Oil Pressure Switch Failure

Ironflight · #1 02-26-2012, 04:13 PM

There I was, high over the Texas prairie, enjoying the view as Junior?s autopilot drove us home after a long (but successful) day of flying the RV0-1 in Fort Worth. Suddenly, without warning ? the oil pressure annunciator light comes on! Now THAT got my attention, so I looked for a confirming ?Master Alarm? light from the EFIS?.and none came. I looked at the oil pressure on the panel ? nice and steady at 78 psi. Well, it varied just a little ? enough to tell me that it was, in fact, reading correctly, and still showing plenty of pressure. The annunciator is driven by a simple oil pressure switch screwed into the transducer manifold, and obviously, something was wrong with the circuit. I put it out of my mind for the rest of the trip, and when I got home, I pulled the cowl to check it out.

The pressure switch that I use is a regular automotive switch, used almost universally in cars, trucks, generators, pumps ? any place where you want a switch to complete a circuit when you have less than 5 (or so) psi of oil pressure. I?ve never had one go bad in decades of using them in an airplane ? but this one really had an interesting failure mode ? not only didn?t it work electrically (I checked the circuit quickly by pulling the wire off - the light went out- and grounding it to the airframe -the light came on), this one also failed with a LEAK! The case of the switch was wet with oil, and a drizzle of engine oil ran down the firewall. I check the connections, and everything was tight ? the case of the switch itself was leaking. There was no general spray of oil, so the leak was really tiny ? but it clearly had failed in a bad way, and I quickly replaced it with a spare.

This is one of the reasons for the restrictor orifice in the oil pressure fitting on the engine ? in case of a gauge leak, we won?t lose much oil. But it is something to check, as I know that many, many people use these automotive switches for this job. I know that some will say that you shouldn?t use an automotive witch in this application, but I have looked at the ?aircraft? ones sold by the popular supply houses ? they appear to come from a very similar source. This certainly isn?t a catastrophic failure (so long as you have the orifice fitting), but it is something to note the next time you have your cowl off.

Paul

jbagley · #2 02-26-2012, 04:57 PM

Is this the same oil pressure switch that Van's sells with their Hobbs meter?

This is what is in my airplane.

Ironflight · #3 02-26-2012, 05:14 PM

I'd have to see a picture James - but generally, a Hobbs pressure witch turns ON when you have pressure - the one I am talking about turns OFF when you have pressure.

Paul

Lars · #4 02-26-2012, 09:17 PM

Quote:

Originally Posted by Ironflight

I'd have to see a picture James - but generally, a Hobbs pressure witch turns ON when you have pressure - the one I am talking about turns OFF when you have pressure.

Paul

Hobbs makes them both ways, and even sells a couple with both normally open and normally closed contacts. I've never had one fail on an airplane (given the laughably small number of hours in my logbook, that's not surprising) but I've had a new switch fail in a car, within hours. Don't know if it was made by Hobbs, but the failure mode was just as you describe.

jbagley · #5 02-26-2012, 09:28 PM

The one I have (from Van's) does both. DPDT switch. One side turns off at 15psi and the other side turns on. Makes for a nice double function; turn the hobbs on and turn the idiot light off from one pressure switch. Has worked flawlessly for the last 115hours and 10 months. The casing seems pretty heavy duty so i'm not worried about leakage. But, I still have a restrictor just like your suppose to.

Sounds like a very different unit than what you have.

Quote:

Originally Posted by Ironflight

I'd have to see a picture James - but generally, a Hobbs pressure witch turns ON when you have pressure - the one I am talking about turns OFF when you have pressure.

Paul

turbo · #6 02-27-2012, 06:55 AM

oil pressure is a required instrument. the builder of my plane elected to install a non electric oil pressure gauge along with the electric sender type. the non electric is rock solid. i can not say that of the other. i am glad i have both. safe flying.

chaskuss · #7 02-27-2012, 08:31 AM

Paul,
You can thank NAFTA for that one. I belong to an international automotive mechanics forum. The purpose of which is to aid each other with difficult repair problems. I am AMAZED at the reports of brand new defective aftermarket parts. This even includes well known, well established brands.
I have seen and experienced this problem many times myself. Most aftermarket parts are no longer made in the USA, Japan or Canada. Most third world produced parts are absolute garbage. Suggest you look for an automotive replacement directly from a manufacturer. OEM is the only way to insure quality automotive parts these days. It's a sad fact of life now.

Charlie

fl-mike · #8 02-27-2012, 10:06 AM

Quote:

Originally Posted by Ironflight

this one also failed with a LEAK! The case of the switch was wet with oil, and a drizzle of engine oil ran down the firewall. I check the connections, and everything was tight ? the case of the switch itself was leaking. There was no general spray of oil, so the leak was really tiny ? but it clearly had failed in a bad way, and I quickly replaced it with a spare.
Paul

Paul, have you never owned a Jeep? Oh, maybe you have, I see you had a spare...