Monthly Archives: April 2010

BOOM PUMP: Proximity Switches

Your Putzmeister boom pump uses proximity switches (aka prox switch) to cycle the pump. Some quick tips to keep them from being a misunderstood part on your pump.


What might go wrong?

Prox switches are screwed directly in to the drive cylinders and are subject to high pressure hydraulic oil. It is common to over tighten them, they take 14 foot pounds of torque which is a relatively low torque. Because they are in the pump hydraulic cylinder subject to high pressure oil they are quite often over torqued, this distorts the switch and it will fail early or not work correctly. The prox switches have a high pressure seal to handle the pressure subjected to them.

Next prox switches are grounded through the cable not the cylinder, so to test the system you can take a new switch, plug it in, touch it to metal and it will trigger. The switch works from magnetic principle, the piston is steel and the switch is triggered by this steel piston passing under it.

PMA Prox Switch Testing

PMA Prox Switch Testing

Deciphering the Light and Wires

On top of the plug you will see two LEDs one is green and one is yellow, The green one indicates that power is getting to the switch, the yellow one is the trigger signal that indicates the piston is under the switch sending a signal to the relay.

The prox switch wires go back to the distribution box, this box combines the signal to send it to the stroke change relay, LEDs are located at this box to tell you if they are working, you might have four or six switches on the standard units.

We encourage you to watch the lights when you are cycling the unit to clean the water box. You should see the yellow LEDs flash when the piston is passing under the switch; you have two switches at each position. They are working together as a backup which means if one fails you are still pumping.

PMA Prox Switch LED Lights

PMA Prox Switch LED Lights

Ways Prox Switches Fail

Switches can generally fail two ways, the yellow LED does not turn on when the piston passes under it, this is the most common failure. They can also fail by being stuck on. The yellow LED will be on even though the piston is not under the switch. In the first case no action is necessary, but if the switch is stuck on it will need to be disconnected from the system as it will interfere with the cycling of the relay. Simply unplug the one that is failed on, and resume pumping.

We all know that if it can go wrong it most likely will, this is the reason for the redundant-style system. However, quite often due to lack of maintenance or lack of understanding one switch will fail, then at some point the other switch will fail and now you have a problem. So familiarize yourself with the system and check it frequently so you can avoid being down due to prox switch failure.


The pictures show a model used in the Putzmeister Service School class room to demonstrate the switches and the cylinder position, as well as, testing a switch not installed on a unit.