Push pedal five full pushes if it has a solo self adjusting clutch and if it works properly
I've never heard of an Eaton Fuller 12 speed, nor am I able to find any references to one via Google. I've driven Eaton Fuller 8LL, 9 speed, 10 speed, 13 speed, and 18 speed transmissions - could you be thinking of one of these? IIRC, Volvo marketed a proprietary 12 speed transmission, and Meritor offered a 12 speed automatic, but not Eaton Fuller.
Installation of countershafts
Remove inspection plate on bottom of transmission, bar the engine until you visualy see the adjuster on clutch, press clutch pedal, push in and turn adjuster clockwise until about two inches of free travel at clutch pedal is achieved (5/8 socket for adjuster)
It's the same as the shift pattern for the Eaton Fuller nine speed transmissions, albeit with additional gears on the low side. For the sake of not wanting to type out a long winded explanation, there is a related link posted - 'Eaton Fuller 18 speed shifting diagram'. Click on that link to see a visual representation of the shift pattern.
Either a proprietary Mack 18 speed or an Eaton-Fuller 18 speed. Various models of the Eaton-Fuller were available, from 1600 torque to 2100 torque.
Assuming it has the sixteen speed Eaton box it will have a twin plate clutch. Measure the gap from the between the release bearing and the front of the gearbox, it should be 13mm, i use a 1/2" ext. To adjust it you need to turn the engine until you can see a removable locking plate held in with a 9/16" headed bolt. remove this plate and with the clutch pushed down you can turn the inner ring, there is a proper tool but you can do it with a small strong bar. Remeasure it with the pedal released, turn the ring clockwise to decrease the gap. When the locking tab is getting to point where is almost proud of the castellations on the inner ring it is time to change the clutch. If it has the zf gearbox there is no adjustment.
Grey, both on an Eaton-Fuller and a Mack 18 speed.
On all Eaton-Fuller transmissions, the fill plug is on the left side of the transmission. You insert the drive end of a 1/2" socket into it, and turn it counterclockwise to unthread it. That's where the transmission oil goes. Eaton-Fuller recommends you use either 50W synthetic, or 80W if you're going with mineral oil.
Keep rpm between 1400 to 1600
Fuller Transmission shift diagrams.There are several places on or offline where you can obtain and Eaton fuller 15 speed gear shifting diagram. It depends what method you prefer. such as from your Truck dealer or a Truck Transmission Re-builder or someone who offers fuller transmission parts and service. For more information of sources online, see the Related Links section further down this page.
Eaton Fuller recommends 50W synthetic. Not sure about other transmission manufacturers.