When you insert the key, you are enabling the start switch by closing the circuit from the battery. The start switch on the handle is closed when you press the start button and then pull the bail bar back. When you release the bail bar, the start switch is supposed to open, shutting the mower motor off. If, however, the switch is faulty and does not open as it should, then the motor will continue to run until you open the circuit by removing the key. Either the switch is stuck closed or the contacts have fused together due to a short circuit. Disconnect the battery ground cable, then check the switch wire connecting points with an ohmmeter. If the ohmmeter reads zero Ohms, then the switch is closed in which case it is faulty and needs to be replaced.
Here is a link to the parts list for your mower.
https://www.ereplacementparts.com/b...-36v-19in-mower-parts-c-4167_9514_173260.html
I don't know if you have a CM1936 Type 1 or Type 2 (shown), but they should be pretty much the same. The electrical switch I am referring to is part number 2 in the diagram, and there should be three wires connected to it. It is most likely the part that your son thought was a relay. The single red wire probably goes to the positive terminal on your battery, and the two other wires with a single connector probably go to the motor. One of the motor wires should be black, and that wire goes to ground. The other wire will be either red or white, and it is the hot wire to the motor. To test the switch you must first disconnect the battery negative (black) ground cable to prevent an accidental short circuit or startup. Next, disconnect the wires from the switch, set your meter to read resistance in Ohms, touch one meter probe to the switch terminal that was connected to the single red battery wire, and the other to the switch terminal that was connected to the red or white motor wire. The meter should read OL (overload), indicating infinite resistance, because the switch should be open. If the meter reads zero Ohms, it means that the switch is closed, and the switch is faulty requiring replacement. If the meter reads OL as it should, then your problem lies elsewhere.
As a clarification, resistance is measured in Ohms and current is measured in Amps. The resistance across the contacts of a closed switch will be zero regardless of whether or not there is any current flowing in the circuit. The bail bar and button are part of a safety startup mechanism, that prevents accidental startup if either is moved individually. When depressed and held, the button pushes the bail bar into a mechanism that actuates the switch when the bail bar is pulled back. Once the bail bar is pulled back, the button can be released. When the bail bar is released, the mower is supposed to shut off, preventing the operator from walking away from the mower while it is still operating.
Thank you very much, Mike! This gives us vital info. Like "what is the switch". And how to test the switch. If the switch is good, it's going to need more sleuthing, but we'll cross that bridge. Come next weekend we're going to give it a whirl. Really appreciate your help. By the way, my son was wondering if the way it's working now hurts the battery/mower at all -- it's like you're completing the circuit when it's at full speed. But that reminds me of the way I've seen a Tesla go from 0 to 60, or the way this lawn mower, even, goes from 0 to "full throttle" fast.
Also, we were wondering if there's a way to know exactly how the torsion spring (part 23) and crank (part 7) are oriented when you put it back together. I think we got them back in the right position, but I'm not 100% sure, and I wonder if this could be the issue. If jiggling shook something loose, then maybe. Sounds like maybe it's not the issue.
Thank you again. Unfortunately, I already did charge the battery. Just went out and unplugged it, when I read your message. (As you know, the mfr. recommends leaving it plugged in [when everything is working normally]). Now I'm wondering if (a) the battery will be OK after having been left plugged in for a couple days; (b) what exactly the problem is with plugging it in to charge while the motor is live; (c) whether a problem with the switch or a short circuit caused the battery to drain before it should have. The mower/battery died, at first, while my son was mowing. It just suddenly died. At that moment, the bail bar was working properly. I guess that's a clue. I've got to think about this. But I'm glad you're recommending fixing the switch/short issue before we go on.
The reason I said not to charge the battery with the motor switch closed is that the charger for your mower plugs into the same circuit that the key uses to enable the motor circuit. Depending on how the mower is wired, this could possibly damage the charger. You might want check the battery with a voltmeter to see if it actually was being charged.
^^^^ that - precisely
just mentioning that, a possibility is that a wire in the harness was pinched or misplaced at the factory and has taken some time to abrade or cut thru 'enough' to actually short the system. Good job on the tape. Hopefully, no conductors inside were severed.