Does the HD sending unit mount in the oil pan like the McEwan sending unit? If so, and should it become necessary, would it be feasible to retrofit by running the ground wire to frame from both the HD sending unit and HD gauge, thus saving the signal wire and the trouble of routing it again...........or would that be too dirty?
Henry, the spec sheets provided with the HD gauges do not put the sending unit in the pan. Instead placing it in any of several different locations on different year/model bikes. None of those locations are as effective or as easy as putting them in the pan. And that's what many here have done.
The HD spec sheet also has a much busier new harness than is necessary if you're replacing a pre-existing rain gauge. The new install specifies routing to the fuse panel for power, to the newly installed sending unit and on to the instrument in three legs off the supplied harness. Humbug.
At the existing gauge there's already power and ground for instrument and light. So all that is needed is the signal from the sending unit. You could take out the single wire sender, put in a two wire sender, use the signal wire and simply add a ground. Would work just fine.
You'll snip the terminal or socket (whichever it has) off the end of the old signal wire to use the two wire socket matching the new sending unit. Then just take its ground side to a good ground.