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Yet Another OBA System

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Joined: 10 Jan 2006
Location: Iceland

PostPosted: Mon Feb 11, 2008 1:46 am    Post subject: Yet Another OBA System Reply with quote

Hi everybody,

I have just finished (for a certain value of finished; I have yet to install the control switch and wiring) converting the AC in my XJ to on board air. This has of course been done, and documented, numerous times before. Here are a few links I found useful; there are more out there:

Update: I have now finished the control circuit also, See below.

* Note: English is not my native language, and I may have used the wrong technical terms to describe some components here. Give me a shout if something is incomprehensible, or correct the terms if you can guess what I'm trying to say Very Happy *

Now, my chief objective was to do this as cheaply as possible. Also, I only wanted to be able to air up my tires; air tools are not on the agenda for now. Given this, I decided to go with an "open" system: Instead of using a pressure switch to shut off the compressor when a certain pressure is reached, my system is constantly pumping as long as it is switched on. There are no valves on the exhaust side, so no pressure is built up, except in the tire while the chuck is on the stem. It should be noted that this system is completely unusable with just about all air tools, as they are designed for conventional, constantly pressurised, systems.

So, what components were required?

First of all, a safety valve. While the system was supposed to be open, it would only take a kinked air hose to change that, so this was a must.

Secondly, an oil seperator. The Sanden pump spews a considerable amount of oil out with the exhaust, and I would prefer that it didn't wind up inside my tires.

Thirdly, some kind of oiling mechanism for the pump. There is not a great deal of oil in the pump, so it can run dry in a short time. Some webpages suggested that for occasional use it would be sufficient to give a couple of squirts from an oil can into the intake before each use, but a helpful salesman in an air compressor shop suggested an alternative: Why not recirculate the oil from the drain cock on the oil seperator to the intake via a "backfeed" line and restrictor? Why not, indeed.

Finally, various fittings, couplings, et.c. which I will try to detail below.

Here are (almost) all the shiny new components laid out:

1: Intake air filter / silencer, 1/2" male thread.
2: T piece, 3 times 1/2" female thread.
3 and 4: Hose barbs, 1/2" male thread, to fit 12mm ID hose.
5: T piece, 2 times 1/2" female and one 1/2" male thread.
6: Oil seperator, intake and outlet 1/2" female thread, drain cock 1/8" female thread.
7: Quick release coupling, 1/2" male thread.
8: Quick release adapter w. hose barb, to fit 12mm ID hose.
9: Tire valve chuck, with disappointingly narrow stem, to fit 8 mm hose, I think.
10: Reduction, 1/2" male thread to 3/8" female thread.
11: Safety valve, 3/8" male thread
12: Reduction, 1/2" male thread to 1/8" female thread.
13, 15, 17 and 18: Quick release adapters for nylon pipe, 1/8" male thread.
14: Restrictor valve, 2 times 1/8" female thread.
16: Nylon pipe ("backfeed" line).
Missing are a few pieces I had to add as the project evolved.

It is worth noting here that not all inch-based pipe threads are the same. I had originally intended to mount the intake components directly to the inlet port on the pump, which appeared to be 1" female thread. This, it turned out was incompatible with the 1" components generally available here in Iceland. My guess is that the pump uses US National Pipe Thread, while the standard here seems to be British Pipe Thread. This meant that I had to cut the original AC lines and reuse them with hose barbs to connect the pump:

I had intended to mount the oil seperator behind the grille, with the quick release coupling sticking out through a cutout in the grille, but it turned out that there was insufficient space, so I had to mount it in the engine bay, with a hose out through the grille. The insistence on an externally accessible air connection, by the way, was because I didn't want to be obliged to open the hood in the middle of a howling blissard just to air up. As for mounting the oil seperator, this is what I came up with:

It's a piece of perforated sheetmetal that mounts on the same mounting bolt as the power distribution center and washer bottle, and also the mounting bolt for the coolant reservoir. The black line that is clipped to the oil seperator mounting bolt is the cruise control cable; it used to be clipped to the coolant reservoir mounting bolt. This mounting is not overly sturdy and may need to be updated; we shall see how it turns out.

At this point I found that there would only be a very short stub of hose between the hard line from the pump exhaust to the safety valve / oil seperator intake. I was not too happy with this, given the extreme stiffness of the AC hose. I therefore decided to extend it with the much softer air hose I had bought, and make a loop to take up any vibration and movement of the engine mounted pump, relative to the oil seperator:

A double ended hose barb would have been the proper tool for this, but as I couldn't find one of a suitable diameter, I used a quick release adapter w. hose barb. I think the protusions of the quick release adapter are almost as good as the barb for retaining the hose anyway.

On the intake side I only used the original AC hose. For one thing I have a bit more length of hose to take up movement there, for another the intake components are not rigidly fixed, and finally the intake hose is thinner walled and hence more flexible than the exhaust hose.

