Hi Tansel a,

The 5.2s is a 48 volt truck so if you are reading 51 volts, all would seem to be well with the Charger and the battery. Make sure you still have around 48 volts with the battery disconnected from the Charger.
The 'clunk' you hear when you turn the keyswitch is the Line Contactor operating. This contactor connects battery Neg to the circuit. It is situated in the very worst place for checking and maintenance. It is under the battery, on the right hand side and about half way towards the rear of the truck. The Contactor is in a tin box with a cover that has to be lifted off, UPwards so the battery needs to be out of the way. For identification of the correct box, the battery grey plug Negative cable goes directly into it. Problems you will encounter with this unit::: The moving arm that carries the contacts pivots on a nylon shaft through an ally housing. The nylon swells and makes the contact stick. (wont pivot off to on and on to off. ) Check the Positive voltage AT the contactor coil at the pink wire.. Check for something like 46 volts when holding the key over onto its spring position, When switched to first position, expect to find about 18 volts at the Pink, not enough to operate contactor. (18 volts hold contactor 'in' once it has been brought in by the 46 + volts) If no 46+ volts, check first at keyswitch... for battery voltage and trace through switch to contacor. It is usually the sticking pivoting nylon rod that stops the contactor operating. When you switch the key off after a normal day, the Contactor moving arm (and that nylon rod) dropps out and comes into contact with a small, oblong contact in the off position. A sticking Contactor can prevent this 'Normally closed" contact from contacting and you get no keyswitch voltage.....
Finally, check the small glass fuses (2) in the front of the panel under the floor, on the right. One fuse feeds the Contactor Normally closed contact circuit.... Let me know how you go. Be safe if lifting the battery off..... I have jacked up just the right hand side to gain access BUT... BUT... BUT, the truck has to be very nearly against a wall on the left to stop the battery from sliding off sideways.... as you tilt the battery. best to move the whole battery sideways to the left and keep it level...... You need to clear the Line Contactor box and also need to be able to maintain battery plug into circuit.... You also need to JACK THE LEFT HAND WHEEL OFF THE GROUND, (OR BOTH). Left hand wheel because you are going to be working on your knees, on the Right and you dont want a turning wheel gouging out your trousers.... !!!