Subaru Outback Forums banner

1 - 6 of 6 Posts

·
Registered
Joined
·
18,179 Posts
Discussion Starter #1 (Edited)
So, how does the oil pressure sensor switch and others like it “click”, that is, work, and more to the point, go bad? I had some ideas but when the switch on my neighbour’s 98, 2.5, 4EAT Outback failed, I thought I’d look into this, literally. I found what makes them tick, or “click” and what went wrong in this instance.

Some background:

The oil pressure warning light didn’t come on when the key was turned to ON but the engine was still not started. The immediate cause was thought to be the bulb but after removing the combination meter and swapping in a known good bulb, there was no change.

The next likely cause was the oil pressure sensor switch. With the engine off, there should be continuity between the single terminal on the switch and its metal case/engine block. A quick check revealed a very high resistance. In addition, the voltage between the wire connector and ground, with the ignition at ON, was 12 V so there was no problem in the wiring.



The switch was replaced and all was good.

The breakdown:

The next step was to examine the defective switch more closely.

I found that there was oil showing in the small gap between the terminal and black plastic top. Either this was coming from fumes around the engine compartment, or from inside the switch. (Turns out it was the latter, but we’ll get to that.)

The switch consists of a metal base and a black plastic part at the top, with the single terminal coming out vertically.



Looking at the fitting that is attached to the engine, we can see a tiny hole in the end. This is a passage to the main switch area. The tiny opening limits the amount of oil that can pass through in the even the switch develops a leak at the top, and perhaps slows the variations on the other side so that the switch doesn’t give a false indication.



The black plastic part is held to the metal base by the round crimping around it’s circumference. To get inside, I used a rotary tool to cut that upper part away.



With the cut completed, the loose ring was removed, and then the black plastic part pulled out easily. This is what I found:



The terminal at the top comes through the plastic and bends 90 degrees, and is riveted to the plastic. There’s a small spring that at one end contacts the terminal. The black mass inside the spring (at 7 O'Clock) looks like sludge, but I suspect it was some grease that was probably originally applied to this area of the switch.

The metal base side revealed a rubber diaphragm inside, with a brass or copper cup-like part riveted to the center.



The rubber diaphragm pulled out easily, leaving three parts:



The other side of the rubber diaphragm (and the other side of the rivet) is actually a flat disk.



This revealed how the switch works. When the diaphragm is in place with the spring held in behind by the plastic top, the flat metal disk at the center of the diaphragm is pushed against the center of the base. At the center of the base is a copper-colored part that has three raised pointed bits. That part is crimped into the metal base.



So the flat disk is in contact with the pointed piece. All together, there is continuity from the terminal, through the spring to the cupped metal part, through to the disk on the other side of the diaphragm and from there to the pointed contacts on the metal base. This closes the circuit for the engine oil pressure warning light, turning it on.

When the engine starts and the pressure builds up, the oil from the engine goes through the tiny hole and pushes against the metal disk and diaphragm. With enough oil pressure, the disk lifts up, separating from the three raised points. This breaks the circuit and the light goes out.

So what went wrong?

There was good continuity between the metal base and the three pointed contacts (they are crimped together). There was also good continuity between the cup-shaped piece and the flat disk on the diaphragm (riveted). The problem, however, was that the flat disk had become tarnished, and this acted as an insulator, preventing a good electrical connection between it and the pointed contacts on the metal base. There was also tarnish or some sort of dark coating where the spring contacts the terminal and where it sits in the cup-shaped piece -- these contacts were electrically intermittent. (In the picture I have cleaned off half of the disk to show the difference.)



Why did the contacts get so tarnished? Hard to say. There’s oil on the flat disk/pointed contacts side of the diaphragm all the time, but it’s also exposed to the heat of the engine. Perhaps it was the combination of heat, contaminants in the oil, and time.

There was also oil on the opposite side of the diaphragm (where, I believe there should be none), and as noted above, oil visible at the base of the terminal on the outside. It appears that the diaphragm wasn't sealing against the metal base as well as it should. Perhaps this oil contributed to the tarnishing of the spring contact surfaces in the same way as with the flat disk on the other side of the diaphragm.

