Joe,
If you have a solenoid with 2 terminals at the end, then the coil inside is isolated from the metal casing.
On my 230SL, the previous owner removed the solenoid (what on earth for...
) and fitted a cover plate on the injection pump.
Also cut the wires to said solenoid and taped it off to fit a really stupid switch under the dash for the CSV only.
Here's what I did when I eventually found a used and fortunately working solenoid:
a) remade the small wiring harness for the solenoid
b) using separate wires, connected the solenoid to the battery to test polarity and stroke
c) I alternated the position of the positive and negative wires relative to the terminals
The combination that gave the best "push" of the plunger was marked and the wiring harness connected accordingly.