Here are all the hints I gathered to check and adjust the monkey sense switch.
Prop the playfield up at the front so the ball will rest on the monkey
magnet. This way you don't have to try and hold the ball on the magnet
while initiating the test. Go into tests, Top Lane Magnet Sensor Test.
Close the coin door or enable power to the coils with the interlock
switch. Place the ball on the magnet and press enter. You should see the
Monkey Magnet Sense switch activate when the magnet turns on.
I had a Non-working sensor, which in turn gave me a error dot/report on the LCD (lower right corner/red square).
I flipped the board over (circuit board turned in) and then got a over-sensitive (working) reading.
I then started moving the board to different locations to try to find a working one (circuit board still tuned inward)
After moving the board to 3-4 different locations I came across this one that seems to work great (see pictures)
WARNING: when testing the magnet gets VERY HOT rather quickly while in test mode. I personally would suggest backing out of the menu (one layer) and unplugging the sensor in between adjustments.
If you are unsure of just how hot your magnet is, AT YOUR OWN RISK, touch it, if it burns your finger/hand you should probably back out of the test menu for about 10 minutes or so to let it cool down.
Remember, a hot magnet may give a bad reading.
My Monkey Magnet Sensor was also not working after upgrading to 4.0. All it needed was for the magnet to be flipped.
1) lift up the playfield all the way
2) unscrew the 3 screws holding the monkey magnet. there is no need to unplug anything.
3) carefully remove the magnet housing and center pole and you will see the donut shaped magnet
4) remove the magnet off the pole and flip it over and reinstall
To test the Monkey switch, you need to test it with and without a ball. If you go into the monkey magnet test (under device tests), you can hit the start button to activate the magnet. The switch should NOT be active when this happens. In the test, you should see the box for the magnet active, but not the switch. Then, hit the start button again, but this time with a ball on the magnet(easy to test this by having the playfield pulled up and on the first rail position). Now, your monkey magnet detect switch should be active along with the magnet. This switch will only ever activate if there is an active magnet present. Just putting a ball next to it won't do anything. Some of the games were built before this mode was fully vested out, so the switches may not be calibrated in the right position, and could use adjustment.
If your switch is always active when the magnet is on, you will need to re-position the switch. Try holding it in a place you can mount it, and try the magnet with and without a ball on it to see if it trips the way it should (only with a ball on it). Once you find a good spot, screw it back down and test it again.
Another complication is that the switch is a Hall Effect sensor, and will only sense a certain magnetic field orientation. If the big magnet underneath the playfield is not North South relative to the playfield, your hall effect sensor will not register. Because the magnet can be mounted in either direction, and there is no indication, there are a couple things you can do to make sure yours is correct, or simple ways to change it. Easiest thing to do is just flip the monkey magnet detect board over (so the hall effect, resistor, and connector are now away from the panel instead of against it, or vice-verse).
LTG : )