It's not unusual for plastics, and rubber, to age and undergo dimensional change. If the plastic part were to shrink it’s pressure against the outer edge of the diaphragm would be compromised, and this would loosen the diaphragm seal allowing oil to get past it. This would show up more often on older vehicles. However, there are several reports here of the oil pressure sensor switch on the variable valve lift mechanism (2006+ 2.5) leaking significantly, and I suspect there was a similar, but premature, loss of seal effectiveness inside the switch. All to say that if you see any sign of oil at the base of the terminal where it comes out of the plastic part, oil is getting past the diaphragm and this might be a signal for an eventual failure of the switch to work properly electrically, and worst case, develop a significant leak.

So there you have it. The ins and outs of the oil pressure sensor switch. I hope that you don’t ever have a problem with it, but if you do, I also hope this provides some insights into what might have gone wrong.

Some additional notes

The switch had what looked like the remnants of Teflon tape on the threads, so it might not be the original switch, or even a genuine Subaru part. (My neighbor didn't own the car from new.)

The design of the inside components might be different depending on which company made it, and when.

The Subaru spec calls for the switch to function at around 2 psi. I had no way to check this, but it's apparent that the spring and the rubber diaphragm work together to hold the flat disk against the pointed contacts. I found that neither the spring nor the diaphragm seemed to have a lot of resistance to movement, and this seemed to be consistent with the very low psi switching point.
 

·
Registered
00 OB 07 OBXT
Joined
·
5,538 Posts
Interesting, nice writeup.

I think about this sort of thing all the time, sometimes I toss it on the work bench with intentions of messing with it but I rarely follow through. Even when I do, it's hard to find anyone else who actually cares enough to listen to the explanation.

I didn't think my wife really listened to my ramblings but, the other day I was telling her about what I was doing to the car [struts, trying to adjust out the positive camber] and I used Formula One cars as an example [she watches it with me] ... Out of the blue she says " Don't they normally run negative camber on those cars" ?

Thanks for taking the time
 

·
Registered
Joined
·
2 Posts
I'd love to see the photos that go along with the post, but they are super blurred out. Do I need to have a photobucket account to see them properly? I know this is an old post, but I see it referenced from time to time. I'm getting ready to replace a leaking oil pressure sensor and reading all that I can find on it. Thanks!
 

·
Premium Member
2001 VDC/SC One of a Kind
Joined
·
14,322 Posts
I'd love to see the photos that go along with the post, but they are super blurred out. Do I need to have a photobucket account to see them properly? I know this is an old post, but I see it referenced from time to time. I'm getting ready to replace a leaking oil pressure sensor and reading all that I can find on it. Thanks!
The oil pressure switch is not difficult to replace. And it's cheap.
Remove the alternator belt and tilt the alternator up and use something to hold it up; or you can take it out altogether, but if you do please disconnect the battery.
The OP switch is a 24mm hex head. Unplug it by just pulling up on the wire connector. Use a deep socket and an extension and take it out. The new sensor will have thread sealer on it and you just screw it in snug. Put the alternator back down, attach the belt, connect the battery, and away you go.
 

·
Registered
Joined
·
18,179 Posts
Discussion Starter #5
I'd love to see the photos that go along with the post, but they are super blurred out. Do I need to have a photobucket account to see them properly? I know this is an old post, but I see it referenced from time to time. I'm getting ready to replace a leaking oil pressure sensor and reading all that I can find on it. Thanks!
When the photos were first posted, they were stored on Photobucket, and we were allowed to link the stored photos into posts here. Photobucket was free at that time, but then restricted that type of linking to paid memberships (by the original photo holder).

I have the photos, but unfortunately can't now insert them into the first post (above). But I can post them here in the same order.

Oilpressureswitchcircuit.jpg Oilpressureswitch.jpg Smallholeinfitting.jpg Retainingcrimpcut.jpg Plastictoprevealed.jpg Metalbaseopened.jpg Threepartstogether1.jpg Threepartstogether2.jpg metalbasecloseup.jpg Tarnishremoved.jpg

Please note that this is the engine oil pressure sensor (switch); as @cardoc noted, it's the one at the top of the engine somewhat below the alternator. Depending on the year and engine, there could be more than one oil pressure sensor; some engines have them with the AVLS (variable valve lift) systems. Those pressure sensors are similar, but not the same; they work at different pressures, and, have a different connector at the top.

Also, when installing the new one, be vary careful not to overtighten. There's been cases of cracking the aluminum casting (block) where the sensor is attached.
 

·
Registered
Joined
·
2 Posts
@plain OM Thanks for reposting the pictures! I love seeing how stuff works. I'll be very careful not to over tighten. This forum is an excellent resource.
 
1 - 6 of 6 Posts
Top