Compressor not stopping at 120psi

Our compressor want stop at 120 psi for some reason and i was wondering why it wouldn’t…Please i need help

What is the wiring on your pressure switch?

What are you doing in programming to stop the compressor?

These are just a couple of pieces of information that will help troubleshoot.

Also, is your relief valve tripping at 125 PSI? (This will be inspected at competition.)

First - check across the terminals of the pressure switch with an ohmeter while you are pressurizing the system. I think it is a normally closed switch so you should read some resistance value until you reach about 120, then it should go to off-scale because the switch has opened. If you don’t see that happen, you have a bad pressure switch.

Next, check on the driver station what you are seeing from the digital I/O port where you have the pressure switch as you go up in pressure. It should read true as you go up then change to false at 120 (or vice versa). If you don’t see this then you have a bad connection somewhere between the switch and the cRIO - check the wiring.

If those are both good, your problem is in software. There is a pressure switch vi that should be running all the time - make sure you have it in place.

447 Had the same problem. We had to replace the Digital side car.

Will the pull-up resistor in the DIO confuse the ohmmeter reading ?


If the controller is on: use the voltmeter to measure 0 or 5v across the switch. If 5v the switch is closed (115 psi) if 0v the switch is open ( less than 95 psi). If that is working correctly then try measuring at the end of your pwm cable (where it attaches to the sidecar), if that is still working do a print statement for the digital input you are using. You should see it go high (1) when the sensor trips at 115 psi. If not, your sidecar has something wrong with it. Try a different digital I/O

Yes, use the voltmeter setting, not the ohmmeter.

It’s the other way around. 5V means the switch is open; 0V means the switch is closed.


I’ve been measing 1.69 volts across the switch even when I change the DIO port to another one. Everything else connected to the digital sidecar works even another limit switch that’s connected to the DIO. The switch itself has the black and white wires from the pwm cable hooked up to it, with the black wire going above the C and the white above the NO. We’ll have to get back to the troubleshooting at competition thursday, but these are great tips thanks!

Also, do you think we should change the Digital Sidecar as mentioned earlier, even though everything else is working? We appreciate everyone’s input!


You cut the center wire back and insulated it correct? You are looking for the switch to be wired from the black and white wire of the PWM cable you are using. The pullup to 5 volts when the switch is open occurs via a resistor in the digital sidecar.

I think the short answer is yes but you don’t care. We were debugging and used this process, and there was definitely a difference in the resistance with the switch open and closed.

I’ll vouch for that one. Ran into a rookie team at a regional last year that we couldn’t figure out why their compressor wasn’t shutting off. Eventually they replaced the sidecar and it fixed the problem.