I was told there are three different sensors involved with the glow plugs, ranging from expensive to outrageous, and installation ranging from easy to impossible. They kept malfunctioning in my '85 6.9, resulting in glow plugs getting energized at 70 MPH on the freeway, and burning them up. After a couple rounds of that at about $300 a pop, I wised up and bypassed the sensors completely, and just ran #10 wires from the big poles on the relay to a push button switch that I mounted under the dash. Press it for about 5 seconds when engine is cold, if truck doesn't start, repeat. End of problem. You'll drive yourself (a) crazy and (b) to the poorhouse trying to keep sensors in that rig. Just do the pushbutton and you'll be fine.
Also, there is a procedure for checking the glowplugs in place, with an ohm meter- I think good ones will show 2 ohms from tip to ground, bad ones will show open circuit (infinite ohms). I've got the Navistar service manual out in the shop- I'll go confirm that if I can work up the energy.