I have a 753 with around 3000 hours on it. I recently bought it off of an friend of mine that I grew up working with when I was in high school and college. I learned pretty much everything on this machine and was happy to get the oppertunity to buy it for my property. He used it primarily for the aux hydraulics - drilling fence posts for his fence business. Honestly i i think i saw this machine with a bucket on it 5-10 times. Here are my issues - I was changing the fluids and noticed the two wires that go to the charge switch were unplugged - i think i got the right name its the sensor to the right of the hydraulic spin on filter in the back of the unit. When i plug the wires onto the sensor i get an error message that beeps and eventually shuts the machine down. Unplugged the machine runs fine like nothing is wrong. Tried pulling the sensor off and cleaning the small 10 micron filter behind it and reinstalling - same issue is present. I am not sure if this issue is related but i cannot start the machine without first keying on the ignition, hitting the aux buttons so both lights go green then firing it over. If i do not hit the aux buttons the machine will start then die right away.
sounds like 2 seperate problems to me.. does it have an aftermarket ign switch installed? you may want to check up inside the pressure switch and see if it's leaking hyd oil when the engine is running....
i read a little on the ignition since your comment made me wonder why that would matter. should those aux buttons see 12v switched or 12v constant? How should these be wired from the factory? i can pull the panel to check tomorrow but i am 99% sure its OEM ignition.
aftermarket ign switches in C and F series machines (rectangular headlights) play hell with the aux functions. not sure why, but this a fairly common problem. in the past ive had to remove the bronze filter from that fitting to get the warnings to go away. the BOSS system wasnt the greatest thing even when it was new.