Mark VII Low Pressure Pump Issue

[Austin603]

My Mark VII Choice Wash XT is having an issue with the low pressure pump. It’s not turning on. I’m only getting 80psi on low pressure settings. This machine is only a few years old… so hard to believe the pump is out. Mark VII told me it could be the pressure switch (pictured). Has anyone had this issue? I thought I’d ask here before I sell my kidney to afford Mark VII coming out to fix it…

 

[Fatboy769]

First off, I know very little about Mark VII equipment, so take this for what you paid for it. You say that the pump is not turning on, but in the next sentence, you say you’re getting 80psi on your low pressure settings. I don’t see how both of those things could be happening. Every pressure switch that I have dealt with is just a normally open switch with power to one side of it and when it detects a certain amount of pressure, it closes and sends that signal to a input on the machine. I don’t see how that switch would have anything to do with the pump not coming on or putting out low psi, but like I said, I know very little about this particular machine. Maybe @Keno could shed some light on this.

 

[Keno]

That is a pressure transmitter and is an input to the vfd that controls the low pressure pump psi via frequency control - it is not a simple pressure switch. They do go bad, and can cause the issue you are describing. The same sensor is available from other vendors at a significantly lower price. Its a very easy part to replace. Google the model number on that sensor in your machine and you should find some other sources. Be sure to buy the exact model, they have quite a few variations in that family of sensors. I think we paid around $100 for the last one we had to replace.

It's a pretty simple system, not much to diagnose, but parts are pricey, so a proper diagnosis should be made before throwing parts at the machine. To diagnose and ensure it is actually the sensor, you can override the vfd to about 40-45 hertz to test that it is actually the sensor not sending correct data and not something else. Pressure should go up at the higher manual override hertz. You can also check on the vfd what hertz the pump is being commanded to run at currently (this should vary based on which chemicals are being called for to maintain pressure around 200psi). Both will be big clues on what is going on. Google the vfd model for a manual to see how to do all of this. This can be also used as a short-term, temporary fix until your new sensor comes in, but there are risks involved with that also, wouldn't reccommend as a permenant fix.