Compressor not shutting off at 115 psi

FIRST General Forum
1

We have, last minute, got our compressor working and it is stating up well. Our problems is that it is not shutting off at what we think should be 115 psi. Any suggestions? There doesn’t seem to be a leak in the system. Could the problem be in the wiring or programming?

Thanks for all your help to a very rookie team.

jalmos

2

When does it shut off? Also what is connected to what, please provide details on how the compressor is wired and how the pressure switch is wired.

3

Is it reaching 115 and going no higher while the compressor runs? Your 120 PSI pressure relief valve might be set too low, not letting the pressure get to the point where the pressure switch is satisfied.

Or is it continuing to rise past where you think it should go? Your pressure switch might not be connected properly, either pneumatically or electrically.

4

Thanks for the quick response.

The compressor doesn’t shut off and gets to a pressure of 100 psi.

We have the compressor wired into a spike replay, which goes back to the power distribution board. The pwm wires do to the DSC.

The pressure relief valve is wired to the DSC using a small red and white wire, only two not three. Would there be a ground missing? Please let me know if you need more info. Thanks so much!

jalmos

5

Thanks for the info. How do we configure the relief valve. The pressure will not go over 100psi and it continues to run.

Should there be a ground connect. Now we have it connect to our DSC with two small wires.

Thanks so much

jalmos

6

Configuring the relief valve: screw the large part at the end out or in (I forget which is the raising direction) until you get to the correct pressure (125 PSI). Make sure you keep an eye on the gauges. Once the large part is in the right place, jam the nut next to it into it.

7

The relief valve has no wires attached. It is a brass poppit valve that looks kinda like a brass plug with a hole in it. To adjust it you need to turn the “cap” portion.

The pressure switch has 2 wires that run to it. It is preset to 115 PSI and will not activate (shut off your compressor) until the pressure gets to 115PSI (which your system is not doing).

It sounds to me like you may have a leak in your system that is preventing it from getting to 115PSI. When you shut off the compressor does the pressure immediately fall to zero? If so you have a large leak.

8

Ah, so the pressure never gets high enough to trigger the pressure switch. This means you likely have a leak in your system or the blow off valve is venting prematurely. If you turn your robot off how fast does the pressure drop?

9

WE will give all of the things a try. Thanks for your help

Jalmos

10

Jalmos,
The pressure switch is not an accurate device and may not shut off the compressor until it reaches 120 psi or slightly more. If you are removing power before the compressor shuts itself off, try letting it run. If it should rise to 130 psi then disable the robot, bleed the system and check your wiring. The switch should connect back to the DSC but must not connect the red wire in the PWM cable if you are using one. As a second check, use an ohmeter or continuity checker and see if the pressure switch changes state at 120 psi +/-5 with the connector pulled out of the dsc. This most often occurs with a wiring error.

11

We had the same problem last night. At 130 we disabled and checked the wiring, fiddle with the screw then we replaced the pressure sensor with another and it worked correctly.

12

a couple of days ago I was spraying primer on a few car parts and heard a loud “pop” or “boom” so I immediately stopped and shut off the compressor. A while later i started it up again and noticed the main pressure gauge only went up to about 90 psi instead of the normal 125 or so psi and the compressor did no seem to want to shut off. I manually shut it off and went away for a while. The gauge stayed at about 90 psi and I could not hear any air leaking. I have read a few other forums and am a bit confused as to the possible causes that range from replacing a number of switches (including the pressure switch) to possibly a blown gasket or baffle somewhere that may be preventing the pressure from getting above the 90 psi. Any help would be greatly appreciated. Thanks

13

You should be using the black and white wires on the PWM cable, not the red one. It’s important to have the pressure switch wired up to the DSC with Ground and Signal. The digital inputs on the DSC all have “pull-up resistors” - this means that when there is an open connection, they’ll read (in the code) as though the signal was connected to ground. So when your pressure switch closes, you want it to connect ground to signal to pull it down.

Once you get that fixed, check to make sure the code is working. Do this by removing the wires from the pressure switch and manually touch them together. when touched together, the compressor should turn on. When you pull them apart the compressor should turn off. If this happens, then you know everything is set up correctly both in the code and electrically!

Once you know all of that is working correctly, hook the pressure switch back up and address your issue - you can’t generate more than 100 PSI, so the pressure switch never trips! As others have suggested, the blow-off valve is likely the issue. When you run the pressure up to 100 PSI, put your finger over the hole - you’ll be able to feel the air pushing out if it’s tripping early. To set it properly, do the following:

  1. Look at the valve. you should see a big nut on the end (where the air comes out), a thin nut in the middle, and another nut on the other end that you used to tighten the valve to the connector.
  2. grab the big nut with one wrench and the thin middle nut with a second wrench. Give them a twist to loosen the middle nut, and spin it down a bit to give you room to work with.
  3. Turn on the compressor and let it get up to the max it can. Slowly twist the big nut (by hand, it should move pretty easy) to increase/decrease the pressure. Assuming you got the code/wiring figured out already, you should see the compressor turn off around 115 PSI!
  4. Once the compressor turns off at 115 PSI, grab a screwdriver or a wrench and lay it across the two prongs on the pressure switch to turn the compressor back on. Turn the blow-off valve until it starts letting air out at 120 PSI.
  5. Lock down the blow-off valve. Hold the wrench on the big not completely still, don’t twist it at all! tighten the thin middle nut against the big nut to ensure that neither one will move on its own.

As a side note, at the MN State Championship I had a team that appeared to have a bad pressure switch - from all indications it wasn’t turning off the compressor until they hit 140 PSI. We tried swapping it with two other pressure switches, but neither of them seemed to work (we turned off the robot after passing 125 PSI for each of them). We finally realized the issue was with the pressure gauge, not the switch! Once we swapped the gauge out, we saw that the original pressure switch was working perfectly correctly.