Pneumatics Compressor not turning on automatically when plugged into pcm

We are a rookie team. We are working on our pneumatics system and we are having trouble with our compressor. We have 2 double solenoids instanciated, and the lights on the solenoids change according to which one is open, so we know there is nothing wrong with the code, but the compressor does not start working after we clicked enable on the driver station. We think there may be a problem with the pressure switch but we don’t know how to troubleshoot it, and we can’t tell if there actually is anything wrong with it. The pressure switch stays at 0, there is no miscomprehension making it seem as if the pneumatics are full. We have checked the wires over and over again, and cant find anything wrong.

I’d start with breaking down and verifying each of the elements in the system to make sure they work individually: If you connect the compressor directly to a 12V battery does it run (it should)? If you don’t have anything plugged into the Voltage Regulating Module (using the volt settings on a multimeter) what voltage does each port supply (it should be 12 for the ones you plug the compressor into)? Etc.
Make sure your components all work individually then start rebuilding the system step by step, checking that the outputs of that part of the system are what you expect them to be, and eventually you’ll either find a problem or it’ll be working.

Thank you very much! We have been doing some of the things you mentioned to a certain degree, but just curious how would one test the pressure switch?

I am admittedly a bit rusty on the electrical side of pneumatics, and I’m not in the lab to look at our pneumatics system which is working. But there should be some way of testing it.

Easiest way would be to replace it, see if it works, if it isn’t then replace it a second time, if it’s still not working then the change you have 3 broken sensors is near 0, and the problem is elsewhere (though you could have wired it wrong). If it was a problem elsewhere then at least you have a couple spares for other robots or in case it does break in the future. A local veteran team might have a spare they’re willing to let you borrow. I know we do.

Next easiest is to poke at the terminals with a multimeter to figure out how it works. It clearly is supposed to change state somehow when the pressure gets to ~125 psi, so if you probe the opposite leads and see the resistance is low (~0 Ohm) or high (> 1 mega-ohm), then pressurize the system to 125 psi (manually by connecting the compressor directly to the battery, don’t forget to have someone ready to disconnect those wires when you reach the target pressure) and see if the resistance changes. If it changed then it’s probably working. If it’s a digital sensor then debugging gets a lot harder

Harder still is to have someone go hunting in the datasheets for how it actually works (or wait until someone on CD pipes up) and come up with an actual testing plan based on knowing what it does.

I’d recommend replacing anything you even suspect is broken, and then having someone test to see if it’s actually broken when you have time. I imagine your priority is to get your mechanism working, not necessarily finding out what’s wrong, though knowing why something broke is a much better solution.

Honestly my bet is there’s a loose wire somewhere, so if you run around with a multimeter measuring the ends of wires to see if there’s a break (>1 mega ohm across the wire when there should be basically none) and gently tugging on things, you’ll very often find electrical problems.

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The pressure switch is normally closed. That means that when there is no pressure (or less than 120 psi) in the system, its terminals form a closed circuit. When it exceeds 120 psi, the circuit opens. So you can test if your pressure switch works easily using a multimeter. If you set your multimeter to beep mode (aka continuity mode) and touch the multimeter leads to the pressure switch leads, it should beep when the system has less than 120 psi and stop beeping when it gets to (or over) 120 psi.

If your pressure switch is working correctly, you should check what the PCM thinks is going on via the Phoenix Tuner. You can run a self-test on the PCM to see the state of the pressure switch, compressor, solenoids, and a bunch of other stuff.


@AriMB 's test looks good. If the pressure switch passes the test, but the compressor does not turn on, try a direct short of the pressure switch terminals (e.g. take a piece of paper clip and bend it and stick one end into each of the terminals). If that still does not work, check that when you do this, you get ~12V at the compressor output terminals using a meter when you provide 12V input. These tests will help determine if the issue is somehow inside your PCM; not common, but not unheard of.

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Only run AriMB’s test with robot power off! Otherwise you may get an invalid reading or damage your multimeter.

Riddle me this, Batman! How do you get the pressure up to 120 PSI without powering up the robot?

I would hope you wouldn’t damage your meter with a 12v robot. If you are frying multimeters you may have other problems.

Only use a multimeter in resistance mode on an unpowered robot for a valid resistance measurement.

Unfortunately there are a lot of really poor quality multimeters on the market. Caveat Emptor!

The op is checking if his switch will stsrt a compressor at 0 psi, not if it will stop it at 125 psi. One step at a time.

Yeah we recently had the same problem and it turned out that in the can bus the pcm was not showing up. We fixed the can bus for the pcm and it did the trick.

What we do for the compressor is instead of plugging it into the PCM where it says compressor out, we plug it into the PDP WAGO ports instead so that it runs try seeing if that works for you

What we do for the compressor is instead of plugging it into the PCM where it says compressor out, we plug it into the PDP WAGO ports instead so that it runs try seeing if that works for you

This is illegal on a competition robot - but an OK way of verifying that the compressor is not dead.

What are you programming in? It very well may be the switch, it could be the compressor, it could be the PCM, or it could be the code. I believe that you can go to the phoenix tuner and get some information from the PCM ( and that should hopefully verify that you see the switch change from FULL to (something else) and vice versa (we usually just short with a screw driver)

It’s the only thing we’ve ever done so we haven’t used it for a competition robot it was only a way to see if it works before addressing other issues @ahartnet

We have ran several tests and are fair;y confident that the source of the problem is the pressure switch. We know the compressor, pcm, and code have no problems, so after trying the trick where we close the circuit on the pcm with a paperclip, the compressor turned on. It seems the pressure switch was the problem. Thank you all for your help!

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