Some injection pumps have them, some dont.
 

Injection pump on a 300sd has a small device on the injection pump ,it rest between the block and the pump and has a thick wire that runs to the interior somewhere.My other cars lack this piece ,what is it and where does it go?

What color is the wire?

and/or can you post a picture of it?

What year?

My 83 does not have said wire.

its a rack sensor for emissions control.

there is some speculation that you can adjust the idle with it by opening the passenger kickpanel and adjusting a pinned cap with numbers on it.

I don't know how valid that is though.

It's probably an 85 and the device is probably the same as is on an 85D It's a rack position sensor that controls the EGR and Klima relay. On the D it feeds to a computer on the right side of the passenger footwell behind the kick panel. It cuts out the AC compressor under heavy acceleration. Don't know what it does to the EGR.

Superficially it sounds plausable. Nice to know.

What year 300SD is it? If it is 86 or newer then it is probably the idle adjustment.

1985 ,thought I had that ,have an 84 300d with it also.

Anyone know how this interacts with the EGR?

I just opened one.

It cannot affect the rack at all.

glad thats finally solved!

My 84 CD doesn't have that either, are these California cars?

Don't think it has anything to do with California. I think all 85 D's had them. Maybe it extended back to late 84. I think they just open and close the EGR and stop and start the AC compressor based on load on the engine.

This too sounds plausable and adjustment may still be benificial for some users. Depends on the range of adjustment I supose.

Does it look like it just does this based on rack position or are other factors involved? I guess what I mean is, is it just a limit switch?

YEs if you remove the egr it just disables it-you could even disconnect the wire ath the harness connection but theres no reason to-and also the part about the klima cutting out the compressor on acceleration--it gets the signal from the micro switch on the valve cover.

Theres a good thread about removing all the extra garbage on the left fenderwell when U remove the egr--and a vacuum like rerouting diagram.