What is this?
 

Round thing with electrical connections mounted on the driver's side fenderwell, under the hood.
Thanks

that's the diagnostic port

xp

If you're talking about your '83 SD, I beg to differ with xp190. There is no diagnostic hook-up on your car car. The part in question is the "tach amp". If you remove it, the tachometer won't work. If the tachometer doesn't work, that part is a common cause for this problem.

Yes, this is from the 83 300sd.....
How can there be a diagnostic port when here is no ECM?

sorry I was only going on what I know about my car :)

xp

It diagnoses RPM.

yep in my old 300SD this little jimmy came loose a few times and I had to tighten her back up to get my tach going. :cool: