Air Compressor puzzle.

[Cuppa]

The air compressor in our car feeds into an air tank which has the usual pressure cut off switch & a pressure relief valve (PRV).

Recently I noticed that the compressor was continuing to run after it reached the cut off switch's upper limit, & would continue to run eventually causing the PRV to vent.

I assumed that a new cut off switch was needed, bought one & today I fitted it.

BUT it has made zero difference. The compressor still doesn't cut out when it reaches the switch's pre set pressure.

I''m wondering of I have a faulty new pressure switch but also whether there might be some other cause. Can't think of anything else though. The hole the switch screws into isn't blocked.

Any ideas?

[mudski]

I would assume the signal line to the compressor from the switch is corrupt in some way. Is the switch single or two wires? If its two wires there should be current running through the switch when the compressor is on. If its single wire, it will just be an earth point. But from memory its two wire. My knowledge is very limited to electrical stuff, does the compressor run if you disconnect the switch?

[MudRunnerTD]

Have you made any other changes or additions to your system? Is thebpressute switch going to a relay? It is a simple positive In Out switch. Run a got wire from the battery to the pressure switch to the compressor and isolate it from the rest of the car and test. If it turns off it is your loom, if it runs on then the new switch is knackered too. Is the pressure switch bolted to the compressor? Change ports.

In fact...... change ports first. Bet that is your trouble.

[Brissieboy]

Unlikely a new switch is faulty - but not impossible.
Just disconnect (at least) one wire from the pressure switch and the pump should stop. If so, it means the problem will likely be the pressure not getting to the pressure switch to operate it. Make sure that air freely escapes from the hole when you remove the pressure switch, and there is no obstruction possible when it is fitted.

[the evil twin]

As others have said... usually a symptom of a faulty switch or blocked pressure port due to corrosion

Assuming the PRV and Pressure Switch are not integrated
1 - remove pressure switch (leave wiring connected), run compressor if tank is empty, either way air should freely escape switch port - if not port is blocked
2 - remove wire from switch, compressor should stop - if not wire is low resistance to earth (pressure switch should be on earth side of comp not 12V)

[Cuppa]

Have you made any other changes or additions to your system? Is thebpressute switch going to a relay? It is a simple positive In Out switch. Run a got wire from the battery to the pressure switch to the compressor and isolate it from the rest of the car and test. If it turns off it is your loom, if it runs on then the new switch is knackered too. Is the pressure switch bolted to the compressor? Change ports.

In fact...... change ports first. Bet that is your trouble.

Pressure switch is on the tank. One wire in, one out. Connect the two wires together without the pressure switch & compressor runs. Connect to pressure switch & compressor runs. No problem with the loom. Suggests it is the new switch I think. Not sure what you mean by 'change ports', ... is that if the pressure switch is on the compressor?

[Cuppa]

Unlikely a new switch is faulty - but not impossible.
Just disconnect (at least) one wire from the pressure switch and the pump should stop. If so, it means the problem will likely be the pressure not getting to the pressure switch to operate it. Make sure that air freely escapes from the hole when you remove the pressure switch, and there is no obstruction possible when it is fitted.

Pressure builds past 150psi (this is the max the gauge reads & is also the cut off pressure for the switch) until the tank releases pressure via the PRV. With switch removed the hole is clear.

[Cuppa]

As others have said... usually a symptom of a faulty switch or blocked pressure port due to corrosion

Assuming the PRV and Pressure Switch are not integrated
1 - remove pressure switch (leave wiring connected), run compressor if tank is empty, either way air should freely escape switch port - if not port is blocked
2 - remove wire from switch, compressor should stop - if not wire is low resistance to earth (pressure switch should be on earth side of comp not 12V)

1. yep air escapes
2. remove wire & compressor stops.

Still seems to me that the new switch is faulty. Old switch had a slight build up of brown water staining, but new switch should have been an instant fix I reckon.

[the evil twin]

Hmmm... does sound like you got a dodgy new switch then Cobber.

