Checking transmission fluid

First I need to know what exactly you are referring to when you say "gearbox".

Do you mean the "transmission" or "differential case"?

If you're referring to the tranny, then I'll need to know your engine model and transmission model.

The differential doesn't have a drain plug. Fluid is removed by either cracking the rear pan or removing the axles and letting the fluid drain. It's refilled while off the car.

Unfortunately, I can't provide you with any illustrations. The service that I get my repair data from prohibits me from sharing their illustrations with anyone.

But there may be another technician on this forum that has access to information that I don't.

As for your tranny fluid change:
Your transmission is located at the back of your engine. (It's actually bolted directly to the back of the engine)

If you get under the vehicle, you'll see the bottom of the engine, and right behind the engine, you'll see the bottom of the transmission.

You're transmission's drain plug is located at the very back of the bottom. The fill plug is located at the side. They look similar. (Round metal plugs with a hex-key opening in the middle)
You fill the tranny through the fill plug by squeezing the bottle into the opening. Fill the level to just under the hole.

TRANSMISSION FLUID LEVEL CHECK
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Tech1Question: fluid

Raise and support the vehicle.
Connect the Jaguar Approved Diagnostic System.
Make sure J-Gate shift selector is in the Park position (P).
With the engine running and the foot brake applied, move the J-Gate shift selector through "P-R-N-D-4-3-2" and back to the "P" position to circulate the automatic transmission fluid until the temperature reaches 30 to 40 °C (86 to 104 °F) on the Jaguar Approved Diagnostic System.
When the automatic transmission fluid temperature reaches 35 °C (95 °F) check that the selector lever is in the ' P' position raise the vehicle.
With the engine running, remove the automatic transmission fluid level tube plug.
Remove and discard the level plug and sealing washer.
Fig. 6: Identifying Transmission Fluid Level Tube Plug
Courtesy of JAGUAR CARS, INC.
NOTE:Use automatic transmission fluid WSS-M2C922-A1 or equivalent meeting Jaguar specification.

Fig. 7: Identifying Transmission Fluid Charging Pipe Cap
Courtesy of JAGUAR CARS, INC.
If the automatic transmission fluid does not come out of the transmission fluid level tube the automatic transmission fluid level is insufficient. If this is the case add the automatic transmission fluid in 0.5 liter units into the automatic transmission fluid charging pipe until the automatic transmission fluid comes out of the automatic transmission fluid level tube.
Remove the automatic transmission fluid charging pipe cap.
NOTE:Make sure the automatic transmission fluid temperature does not exceed 40 °C (104 °F). If the automatic transmission fluid temperature does exceed 40 °C (104 °F) stop the automatic transmission fluid level check and allow the automatic transmission fluid to cool until the temperature reaches 30 to 40 °C (86 to 104 °F).

Allow the automatic transmission fluid to come out of the automatic transmission fluid level tube until the overflow stops at a temperature of no more than 40 °C (104 °F).
If when the automatic transmission fluid level plug is first removed and automatic transmission fluid comes out of the automatic transmission fluid level tube, allow the automatic transmission fluid to come out of the automatic transmission fluid level tube until the overflow stops at a temperature of no more than 40 °C (104 °F).
Install the transmission fluid level plug.
Install a new level plug and sealing washer.
Tighten to 15 Nm.
Fig. 8: Identifying Transmission Fluid Level Tube Plug
Courtesy of JAGUAR CARS, INC.
Install the automatic transmission fluid charging pipe cap.
Fig. 9: Identifying Transmission Fluid Charging Pipe Cap
Courtesy of JAGUAR CARS, INC.
Lower the vehicle.
Disconnect the Jaguar Approved Diagnostic System.