The hose barb had to be improvised, as I had already used the barbs I bought on the oil seperator assembly. It consists of a garden hose barb and a 1/2" male to 3/4" male adapter. I would not recommend this kind of barb on the pressurised part of the system, but it should be okay on the intake. Note that the intake filter lies slightly higher than the T piece and pump. This is neccessary because of the oiling arrangements; otherwise oil would drip from the intake filter when the system is not in use. The adjustment of the restrictor will have to be experimented with. I've currently set it so that there is a barely discernible flow of oil when the system is running open, increasing when it is being used to air up the tires. The target is to maintain a constant level of oil in the seperator in normal use: If the level increases it means that the pump is losing oil, and the restrictor flow will have to be increased. If the oil seperator runs dry (I started off by adding a couple of spoonfuls of oil to the bowl), the efficiency of the system will decrease, and the restrictor flow will have to be decreased.

Here is the connection, sticking through the grille:

It was my buddy's idea to use the quick release adapter, rather than the quick release coupling here. It solves the problem of the quick release coupling blocking the exhaust when the hose is disconnected, and it allows the complicated quick release coupling to be stored in the car, rather than exposed to the elements.

Here is the hose connected:

Here is the business end of the hose:

The chuck originally incorporated a shutoff valve, but I was able to disassemble it and remove the core, so now it is always open. Unfortunately this core also served to depress the core of the tire valve. The system still works; the pressure is sufficient to open the tire valve, but I think it would be faster if the valve was mechanically opened. I will be keeping a lookout for a different type of chuck.

Here is my temporary control "switch":

Just a jumper in the AC relay position in the power distribution center. I will be installing a proper, dash mounted, switch in the near future, and will update this thread when that is done. Update: Done. See below.

When I had finished assembling the system and started to test, disaster! My precious oil seperator was leaking around the drain cock. This drain cock was supposed to be semi-automatic; when pressure was removed from the system it was supposed to open automatically. This would not work with my "backfeed" oiling system, so I had drilled it out with a 2mm bit. Whether it was this that caused the leak, or simply careless handling on my part I do not know. Either way something had to be done. I got some two component claylike stuff I had, and which was supposed to set like steel, and proceeded to close the leak with it:

It has already stood up to some testing under pressure. Time will tell how it lasts.

Altogether, this project has cost me between 20 and 25 thousand ISKR; ca. 300 to 370 US dollars. Bear in mind that those are Icelandic prices; I would guess you could get away with about half of that in the US.

I hope this rambling can be of some use to somebody. Any comments appreciated

Best regards,

Last edited by kaos on Sat Feb 16, 2008 11:14 am; edited 1 time in total
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Joined: 01 Jan 2004
Location: Uzbekistan

PostPosted: Mon Feb 11, 2008 12:01 pm    Post subject: Reply with quote

Great job - moving this to the projects forum. Wink
Nothing to see here.
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Joined: 10 Jan 2006
Location: Iceland

PostPosted: Sat Feb 16, 2008 11:08 am    Post subject: Reply with quote

So, time for Part 2, in which a switch is wired.

Note: This description applies to my 1993 model XJ. Other model years can/do have different wiring and need to be adapted accordingly.

This is really the simplest part of the project. I decided to use the stock wiring as far as posssible. Here is the original circuit diagram for the A/C - heater system:

And here it is with my modifications in red:

As you can see, I modified the wiring in two places: In the instrument panel, where I put a control switch and pilot lamp instead of the switch built into the A/C - heating control center. And in the Power Distribution Center, where I grounded one pin of the A/C relay, thus disabling the A/C pressure switch, thermostat and ECU as control elements.

Here is the A/C heater control center dangling from it's wires and hoses:

The connector I hacked into is on the top right hand side of the center. (N.B. if anybody is curious about the metal panel to the right, see this thread:

Here is a closeup of the connector:

Please ignore the wire splice at the back of the connector. I managed to cut the wrong wire and had to splice it together again Embarassed .
This connector originally had 4 wires, marked A, B, D and E on the wiring diagram. Wire A would have been rightmost on the picture. Pin C, in the center, was not connected. Wire A goes to the A/C relay coil, while wire D is a +12V power source when the key is in Acc or Run position.

Here is the switch I used:

I worried a bit about finding and accessing the correct wire in the Power Distribution Center, but, as it turned out, needlessly.
Note, disconnect your battery before starting this part of the modification.
Everybody disconnected their batteries? Good, then we can proceed.
On the front of the PDC is small cover, and under it, the battery feed connection, secured by two 10mm. nuts. Undo those.

Then, use a flat-blade screwdriver to open the metal catches, one on each side of the PDC, and slide the PDC rearwards until it is free of the fixing bracket.

Now you can undo the smaller plastic catches securing the bottom cover to the PDC, and unhinge it at the front.

You now have access to all the wiring of the PDC. The wire we are looking for is dark blue with orange tracer, and goes to the A/C relay position. Cut, splice a new wire to the end that goes to the relay, reassemble the PDC and ground the other end of the new wire. Reconnect the battery feed connection and the battery itself and enjoy your OBA.

Best regards,
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