450 SEL M117 D-jetronic running badly

Hello all!

As I wrote in my introduction I am having trouble, since spring this year, with an engine that suddenly started to hesitate and stutter. I can also mention that I replaced the spark plugs (bougies) with NGK BP6ES last year June and the car was running very well after that.

My first thought was - as usual - some moisture under the distributor cap is the root cause for bad ignition. So I checked, cleaned and sanded the distributor cap copper surfaces carefully with sand paper 800 and a little 5-56. Also very carefully brushed the tip of the rotor. Cleaned and degreased but all this did not help very much.

My son suggested buying starting gas to rule out either fuel distribution or ignition system.
While spraying starter gas straight into the throttle body (air cleaner removed) the car did run on all cylinders. So the ignition system was not the problem.

I can't remember exactly how but at some point I came aware of the "trigger points" system in the bottom of the distributor body of the D-jetronic system. I have been reading a lot on bad running engines on Peachparts, W116.org, Benzworld and also this place, jetronic.org.

So I decided to remove the distributor body for the first time in my history. But most important: first I carefully documented the exact position of the bottom screw in the slot on the bottom of the distributor AND the rotational position of the rotor arm, BEFORE you start loosening this screw. Make pictures straight from above on the distribitor (without cap) or mark with something that lasts and can be cleaned away later.
The biggest surprise was that - after careful documentation of the rotor arm position - the rotor arm started to rotate counter clock wise while lifting the distributor slowly upwards. This because of the angled teeth of the distributor shaft.
I made a picture of the rotor arm rotated, to know how it should be positioned when I put the distributor body back on the engine. The rotor arm will rotate back to its original position. There are only twenty teeth on the distributor shaft so this is not so difficult as long as you have a good picture of the original position. WARNING: when you put the distributor back the slot postion needs also be very close to original, to determine if the rotor has the same position again.
Do NOT rotate the engine OR start it while the distributor is out! You will lose your original timing settings!

Plug or cover the hole in the engine block (where the distributor was in) so that anything that falls down can not end up in that hole.

Next challenge was to loosen the contact breaker ground cable from the dist body. In my case the distributor has the trigger points assembly with the short cable and 2+3 pin connector. The overmold on the trigger points assembly is often in a bad state: cracked or totally disintegrated. Mine was okay but I taped som electrical tape around it to hold it together while handing the whole dist assembly.
The screw for loosening the ground lug on the breaker point cable is directly behind this trigger point cable overmold, so I decided to take out the trigger points assembly first to get at the ground lug screw. This means you have to hold the distributor and put it on its side between the left valve cover and the top of the self levelling suspension reservoir lid.

When the breaker point cable shoe is disconnected (black and white cable, carefully with small pliers) unscrew the ground lug from the same cable assembly and take away the distributor. Be careful not to lose any screws, washers while doing this.

The Trigger Points assembly was carefully inspected and it looked quite bad, there was swarf on the cams meaning some friction has been there (do not know for how long). I cleaned with isopropanol and a bunch of ear "tops". I also cleaned the cam lobes on the shaft with acetone and "ear cottons". Now I discovered a light surface corrosion on the cam lobes). Regreased the lobes lightly(!)  with Bosch Zündkontakt Fett Ft1v4 (45 ml tube). Smeared some of the same grease on the breaker point lobes on top of the distributor.
         

Next I measured the continuity (Multi-meter) between the pins of the connector to the metal springs of the assembly (4 pcs). As the trigger points assembly did not have an axle in between everything should be connected to each other. But pin #3 did not have a connection to its spring but it was not the cable or soldering. It was the points not connecting. I cleaned this with Electronic cleaner until there was a connection again (lots of cleaner needed, because the clear plastic protection cover is in the way).
I cleaned also the three other points just to be sure and measured again: all the pins where connected in between.

Next step was to make a circular plastic lid (93 mm diam.) with a hole for the rotor and draw a 360 degrees scale on that ( 10 degrees interval plus 45, 135, 225 and 315 degrees). That would be precise enough I figured.


So with the Trigger Points assembly back in the distributor body I measured continuity while rotating the shaft slowly. This I did for all 4 pins (1,3,4,6) towards pin 2 (ground).
The first results were disappointing: pin#1 hardly broke connection, somewhere between 105-110 degrees and between 165-170. So if pin #3 did not give a trigger because of dirt and pin #1 had this intermittant behaviour that would not give enough gasoline to keep the engine running I assume. But the other 3 pins where okay with only one trigger (when points make contact) per evolution. Note that you should rotate the rotor clockwise to simulate the direction of the engine running.

Graphical results of triggers in channel (pin #1, #3, #4, #6) over one revolution of the distributor axle:
 

I took out the Trigger Points assembly and put it back again just to see how sensitive this system is regarding mounting tolerances. Results were now somewhat better for pin #1. The other pins where all within 5 degrees from the first measurement(!). So strange that only the trigger points connected to pin#1 behave so different this time?
Results:
 

 

Unfortunately I was unable to insert more pictures above.

The plastic lid was made of a small 93 mm plastic container with some sauce. The pointer is made of a tooth pick and some tape. I tried to get the pointer aligned with the centre of the rotor arm to get correct angle readings.
 

The sketch I made to figure out what was connected to what is inserted below:
 

After bending a little on the trigger point plate/bleche connected to pin #1, the graphical results are as following:
 

Now there is some new wear buffer on Trigger point connected to pin #1 and the moment of trigger is still quite the same.
I will wait for Norberts measuring gauge to do this work properly.

The engine starts again, because now at least all injector pairs are fireing again. But it does not run very smooth yet unfortunately.
And I have to do something about the surface of the trigger point lobes on the distributor shaft.