Yes, they are pulse modulated which will cause the voltage reading will bounce and the reading to be low.
Also worth nothing that:
- the glow plug system is carefully monitored by the ECU which generally detects any fault to the system
- glow plugs are not actually needed to start the engine except in the dead of winter
- starting fluid and glow plugs are an explosive combination that can cause severe engine damage... I'd strongly recommend putting away the ether.
All of the above is to say if your diesel is having a hard time starting it's unlikely to be the glow plugs, particularly if there's no Check Engine light.
Perhaps back up a bit: what work has been done to this engine, when did the hard starting problem show up, when was the timing last checked, does the car use much oil?

Cold starting issues are generally related to the starter, battery, timing, or compression.