Monday, January 19, 2015

Location, Location, Location...

Things everyone knows about locations:

  - Don't build your house near a pork rendering plant, or (literally) a stone's throw from a high school, the criticality of the latter being dependent on the number and size of glass windows that face it.
  - Don't wear Maize & Blue to the Ohio State / Michigan game when it's in Columbus.
  - Don't buy sushi at a 7-11 (unless it's a treat for your cat).
  - Don't count your cards when you're sittin' at the table. There'll be time enough for that...

And here's one that not many people know, unless they've built and flown an RV-12:
  - Don't mount a $400 oil pressure sensor directly on an engine.

Luckily, there is now a somewhat cheaper replacement for the $400 sensor, and I think RV-12s are now shipping with a different type. That said, I believe the newer sensor is still attached directly to the engine where it is likely to meet the same fate as the two previous models.

In addition to the lower cost replacement sensor, there is also a nice little kit that you can buy that relocates the sensor to a presumably less dangerous neighborhood. It's sold by Aircraft Specialty, the very same folks that sell the fuel and brake lines that I have used to upgrade those provided by Van's.

At first glance, it doesn't seem all that cheap at $144.95, but when you consider it more deeply you will see that it's a bargain. It comes complete with everything you need but minor hardware such as a #14 size Adel clamp, an AD3-5A bolt, a dab of thread sealant, and whatever you like to use to restrain and protect engine hoses.

If your existing Honeywell sensor is already fried, you will also need a replacement. I went with the one that Lockwood Aviation sells since it's a direct replacement for the one I had and I wasn't in the mood to experiment:

Product: Oil pressure sender, 1/8 npt, 4.2ma
Part Number: GAOPSNDHK
Price : $129.95

These are the factors that I considered before parting with the cash for the kit:

  - Even the lower cost sensor that replaces the $400 one is $130. Mine failed at 70 tach hours.
  - Replacing the sensor in it's location on the engine is more difficult that replacing one mounted on the firewall.
  - The wire that runs from the Skyview to the pressure sensor has to be routed alongside the engine. This can be problematic.

For a person of moderate skills, the relocation is pretty straightforward. For those that are more.... challenged when it comes to skills, well..... keep reading.

As I mentioned in a previous post, my sensor died a week after Kyle, Cable & Wiring Guru for The Jackson Two, suffered an identical loss. Always willing to let him figure out the tough stuff first, I drove down to see how his relocation went. As we will see, it was a wasted trip, at least insofar as the wiring goes.

Performing the leak check:

Spoiler: it leaked. He had a small drip where the new fitting attaches to the threaded connector on the new sensor. A little bit more wrench fixed it right up.

I always like to go into Jackson proper for lunch at a great little place called Arch and Eddies. We got there before they were open, so I got a chance to visit one of our customers from the day job.

I was sorely tempted to buy some of the Crick-ettes, simply for the raw enjoyment to be had by pre-placing one somewhere in the break room at work and waiting for a co-worker to notice it, at which point I would casually just pick it up and eat it.

That plan fell through when I realized that I really didn't want to eat a cricket, no matter whether it came in a box as a "food product" or not.

I don't think I'm alone in that; I couldn't help but notice that they had plenty on hand. As in, all of them - not a single box sold.

When I got around to doing my own installation, I started by removing the big oil hose fitting that sits directly under the old oil pressure sensor to provide better access for wrenching it off of there. I figured there might be some oil in the line that would prefer to lubricate the hangar floor, so I used a food storage bag as a prophylactic:

The fitting that goes in where the old sensor was is one of those that go in with quite a bit of resistance and also tend to leak a bit. I learned with the brake fittings, which are very similar, that using a little thread sealant with a Teflon additive addresses both of those issues:

With the lubrication provided by the thread sealant, I got a nice, tight fit.

