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.
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I will assume that you disconnected the switch from the circuit when you did the continuity test -- that is really important.
If so, then i think that the relay inside the top of the unit is stuck on, or the low voltage wires (inside the top) going to the relay are shorted together.
You can easily remove the top after removing 3 (or 4) screws from the upper side of the unit. Be careful to align the vents in the top with the cooling ducts from the motors when replacing the top.
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.
Some models have an on/off switch on the power unit itself. This switch is used to turn your power unit on when you are right at the machine.
If there is no switch on the power unit, unplug your power unit from the wall socket, look for two low voltage wires coming out of the power unit. These two low voltage wires are usually either attached with terminals or are tied to two wires coming out with plastic screw on wire caps. Disconnect the 2 low voltage wires. Plug your power unit back in. If the power unit does not come back on then yes, the problem is most likely a short in the outlet wiring. If the power unit still comes on with the 2 low voltage wires unattached, then the problem is in the internal circuitry of the power unit, usually a relay or some units have a circuit board with relay, circuit breaker and other components on it. The relay or circuit board will have to be replaced.
It's a little difficult to check for a short if you have multiple outlets, but if you've recently done some renovations you might start checking in that area.
There are only two possibilities. One, your relay board has gone bad and if this is the case you'll have to replace it with a new one. Two, somewhere in your system the low voltage wires are touching. An easy way to determine which it one it is, is to go to your tank and you will see the low voltage wire the is connected to the tank by two clips (the wire actually has two wires in it). Unclip the two wires from the tank and plug the power cord back in. If the tank turns on without the two clips connected then it means you need a new relay. If the tank doesn' t turn in with the clips out then that means that the low voltage wiring is causing the problem. This is rare but if they are touching somewhere it is usually where the low voltage connects to the inlet covers in your house. If you can't find where they are touching call a service man they can usually find it.
Start with this. Unplug the vac to shut off, then disconnect the low voltage wiring at the vac.Plug the vac back in. If the vac still runs you have an internal short on the relay or wiring inside the vac. If the vac stops then the vac and final wiring is OK and the problem is in the wiring in the house. Have you removed any inlets for painting or had any renos done or pictures hung. If so undo what you did last. If inlets were disturbed remove them and look for low voltage vac wires touching metal backing plates in the wall or touching the inlet mounting screws. If you have done a reno or hung pictures you may have pierece the wire for the vac creating a low voltage short which will keep the vac running until removed. I have seen this happen with freshly installed 1/4 round molding along a baseboard
This sounds like a relay sticking on the control board inside the unit. The only other thing I've seen cause this, would be a short in your low voltage wiring. I had a service call where the wires were laid across the top of metal studs and the insulation had been damaged to the point that the wires would short out on the stud. To be sure it's not something in your wiring, disconnect the low voltage wires from the unit while it's "stuck" running. If it continues to run, then have your control board replaced. This is a somewhat common problem on these units. Good Luck!
What I'm hearing is the canister(Not the Powerhead) portion of the unit is running all the time unless you unplug it. this problem could be caused by a short in one of your low voltage lines running through the house. To determine that just unplug one/both low voltage lines from the canister, and plug it back into the recepticle. If it doesn't start up then you figured out that it is in the house somewhere and not the unit. If it is in the house I would check for water in the valves, or you're just gonna have to chase down the short. If the unit is still running with the low voltage wires disconnected then it is probably the relay on the board, which means you will need a new board unless you're expereinced at soldering and want give it a try replacing just the relay. Which you'll probably need a new board by the time you're done anyhow. LOL
Don't know the exact model, but it sounds like the low voltage relay is stuck. This will be inside the unit, probably mounted on a replaceable board. You may be able to temporarily free it with a quick tap on the top of the relay if you locate it. But I would suggest replacing it to avoid further problems. mdvacuum.com would probably have the part you need. Good Luck!!
Mine is the same, and it is the relay welding itself closed. The relay is switched by the low-voltage wiring that connects to the vacuum outlets, and it switches the motor on and off. Mine was welded in the energized position. I have cleaned and filed the contacts on the relay, and that solved the problem for about a week. I suspect the relay contacts were plated, or a worn motor is drawing a little too much current (it is several decades old at this point).