The glow plugs are designed for 11.5 volts. I don't think voltage checks are useful since your car has the glow plugs hooked up in parallel. Each glow plug gets 30 amps to begin. As the coil heats up resistance limits the current. The current spec is thus something like 8-16 amps per glow plug. The indicator lamp will go off after a time determined by the ambient temp inside the relay. Glow will continue for another 20-25 seconds while the key is in position 2 (run), or until the key goes to position 3 (start) and returns to position 2. Glow will continue under the same 20-25 second limit while the key is in position 3.

If your car has the afterglow system, the indicator lamp will go off as determined by engine coolant temp. Afterglow time is also determined by engine coolant temp. Whew.

Sixto
95 S420
87 300SDL