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450 SL D Jet - running weak / possible MPS failure

1K views 4 replies 3 participants last post by  cushjbc 
#1 ·
My 73 450 SL developed a "weak running" condition after not starting it for 3 months.

Some data:

Distributor cap and rotor, spark plugs and wires new.

Engine starts immediately, checked Ohm values for air and coolant temperature, both perfect.

I've renewed all the hoses at the AAV and had the AAV refurbished (was stuck open and running at very high rpms).

No vacuum leaks.

I assume it cannot be the trigger points, as it would run "richer" as they wear out.

Could the reason be a failing MPS, what are the symptoms of a failed or malfunctioning MPS?

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#2 ·
I don't know what symptoms a failed MPS exhibits other than running poorly as you describe. Have you tested your MPS ability to hold vacuum? The other test involves measuring resistance between terminals on the MPS:

350 ohms between terminal 8 and 10
90 ohms between terminal 7 and 15

Has you MPS been tampered with (adjustment screw cover missing)? I have experimented with several MPSs that have been tampered with comparing them to 2 on hand that have not been tampered with. In each case, the tampered with MPS had been adjusted by a huge amount in the wrong direction for normal wear and tear - tighter (clockwise) is not better on this device.

The very last tuning item that should be attempted on a D-Jet system is the MPS. It should only be made when absolutely everything else is in specifications. The fine tuning adjustment in this case would be a very slight (1/32) counterclockwise turn on a high mileage engine.

The MPS produces an inductance signal between the coils to the ECU. 5 years ago, I made inductance measurements on the 2 factory set MPSs I own. I would love to have a bigger sample size for this. I did this at ambient pressure, -4"Hg and -15"Hg to form a baseline for adjustment of mishandled MPSs. I then corrected that data for altitude since I am at 6000' in ABQ. I was able to revive 2 MPSs back to a decent operating status. That data is attached.

If your MPS passes a vacuum test and the resistance on the coils posted above is in spec, evaluate the possibility of your MPS being improperly adjusted. Source an inductance meter and have a look at your MPS against these numbers.

I am away from home at the moment. I cannot remember my method for taking the readings - across the coils as the resistance test is done, or between the coils 7 to 8; same as 10 to 15. I would have to put that meter on my spare to verify. The readings I took are packaged with each spare I have on hand so I can figure it out.
 

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#3 ·
Thank you for the reply.

I will measure the values at the MPS and post the results.

The MPS has not been tempered with - it was running perfectly last time I started it 4 months ago.

It is probably related to fuel supply, but I'm not sure. Where should I test for fuel pressure and delivery and how can I short the fuel pump so it runs with the engine off?



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#4 ·
Here are the values I got for secondary coil inductance following cushjbc's method (measuring between terminals 8 and 10 if I interpreted it right):

Secondary Coil Inductance (8 to 10)
Ambient 1.087
-4 in HG .961
-5 in HG .933
-10 in HG .800
-15 in HG .691 (OK)
 
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