OK, I could not leave well enough alone, so I bought an ECU from a salvage yard. That ECU is from a '73 SLC and it has a different part number: CU36X (0 280 002 005), while mine is CU37X (0 280 002 008).

The weird thing is that when I look online, most parts list my '72's ECU as fitting '73 or '74.

How can I find out what ECU I'm supposed to have? And will the '73 ECU work in my car? As an interesting side note, both ECUs are remanufactured.
bye,

Csaba
72 280SEL 4.5

If it's an 8 cylinder ECU, it should be plug and pray.. i mean play :)

Most ecu's I've co e across have the remanufactured sticker on them. The one I was going to lend you does as well.

Let us know how it works!

I've used a 3.5 ECU on a 4.5. They are all pretty much plug & play. The 3.5 has different mapping from the 4.5. I couldn't honestly tell you the difference between one 4.5 ECU and another.

You can find the original part numbers on a site known as Bosch Tradition. It lists the Bosch part numbers for all Bosch parts used on many cars including Mercedes.

Bosch Automotive Tradition - Knowledge

If you look under Knowledge, you can find an equipment list for your car. I attached one below for a 1972 280 SEL. It is in German, but with a little guessing or help from an on-line translator, you can figure out most part descriptions.

It says that correct part for your car is 0 280 002 005. That would be same as ECU on my 72 350SL 4.5. But 008 is probably same internally.

This thread may be useful. Here is the key info:

Graham,
Thanks for the info!

Today I installed the 'new' ECU, idled the car until it was up to temp. The idle mixture plug was set all the way CCW (lean). Once the engine cooled down I pulled a plug and it was black, ie still rich.
I checked the coolant temp sensor resistance at the ECU with the engine at ~180F and it was just 395 ohms. It was 380 at the sensor. I have not checked the graph, but that sounds to be in range. I will also check the air temp sensor tomorrow.
bye,

Csaba

Does the ECU idle screw affect the idle? In other words, if you turn it the other way, is there a change in way engine idles? Reason I ask, is that if the Throttle Position Switch is not set properly, you may not be using the idle mixture control circuit. From an earlier post:
Years ago when I had fouled plugs and overly rich mixture, it turned out to be worn trigger points . As the fibre cam followers wear, the angle the points stay closed gets bigger and bigger. At a certain point, the ECU gets confused. More in this thread and elsewhere here and on BW.

check the ignition system... the resistors next to the coil... and the switch gear in the front left next to the radiator... check it. make sure its sparking bright, even just ground the coil wire to the valve cover and look...

Graham,

>Does the ECU idle screw affect the idle?

Yes, if I richen the idle mixture from full lean the idle speed drop significantly.

> it turned out to be worn trigger points

I checked them when the engine was apart ~500 miles ago, and their dwell seemed well within spec. I do have two other used sets. One looks brand new, but has a different style connector in it, the other is just used. I'll see if swapping it out makes a difference.

Harry,
>check the ignition system...

It is converted to Pertronix. It gives off a nice strong spark.
thanks,

Csaba

We should have an on-line check-list for D-Jets. It is hard to remember just what has already been done or suggested.

But going on. Injection system things to check:
- Fuel pressure in rails. Spec is 28-29psig, but 32-34 is better for hot starts. But this gives richer mixture. Need AFR or co meter to readjust.
- Have injectors been calibrated or at least checked to have design flows?

Sorry about the delay, I've been busy...

>But going on. Injection system things to check:
- Fuel pressure in rails. Spec is 28-29psig, but 32-34 is better for hot starts. But this gives richer mixture. Need AFR or co meter to readjust.
- Have injectors been calibrated or at least checked to have design flows?

Yes and yes. Fuel press is 29psig and the injectors were reman'd while the engine was apart.
I checked both fuel and air temp sensors and both are within range. I checked the dwell of all 4 trigger points and they are all on for 120-140 degrees of rotation. I replaced the hose going to the MAP.
I have a spare MAP but I have not had time to swap them. Maybe on Sunday.
Thanks,

Csaba

May have found it
 

I have been re-testing things, among them the MPS. After I tested it the car would not start, the same symptoms as before. Then I noticed I had forgotten the plug the MPS back in! Once plugged in the car started right up. So I wonder if I had an intermittent connection at the MPS all along. Only time will tell.

Csaba
72 280SEL 4.5