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post #1 of (permalink) Old 03-27-2001, 03:48 PM Thread Starter
 
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OBA Unloader valve

I just purchased my pressure switch for my onboard air installation. It came with an "unloader" on it.

I really don't understand its function. I believe its purpose is to vent pressure from the switch back to the tank to somehow ease the starting of the compressor.

Do I need to hook this up?.

BigHG
84 CJ7
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post #2 of (permalink) Old 03-27-2001, 04:02 PM
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Re: OBA Unloader valve

Yep, that's right...the unloader vents the pressure in the line between it and the compressor, so when the compressor re-starts, it is only pushing against atmospheric pressure in the line instead of pushing against 100psi. Do you need to hook it up? It isn't absolutely neccesary, but it would ease wear on the a/c clutch...your OBA system is powered by an engine driven compressor, right?

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post #3 of (permalink) Old 03-27-2001, 04:14 PM Thread Starter
 
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Re: OBA Unloader valve

Okaaaay, I get the concept but am not sure how to make it work.

The switch I have has 2 connections on it. One that looks like a diaphram that senses the pressure and then the unloader. This means I will need to put in a check valve between the two? With the unloader hooked to the compressor side of the check valve?

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post #4 of (permalink) Old 03-27-2001, 04:37 PM
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Re: OBA Unloader valve

To make it work you'll need a check valve with an unloader port. Then just connect the unloader on the pressure switch to the check valve with a piece of copper tubing.

'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274
post #5 of (permalink) Old 03-27-2001, 04:38 PM
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Re: OBA Unloader valve

[img]/wwwthreads_images/icons/tongue.gif[/img] The unloader valve connects to any part of the line between the compressor and the check valve. The diaphram port of that switch connects to the tank side of the check valve. So the pressure switch diaphram is "feeling" the tank pressure, and the unloader valve opens on compressor shut down so the compressor has at least a little empty pipe to pump into when it restarts. On some compressor setups they even have a little chamber to make even more empty space so the compressor can start easier.[img]/wwwthreads_images/icons/crazy.gif[/img]

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post #6 of (permalink) Old 03-27-2001, 05:32 PM
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Re: OBA Unloader valve

NAPA has a combination check/unloader valve that sells for approx. $15.00. Ask for a combination valve. The valve with a 1/2" compression fitting and a 3/8 NPT fitting is part number 8212. They also had a regular check valve that was about the same price.


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post #7 of (permalink) Old 03-27-2001, 08:32 PM Thread Starter
 
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Re: OBA Unloader valve

Okay I fully understand it now. Thanks everyone!

Jeeperjohn got me to thinking a little bit more. I am building a manifold to make the installation look good. I was planning on using all 1/4" npt fittings as I have a tap that size. Do you think I will lose a lot of CFM by going with this size as opposed to 3/8". I am pretty sure most of my existing quick connects are all that size.

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post #8 of (permalink) Old 03-28-2001, 07:02 PM
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Re: OBA Unloader valve

I'm not sure. It is a fact that a 3/8" hose will flow more than a 1/4", but the unknown is how many cfm is the compressor capable of putting out. I am using 3/8 on my OBA project, but I always over build.


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post #9 of (permalink) Old 03-28-2001, 10:08 PM
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Re: OBA Unloader valve

[img]/wwwthreads_images/icons/smile.gif[/img] And the survey says.................1/4" will do just fine.[img]/wwwthreads_images/icons/smile.gif[/img]

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post #10 of (permalink) Old 03-29-2001, 11:14 AM
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Re: OBA Unloader valve

I'm with Dave. The majority of air tools, inflaters, blow guns, etc. use a 1/4 NPT fitting on them.

'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274
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