The new sensor does not come with the wire needed to attach it to the Skyview, but I figured I didn't need it - I would just cut the existing wire off of the old sensor.  With that done, the only meaningful purpose for the old sensor was to cut it open and see why something that simple costs over $400. After all, they're like $35 for a Lycoming engine.

Well, it all comes down to analog versus digital, it seems. The sender for my Lycoming was purely mechanical - all it had to do was deliver a variable resistance to a mechanical gauge. The Skyview, being nothing more or less than a digital computer, needs that analog signal converted to a digital representation, and that require some electronics. Not $400 worth, of course, but maybe $129 worth.

I went ahead and pulled the original wire back away from the engine. It was attached to the Skyview wires with a pair of terminals, which made it easy to remove it entirely. I propped the two Skyview wires against the engine mount to keep them from falling back into the bird's nest of wires down in that no man's land.

I pared back the insulation, then cut off the unneeded black wire.

This is a very important part of the instructions:

The sensor comes with a plug and two crimp-on terminal female pins. If you screw up attaching one of those pins, you're done until you can get replacements. If you crimp them on before pushing the wires through the holes in the plug, well, you're done for awhile.

You also want to be very careful to get the correct wires in the correct holes, for the exact same reason. With that in mind, I very, very, VERY carefully lined my wires up to match the drawing of the plug provided in the instructions:

I also bent the exposed wire double to make sure there was enough wire for the crimps to grab ahold of.

With that done, I fastened the new oil line to the fitting on the engine. I figured that oil might have trouble getting through the line if it had air in it, so I prepared the open end of the line to allow for a little overflow as I pumped oil into it.

I had decided that I did not want to drill a new hole in the firewall to mount the new Adel clamp, so I chose an existing bolt to use as the mounting location. The easiest place to put the new Adel appeared to be adjacent to the brake fluid reservoir. I routed the new oil line up and across the engine.

To pump oil into the new line, I just moved the prop the same way I would if I was trying to 'burp' the engine. After five or six rotations, I was surprised to see that I wasn't getting any oil at the end of the new oil line.  I went around to the other side of the engine to see what could be wrong.

Remember that I removed the input line and stuck it in a baggie?  You do? Well, you could have spoken up!

With the input line attached, it took just a few more turns of the prop to fill the new line.

It was a simple matter to remove one of the bolts from the brake fluid reservoir.

The additional thickness of the Adel clamp meant that I would need a longer bolt. This is why I keep bags of them handy.

It was just as easy to mount the Adel clamp and the sensor. You can see the fitting here that also benefited from some of the thread sealant - it's the white band between the hexagonal  top of the sensor and the new fitting.

Unfortunately, the new hose was about an inch too short to let the sensor sit vertically, but a little offset was enough to allow the attachment of the line and still provide enough slack to account for engine vibration and/or movement.

If you're wondering how much the engine moves, consider the other problem that I fixed while I had the cowlings off:

There was a pretty good gap there already, but clearly it wasn't enough. I used the ScotchBrite wheel to make more room.

With everything all plugged in and wired up, I cranked up the Skyview, only to be faced with a severe disappointment:

That big red X meant that I had gotten the wires reversed, despite my best effort.  How could that have happened??

Lets's look at that diagram again:

Oh, DUH!!!

The diagram obviously shows the wires going into the sensor itself, not the front of the plug as I had mistakenly assumed.

Given that I didn't want to cut off the terminal pins (remember: I had no replacements) to swap their locations in the plug, I reversed the wires with a pair of splices instead. That did the trick!

I ran the engine for a good five minutes during which the gauge held a steady 72 psi, and not a single drop of oil leaked from either of the new fittings.

I'm calling this one DONE!


Kevin said...

70 hours! Sheesh! I hope Vans moves that sensor to the firewall before I order the engine kit. Sounds like I'm going to have to do it sooner or later anyway.

Dent said...

Wow admin,
Such a great post with lots of information for the local people. Great job and cool reviews. Specially the gallery is just awesome. Thank you so much for sharing.

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