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.