1973 450SL D-Jet Frustration

Hi,

New member here. I have a 1973 Mercedes 450SL.

Over the one year of ownership it has run badly (fixed with new spark plugs and distributor cap), then run quite well, then deteriorated (at this point installed Pertronix electronic ignition) then ran well again, then deteriorated again, had a timing adjustment and ran OK if the air intake temp sensor was unplugged and then, after two days, became undriveable. 

All components tested check out for resistance (IAT sensor, ECT sensor, MPC sensor) and for vacuum (MPC). 

The car will idle, but anything more than idle will die, therefore making it undriveable.

I removed the distributor to inspect the trigger points. They are protected by clear plastic covers. I sprayed them with Deoxit contact cleaner and dried with compressed air. After replacing the distributor in the car it now runs even worse (there is a chance I have spoiled the timing). The car now currently idles but dies after a few seconds, or dies as soon as any throttle is applied. 

I have a lot of questions and nobody to ask, but I will start with a simple question regarding the fuel injection trigger points:

If the car will idle on all 8 cylinders is this an indication that the trigger points ore OK and that I should be looking elsewhere for issues? A local specialist suspects that the main "run" circuits of the ECU are at fault. I could throw components at the car until it works but this would be financially ruinous and not very scientific!

I am stuck here in Western Canada with very little expertise around that I can turn to. Can anyone help me diagnose the issue? 

Thanks in advance,

Jeff L

Hi Jeff,

welcome with your brother of my Goldie - a Swiss 450 SL from 1973 - to Jetronic.org .

I do understadn your frustration and we should come to isolate areas of failure and well working on your car before we go into details. I understad that you have cleaned trigger points, I do not read that you also greased them with ignitioion distributor grease. Normal grease is abd as it can melt and drop on trigger contacts.

I also read that you did measure air and engine temp sensors. Did you do so on removed ECU connector? Please do not measure on sensor itself and always include wiring as that is what ECU sees.

What I miss is fuel pressure, igntion timing at idle, 1500/min and 3000/min and verifying ignition retard. I also miss a MAP sensor leak test as described in chapter 4 of my compendium . Also what about black cap on your MAP sensor? Has it been opened or removed? I have not seen  a single untouched MAP sensor for 450 SLs from Americas in my workshops in years now. Somehow it is  a fashion to detune them.

When yur car runs at idle, you can touch all 8 injectors and feel whether they are all ticking. If they do then I would assume that trigger points are okay unless you can do a better measurement with an oscilloscope.

Please also find my checklist

Hi Volker, thank you for the quick reply!

To answer your questions in order:

- I did not yet grease the trigger points. I will buy ignition distributor grease today. To where do I apply the grease?

- I did not test the components at the ECU connector. I have the ECU removed now so will do this. I have a list of terminal functions. Is terminal #1 the TOP terminal as it is oriented in the car, or the BOTTOM?

- Fuel pressure has been adjusted to the correct amount.

- My mechanic checked the MAP for vacuum resistance - I should presume this to be "leak-down"?

- I have not investigated the MAP sensor enough to check the cap. I will do so.

- I will check the injectors by touch. I think they are OK. When throttle is increased the car dies. Can you confirm please - if the trigger points work at idle they will likely work at all engine speeds? Would their operation deteriorate with increase in engine speed?

My other problem is mechanical - I removed the distributor yesterday and I cannot be sure I put it back in the correct place. I am REALLY struggling to set the engine at TDC as there appears to be no way to put a socket on the crankshaft nut and, being an automatic, I cannot bump the car in-gear. Any advice...?!

My wife has told me I cannot work on the car today as "it makes me upset" so I will not be able to look at this until Sunday. I will report back on all these points.

Many thanks again,

Jeff L

Hi Jeff,

if there is a greasing felt, rub grease in there. If not put a little bit on the cams of TC where they touch the distributor shaft.

terminal 1 is the one most away from wiring harness

What do you mean by leak-down?

Touching injectors is to feel whether ECU operates them all. If they run at idle, TC is likely to run at all revs.

Increasing revs by throttle and then dying engine is likely a matter of wrong ignition timing or fuel mixture.

Hi Jeff,
the terminals are marked by numbers and the wires are color-coded.

You can have a look at the compendium or download this:
jetronic.org/media/kunena/attachments/687/DB_M116_350_EN.pdf

Take care the acceleration impulse is correct. Maybe the engine will die when you open the throttle and the extra fuel isn't injected.
I by myself once had a problem with this and the car wasn't driveable anymore.
You can check this function without any testing device. With ignition switch ''on' the injectors have to 'tick' when you open the throttle

Because trigger points work at idle, you can be quite sure they are fully operational - but there are conditions of wear that could prevent them from correct operation at higher rpms.
For a perfect check, I recommend my trigger-points-tool to you, you will find information here: jetronic.org/en/forum/jetronic/6-werkzeu...sdorn-gebiss?start=0

I think, you will be able to turn the engine at the belt after having removed all spark-plugs.

Best Regards
Norbert

Thank you Volker,

I will execute all your suggestions tomorrow and report back. I'm sure my current #1 problem is bad timing, due to me removing the distributor without enough care to mark its position.

To be continued...


Jeff