Glow Plug Voltage
 

I measured the voltage at the connection to the glow plugs. It was 8 to 9 volts when the relay was on.

Is that acceptable to heat them properly? Or is that a indication that my battery is on its way out?

1984 300SD

(I sense a BC suggestion in the offing)
 

Aero,Marine or Automotive application?

Ingenious (If rather cumbersome) method of testing:
All cabling systems in between the Battery and the Ignition Switch and the Individual Glowplug(S).
The PreGlow Relay(Including the 80 Amp Strip fuse).
Various other relays and fuses.
The Battery itself.

I would suggest checking the Electrolite (Fluid) levels.
'Charging the battery overnight (With the Negative cable undone).
A Specific Gravity Test (Cheap tester available at any autoparts depot).
A Load Test(Most parts depots will perform for free)
These actions will provide you with information to use in Judgement of
your Battery's health.

[Since my ElectroMechanical Prognosticator is broken this week,AND you've
concealed the type of Compression Ignition System in question...this is the
most assistance I can offer.]

('Direct answer to your IMPLIED question...
9 Volts D.C. to a Glow Plug will generally allow you to Brand Cattle with it.
'Of course 12 to 14 volts will allow quicker branding.)

You should be getting 12v. You might check the resistance across your glow plug strip fuse. It could have some tiny cracks that are not allowing full voltage to get to your GPs.

Might want to add the model and year to your signature or included it in the thread, so folks know how to respond with accurate answers.