Diagnosing A Safety Limit

Manitowoc ice machines are programmed to shut down under situations that could potentially damage the machine, these are called Safety Limits.  Depending on the model of ice machine that you have, it could contain 2 to 4 different safety limits.  However, all safety limits can generally be broken down into 2 categories.

The first type of safety limit shuts the ice maker down when it's been in the freeze cycle too long and the second is when it stays in the harvest cycle too long.  If either limit is reached, it's an indication that the ice machine is not working properly and needs to be locked out.  It will remain shut down until someone resets the maker, either through the toggle switch or by resetting the power.

Why would a Manitowoc Ice Machine shut down on a long harvest safety limit?

The #1 reason a Manitowoc will shut down on a harvest safety limit is because it's dirty.  A dirty evaporator will hold onto the ice, making it slowly melt away until there is no ice to break the bin switch.  Which leads to the second most common reason for a failed harvest safety limit, bad bin switch.

When Ice falls from the evaporator it opens the water curtain, which breaks the proximity of the bin switch.  If the switch is bad it will not read that the ice has fallen, thus terminating the harvest cycle.  Typically, after 3 consecutive cycles of long harvests, the machine will lock out.

Why would a Manitowoc Ice Machine shut down on a long freeze cycle?

This safety limit is a little more difficult to diagnose than a long harvest, however is most commonly associated with a water problem.  If there is not enough water in the trough or able to be pumped over the evaporator, then it cannot make contact with the ice thickness probe.  The ice thickness probe is what would terminate the freeze cycle and initiate a harvest.

Places to look would be a faulty water inlet valve or a dump valve that is stuck open, causing the ice maker to lose all it's water down the drain, before it can freeze.  However, never underestimate a good cleaning, clogged distributor holes or a clogged up pump can also lead to a long freeze time, thus shutting down your Manitowoc ice machine.  In addition, make sure your water level probe is not dirty, causing it to misread, scale build up on the probe will tell the board that it's full of water.

Most common parts replaced on a Manitowoc Ice Machine:

Ice Thickness Probe
Water Level Probe

Manitowoc Control Board Testing

Manitowoc control boards are arguably the most reliable ice maker control board in the industry.  In comparison to the number of failures of other components within a Manitowoc ice maker, the control board is one of the least replaced.  Usually an external factor will cause the control board failure, like a power surge, not a failure of components.

To test a Manitowoc control board you must first be familiar with your specific models sequence of operations.  The order of which components are energized by the board or sequence of operations, are published online via the official Manitowoc website.  Be sure to reference your specific model number, because not all Manitowoc ice makers have the same operation sequence.

The next step is simply following the sequence and checking to make sure the components that should be getting power, are receiving the proper voltage.  For example, if the dump valve is supposed to be energized, before the freeze cycle initiates, then check to make sure it is getting voltage.  If the dump valve is receiving the proper power supply, you can rest assured the board is working correctly, at least in this stage of the control board diagnosis.

If a component listed on the Manitowoc sequence of operations receives constant voltage or is energized out of sequence, then that's a good indication that you have a bad control board.  Usually, a sticking relay on the board will be at fault, in this case the entire board would need to be replaced.

Physical examination is another method for determining if you have a faulty control board.  Burn marks can be easy to spot and usually indicate that the board is no longer functioning properly.  Again, separate components are the Manitowoc control board cannot be replaced, you must replace the entire board.

Below are the 2 most common control boards in the field, please verify with the manufacturer which board is used on your machine.

Manitowoc S Series Control Board - $291.41
Manitowoc Q Series Control Board - $483.00