@Acrob I hope I get what you're asking... let me try here... You may need a second person to help.
Set your meter to DC in a 12V range.
Connect your meter to the tractor frame (bare metal) with the (Black) negative test meter probe, then touch the positive (red) lead to a glow plug.
Activate that circuit (Starting) and you should see 12V on each glow plug.
Buzzing can be felt by touching each module under the dash. Glow Controller, glow relay, etc. You can do this of course without removing the entire console. Buzzing as mentioned earlier can be due to bad wiring (incomplete circuit - broken or loose wire to whatever is buzzing).
Looking at the circuit diagram previously added to this thread, there is a relay for the glow plugs because of the large current draw to the plugs. That relay moves the electricity to the plugs when the glow controller says "go".
The controller says "go" based upon water temperature (a hot engine doesn't need the glow plugs) and when the key switch is in "on" position. The controller is designed to control when (and possibly how long) to send current to the glow plugs. A light for glow plugs usually tells you "when" - then you turn the key to "start"
All this of course depends on good wiring and good fuses. If you don't get 12V, and eliminating bad fuses and wires, the glow relay could be bad.
This is not to be confused with the starter relay.
A temp sensor that's not working won't send correct info to the glow controller.
And yes, a bad glow controller is a possibility but I think less probable.
You have to trace the 12V either backward from the glow plugs or forward from the key switch to find the issue.