Originally Posted by funola

The glow plug is a very low impedance device. Even though the voltage output is very low, noise is not an issue as long as you have good connections from the DVM direct to the glow plug electrode and ground (no ground loops). The readings on the DVM are very steady and do not jump around. If noise was an issue, I would expect erratic readings

During the 90 mile drive, I got out at fuel stations to switch to the other glow plugs. They all responded similarly, i.e. more engine load, more voltage. Highest voltage was -0.022 volts.

I do not have a 5 position rotary switch wired up yet, so these are readings of 5 glow plugs at idle only:

#1 -0.008 V
#2 -0.012 V
#3 -0.007 V
#4 -0.009 V
#5 -0.006 V

Note that #2 and #5 are at opposite extremes. I will be swapping them with #1,3,4 to see if it follows the glow plugs or cylinders.

What I have observed thus far is that glow plug voltage outputs are predictable and repeatable, and move in direct correspondence to engine loading.