when cranking.

This green wire from the distributor could be a possible suspect, as it was after changing the water pump in same area when this problem arose. However, I don't think that my K-Jet has "ignition points", which I should be tracing the circuit from.

... off to Google

Re: Ignition points.
In the US, 1975 450s were still equipped with D-jet, and the ignition with that system was triggered by points. European cars may have received K-jet before the US. The distributor then, may have either points or a magnetic pickup of eight segments. Remove the distributor cap and have a look.

If you were still seeing battery voltage at all points in the primary circuit while cranking, there was no interruption of the circuit, hence, either no trigger signal, or no amplification of the signal.

Thanks for following up on this one Frank.

Um, the 450SEL I have is an American version from 1975 with a 350 rear subframe and a K-Jet. Learnt all that the hard way through replacing parts through the years, sometimes more "miss" than "hit".

But, anyway, definately got the "star" type ignition trigger system, and, having Googled a little, think I am beginning to see the light.....

...... if I've got this right .....

The negative poll on the coil on the K-Jet is not grounded by default.

When running the "igniter" sends a positive charge to the bottom half of the distributor, the distributer sends a pulse back, causing the igniter to work something like a relay, grounding the coil.

Typing this at 4 a.m. on a phone, hope that made sense.

skalie:

If I may, with all respect, the type of fuel injection system has nothing to do with the ignition system. The two systems are entirely discrete, and not connected to each other in any way. Hence, mention of "K-jet" while discussing the ignition system is meaningless.

The "silver box" which you previously mentioned is a signal amplifier and a primary current controller. The "star" segments in the distributor, when rotating, pass matching pole pieces of fixed magnets in the shell of the distributor. Those pole pieces are surrounded by coils of wire. As the star passes the pole pieces the magnetic field is altered, and the change in the magnetic field induces a current in the coils. That small current reaches the amplifier via the green coaxial cable, where it is amplified by biasing a transistor to switch the primary current through the ignition coil on and off. The negative terminal of the ignition coil is connected and disconnected to and from ground through the aforementioned switching transistor. A charge is not sent to the distributor, rather, a current is generated in the distributor.

Understood, but the fact that the K-Jet and D-Jet have interchangeable ignition systems is valuable information on it's own, and not previously known by the bunch of shade tree mechanics that I troubleshoot with.

This info, however, is golden, and gave me the confidence to start pulling apart things. The green wire ( culprit #1 ) looked good, so I removed the plug from the "silver box" and lo and behold, the thing is full of water. This fits in nicely with the crime scene scenario, the car had been standing for quite a while, why the sudden loss of spark ? Rain.

Thus, next step is to find a compatible "silver box". The one I'm to replace is also from a scrapyard it would appear as it has "450 SEL" written on it's backside in marker pen.

A photo for your amusement....

http://i.imgur.com/i7XbsaU.jpg

With a Pertronix ignition system (<$100) you could throw that box in the trash. Plus you'll never have to deal with points again.

Thanks,

David

the box is readily available on ebay. Often for 30$ here in the US. The fun part is shipping it to your side of the Atlantic. Regarding the choice of replacement, go by what the connectors look like. Also make sure that the replacement box gets well grounded when you install it.

-CTH