Can I turn on glow plugs during idle to do a test? If yes, how?
 

My SDL is running rough at idle sometimes with a slight knocking sound. I suspect it may be incomplete combustion or bad spray pattern. It always runs smooth during the 'after-glow' period ( I installed the 'after-glow' circuit with info from this forum ) then it runs rough sometimes until the engine is hot. I just want to do a test to turn on the glow plugs while it is running rough to see what difference does it make. It would only be a temporary test.

Any simple method?

an even better test would be to get some #14 wire, and one by one insert it into each glow plug in the harness, and see if only certain cylinders smooth out the idle...
hook the wire to the positive terminal on the battery, then pull the harness for the glow plugs off the relay, push the wire into each numbered socket and see if the idle smooths out.