NuTone CV750 Central Vacuum won't shut off. The on/off switch on side of unit has no effect. Did continuity test on switch, it tests ok. I've disconnected the low voltage wiring at the side of unit, that had no effect.

check for a blockage at the fan blades

Disconnect the incoming pipe from the vacuum power unit and turn the switch on (on the side of the unit) while you feel the bottom of the unit with your hand. If BOTH motors come on and sound normal and are producing suction you should feel the bottom of the canister flex or you should feel suction at the inlet connection.
If the bottom flexes but there is no section at the inlet there is an obstruction in the pipes witin the vacuum unit. A stiff wire with a hook bent at the end is usually sufficient for removing the dust stuck in that piping.
You can also unplug the 110v poser then remove the top cover (only 3 or 4 screws) and visually inspect the motors and piping for failures.

Use the guide at builtinvacuum.com/flowchrt.html to find the issue and solution. Most likely you need to test it without any filter and be sure the bottom of the unit is attached :) To get access to the motors you need to unbolt the very top sides.

You probably have a stuck relay. You can try tapping with medium force next to where the low voltage wires are attached to the unit using the handle end of a hammer. Even if this works it's time to replace the circuit board. Take the unit to a local vacuum repair center for service.

if you dont have any suction coming directly out of the unit then you probably have a full container or a dirty filter. look inside the container up by the motor for a filter that can be cleaned or replaced

The motor may have a sticking brush or a dead commutator segment. Turn off power to the unit to check for sticking brushes. Reapply power, does it now run? If not, shut it off, physically turn the motor a few degrees, and reapply power. If it still doesn't run, look for loose wiring around the motor. If it does run, replace the motor armature or the entire motor if the armature is not available as a separate part.

You need to determine if this is a problem with the hose or the unit. I would recommend plugging my hose in the wall sideways. This should make your central vacuum run continuously. If the unit doesn't cut on and off it is the hose. If it still cuts on and off it is probably the relay.

It sounds like the relay is bad. This website has a pretty good trouble shooting page.

https://www.centralvacuumstores.com/cvs/trouble_help.php#wont_turn_on

Could be 2 reasons, since you changed your bag and this started happening, 1 is that you the aut/manual switch got bumped to manual. In manual it will run until you unplug it or set the switch to auto. #2 is there is a relay that has stuck in the on position. Sometimes rapping on it will jar it free, most times t has burnt contacts and will keep doing this, so replacing it is best.

Probably just didn't wire the l/v side of the relay properly. Or forgot to plug the leads from the outlets back in to the vac after you got it all hooked back up. Folks do it to me all the time and then make like it's my fault! Ha ha.

Jump the l/v contacts inside the CV. If it goes: trace outward toward the outlets to see if something shook loose while it was apart. If you can't jump the l/v with a plier/screwdriver at the CV or even inside it, you have an issue on the board (either how you've connected to it or it's defective itself). The prob is that there's dozens of diff board configs, plus the relays and transformers weren't together on the same board in the old days. If you've got one of these models with a massive transformer separate from the relays get out the test leads and make sure you've got adequate voltage coming out of her to power the relay coil.