[Cuppa]

I think I've sussed it, but wont know for certain until I get another pressure switch.

There was another possibility not mentioned by anyone & which I certainly hadn't considered.

I thought that the way Pressure Relief Valves worked was that they would reach their blow off pressure & then WHOOSH, they would 'let go all at once. Well it turns our they sort of do, but not quite in that way.

When getting close to their blow off pressure they, or at least mine, start to let air out quite slowly & gently, & only when the compressor keeps pumping in air faster than the PRV is releasing it does it blow off in a big way.

It appears that my PRV is a 150psi unit, same as the cut off pressure for the pressure switch. So when the pressure gauge is reading 150psi, air is slowly & quietly being leaked from the PRV & consquently the switch is not activated.

I will now get a pressure switch with a lower cut off pressure - the previous one was 140psi & was ok until moisture (I suspect) killed it after 5 years. Fingers crossed.

It wasn't until I turned both the compressor & the car's engine off (I run the car whilst using the compressor) having pumped 150 psi into the tank (according to the gauge which I suspect might not be super accurate at it's upper limit) that I heard a small telltale his of air escaping & sure enough it was the PRV.

[the evil twin]

Makes sense mate... the cutoff should be a min of 10% to about 20% below the blow off so anything between 120 and and 135 will suit admirably.
140 isn't leaving much room and would def not go any higher

[MudRunnerTD]

140psi cutoff is pretty high. Most are set at 120psi.

I mentioned "ports" earlier, this means the hole that you thread the switch to into. If it was screwed into one ofnthey holes in the compressor then swap to a different port of the compressor. There are a few ports on the ARB compressors .

You could unscrew the gauge and fit the pressure switch there to test too.

The PRV you mentioned will bleed then blow as you describe, kind of a reverse of a staun deflator when they are nearly finished bleeding to the set pressure.

Try the switch in a compressor port and see if there is any difference .

[MudRunnerTD]

I am conscious You are on the road mate so trying to problem solve while mobile with what you have access too.

The cylinder on thr side of the ARB Compressor is actually a little tank. The Alan key nuts in the side and ends are spare ports. Any one will do mate.

Hope this helps in some way.

[MudRunnerTD]

http://www.nissanpatrol.com.au/forums/images/imported/2019/10/142.jpg

Those 2 allen keys are ports that you will be able to screw you switch into.

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[Cuppa]

My compressor is a Boss PX07 (https://4wdindustries.com.au/products/px07-boss-air-compressor-12v-3-4-hp-100-duty-cycle-com-px07-bare?variant=7023245951022), doesn't have any ports in it for a switch. Switch goes into receiver tank.

Higher pressure is good for air gun use.

140psi cut off not currently easily available at present.Will use a 110 on, 135 off.

[MudRunnerTD]

Ok. The ports are out of the head. There will be more than 1. Where your hose is coming out is 1, look for a blanked off second one. In the pic below it has 3 I can see bit I expect there is probably 4. Others on google showed no port on 1 face but multiple options certainly.

http://www.nissanpatrol.com.au/forums/images/imported/2019/10/143.jpg

You honestly won't tell the difference out of an air gun between 140 and 110psi mate. The only time you might feel the difference is running an air tool like a rattle gun anything else will just see the compressor start sooner but make little difference to operation.

Most workshop 240v compressors have a 80psi 120psi range. Don't loose sleep.

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[Cuppa]

Thanks Darren. It does look like those are ports, but they are just part of the casting that are unused. Not drilled or threaded, only useable port is the one the hose comes out of. Unlike some compressors this one requires a one way valve in the hose between compressor & tank, so fitting a pressure switch into the cylinder head even if it were possible would probably not regulate the tank pressure very well, if at all.

I take your point about lower pressure cut off's.

[MB]

Crikey, slightly worried now thanks for the thread Cuppa mate! My utes ARB twin has been factory bought I believe set from the get go @ 150psi shutoff?



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[Cuppa]

Issue resolved. Bought & fitted a pressure switch with a 135psi shut off. All good now.