Battery or battery fuse defective backup not possible KRC4 KSS 8.3.17 HF1

if LED is on, then fuse is not good. change it. it seem to be intermittent.

never had to clean CCU. why your need cleaning?

i see no KUKA recommended maintenance that involves cleaning CCU. this part is inside cabinet and that is the clean zone. cabinet door is supposed to be closed at all times - to prevent ingress of dust (or worse) and to ensure proper air circulation for cooling. More over any cleaning need to be done in accordance with requirements. Friction from cleaning (air flow, rubbing with dry cloth etc.) can introduce static and damage board. This is why vac cleaner that KUKA uses is ESD rated. never use compressed air... intrducing other cleaning means (liquids, abrasives, solvents etc) is a whole new game but i guess you would not want to share the picture of the CCU before and after cleaning and what cleaning method you use.

as for fuse, something is not right there or you would not be asking for help.

i have seen personally all kinds of circuits that exhibit intermittent problems - and fuses (and fuse holders) are not immune. one relatively common issue with diagnostics is that people doing troubleshooting do not consider testing thing under working conditions.

For example they disconnect the motor, then check continuity or resistance of windings or brake and it looks ok. but when motor is connected back it may still not work. it is because multimeter uses tiny voltage and current for resistance test. but in real world those motor circuits sue voltage and current that is many times larger. this means the test will fail to detect poor connection. this is not a bad test, it is just insufficient. at best it tells if there is a problem but if result looks good, that is not a guarantee that there is no problem.

and if you see that blown fuse LED light up, that is because fuse or fuse holder are not ok. if fuse was ok, the LED would be shorted and therefore not on. so i guess that one little fuse must be too expensive of a part to replace it as part of diagnostic process or preventive maintenance. better call KUKA and replace entire CCU. problem solved.