View Full Version : Oil Pressure Switch

05-02-2003, 06:49 PM
I think this item has been covered before, I obnviously did not pay much attention.

Every now and again I come across a system tripped on oil pressure switch. A simple reset and away she goes again.

And then you think..............."why did it trip?"
I can think of the following scenarios

Failed oil pump
Low oil level in sump
Excessive washing away of oil on startup leading to reduced oil pressure
Faulty oil pressure switch


Am I right in saying that the oil pressure is approx 30psi over suction pressure

05-02-2003, 08:21 PM
Here's another possible scenario:

The compressor trips on overload, the oil pressure switch times out and trips, then the overload resets itself.

06-02-2003, 04:26 PM
There is another that used to crop up more often than not - on water chillers, in my experience, especially in the UK, where we have a long tradition of oversizing water chillers.

If the chiller runs for prolonged periods at low capacity, this is usually accompanied by short cycling, then the oil is stranded all over the system due to a reduced gas velocity coupled with short run times.

A low oil level without a pool on the floor means it is still lurking in the system somwhere.

The level in the sump goes down to a point where the pump pick up contains gas as well as oil. The trip does what it is supposed to do and stops the unit. At this point, well meaning engineer changes it because it is 'faulty'.

The solution is to let the heat load build up and run the system for 20 minutes or so. The oil will start to creep up the sight glass.

You may need to bypass the oil pressure switch to get it to run at all....

The oil pressure differential depends on the type of oil, type of pump, viscosity, temperature etc, but 30 psi diff is quite usual in my experience. Hydraulic unloaders can sometime play havoc with oil pressures.

Hope it helps
Ferrari 166 Inter history (http://www.ferrari-wiki.com/wiki/Ferrari_166_Inter)

06-02-2003, 07:14 PM
I would warn against bridging out any safety, I personally make a point of never shorting out a safety. Add oil until you see a level in the sightglass, allow the oringinal oil to come back, then drain to a half glass, at all times not allowing the oil level to go above the glass.
Hope this helps, Regards. Andy

07-02-2003, 12:59 AM
........... 'ought dernit!!!!!!!!!!!!!!!!!

We have had a LONG string on this issue....... I've been searching for it..........still looking.........:confused:

11-03-2003, 02:54 AM
I'm just an old country boy but many times I have found low oil switches tripped because of burnt contactors (especially on 3ph units):)

12-03-2003, 12:38 AM
I'm just an old country boy but many times I have found low oil switches tripped because of burnt contactors (especially on 3ph units)
Dang. I thought I was old. Which 3 hp units have oil switches? I can't recall ever seeing one.

Although, Gary's comment comes to mind if you have internal inherent protection, but then again, I can't recall any 3-hp compressors having that either. And I grew up in St. Louis.:)

13-03-2003, 05:10 AM
I guess the salt spray has made your eyes fail! PH not HP

15-03-2003, 04:36 AM
Haha. Yep. Salt.

How do pitted contactors cause oil failures?

15-03-2003, 11:18 AM
Hi Dan,
probably means the compressor contactor pulls in, the comprssor doesnt start but the auxillary on the contactor makes the feed to the oil switch timer circuit and the oil switch takes the safety circuit out after the delay time. No compressor running equals no oil pressure.
Regards. Andy