Need some more help, this time with the pcm

We have a problem with the connection fr the pcm to the compressor. The pcm does not read. We have plugged in everything and there is a true loop shown in the console, but no indication light. The compressor doesn’t turn on when the bot does.

We have tested everything seperately. The everything is powered except the pneumatics. They work with a thrifty throttle though.

We have been using the user’s guide for pcm but can’t figure it out. Does anyone have an idea of what the problem is. Thanks in advance

So to be clear the compressor won’t turn on till the robot is enabled. Have you enabled the robot?

Assuming you have enabled the robot can you use a multimeter to probe the compressor out lines?

Crazy. We have an almost identical problem. Popped up in last match at Beach Blitz this weekend. Since we had no more matches we figured we’d wait until Tuesday meeting to figure it out.

  • No COMP light
  • all wires are good
  • enabled robot has no compressor running
  • solenoids all click when correct buttons pressed.

Is there a floating ground on channel 1? It sort of looks that way. I am not sure that would cause this though.

The compressor doesnt turn on when the robot is enabled. We are trying to use a multimeter right now

There is not a floating ground

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If you run a self-test on the PCM in the phoenix tuner does it say that the compressor is enabled? If it does and the compressor still isn’t turning on chances are your PCM is broken. If it’s not enabled, then you should check the pressure switch wiring to make sure it’s triggering properly

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There were two causes of this behavior that happened to us. I am relatively certain neither apply, but tossing them out in case it can help.

  1. The pressure switch was not wored correctly.
  2. We had not instantiated any valves correctly in our code.

What is the STATUS LED doing? Blinking? Colors?

Is the COMP LED turning green? If so, check the compressor outputs with a multimeter – should see 12V. If not, try shorting the two “PRESSURE SW” with a jumper temporarily. This eliminates the pressure switch and wiring as suspects. BE SURE TO LEAVE THE PRESSURE RELIEF VALVE OPEN when doing this!

Be sure the PCM shows up in the Phoenix Tuner utility. Note this link has troubleshooting steps.

(More detailed info on the LED states is here – section 4.1 is about troubleshooting this specific issue.)

The status light is green. There is no light for the comp

Do you happen to know how to instantiate code for valves in python. Our team has never had this problem before so something like this wasn’t necessary

I edited/added to my answer with some docs on what to do to run this down.

We did not use pneumatics this year, but give me a sec. I will find it. Was it working before? If so, did you change anything in code since?

You shouldn’t have to do anything special in the code to run the compressor. If you are having issues with solenoids, that could be code. What programming language are you using (Python)? The robot has code and things are otherwise working OK?

Here is our instantiation…

def robotInit(self):
    # pneumatics solenoids
    self.hatchcover = wpilib.DoubleSolenoid(0,1)

If you want to see our full code, it is here.

When we did not have a solenoid in code, the compressor would not start (this was a while ago).

This is expected behavior btw.

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Is it just that part of the CAN bus wire isn’t twisted or is the entire CAN bus wire that goes off to the right are not in a twisted pair configuration???

Also make sure that your pressure switch is operational (swap to a different if necessary as a quick test).

[Changed order]
Not likely, but check that your system isn’t already pressurized.
If it is, depressurize it and see if this solves the problem.

Alternately, just insert the same piece of wire in both pressure switch connectors. Don’t run the system like this longer than needed to verify that this is the issue!

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:warning: If you jumper out the pressure sw, make sure you have a relief valve plumbed in somewhere.


Wow! Yeah, our solenoids activate too, that sounds just like our problem. We still haven’t solved it, so please let us know if you do end up solving it in your meeting.