This article is from the Vietnam FAQ, by Brian Ross, John R. Tegtmeier, Edwin E. Moise, Frank Vaughan, John Tegtmeier with numerous contributions by others.
Detailed knowledge of the system required a much higher pay grade than
I had. However, here is what I think I know: it was a device that
would pick up the electronic "noise" of a spark crossing the gap on a
spark plug. I believe that it could work as far away as the horizon,
and I recall the BC operator vectoring us into to convoys. A good
operator could also let us know if the trucks were new or old, and if
they were moving or idling, simply based on the quality of the signal.
It certainly did not pinpoint a truck for us, but got us in the