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Old 07-07-2020, 03:12 PM   #1
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ISL air compressor -- only getting 27PSI on gauges

I have a 2007 Tiffin with a ISL 400 Cummins.
Along with a ride height problem I am now only getting about 27PSI at the tank gauges. suspension not airing up of course. Not even enough air to operate the Parking brake release.

On the ISL ---there is the air compressor driven on engine, with governor on top
Air goes to Dryer then to tanks

At the dryer there are primarily three air lines.
INLET from governor
Outlet to tanks
Governor sensing line.

What would be best method to determnine if problem is at air Compressor/Governor or at dryer?

Release air at sensing line ??? and see what happens???

thx Dale
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Old 07-07-2020, 04:00 PM   #2
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Probably the first thing I would do is make sure you don't have a leak. After getting it to the highest pressure you can shut it down and wait about an hour and see if the air pressure dropped anymore. How long has it been since you serviced the air system?
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Old 07-07-2020, 04:24 PM   #3
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There should be two large 1/2" lines on the dryer. One from compressor and one out to the wet tank.

The governor sensing line comes from the wet tank.

The small line on the dryer is the unloader line FROM the governor. The Governor reaches pressure and sends an air signal to the compressor and unloader. That stops the compressor from pumping and purges the dryer.

The air dryer purge valves stick open and let the air escape before getting to the tanks. Some dryers have threads in the purge valve. A 1/2" plug will fit, as a test.

The compressor output lines rupture and leak. Only hear it with engine running.

The line from the dryer to the wet tank can rupture. Can only hear it with engine running.

Beyond the wet tank, that has a check valve to stop the air from going back towards the compressor, you can hear a leak with the engine off, if you can build some pressure. Try increasing the idle to build more pressure.
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Old 07-07-2020, 05:35 PM   #4
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I have no real leaks if you shut of the engine. but there is only 27 lbs.
If you pump the brake you can hear air on each pump.

I cannot hear any large leaks with the engine running either.

I know nothing about air system servicing since i just purchased a week ago.

I have increased the idle and it only goes from 24 to maybe 27.

Let me reread twinboat's post so I can comprehend it.
twinboat -can the dryer malfuntion so that air does not get to the tanks even if the compressor is finctioning correctly?
This happened suddenly as I just drove it the day before.

Attached is the schematic that Freightliner sent me today given my VIN.
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Old 07-07-2020, 05:52 PM   #5
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Yes the dryer can fail and all of the air goes out the bottom and not to the tank.

That's why I suggested plugging the hole in the bottom.
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Old 07-07-2020, 06:44 PM   #6
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got it --- thx Dale
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Old 07-07-2020, 07:29 PM   #7
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Does the parking brake disengage? If so, you have more than 27psi, and it's a gauge problem.
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Old 07-07-2020, 07:49 PM   #8
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twinboat is spot on. My gut is also the dryer...you know what kind it is? Haldex?

Take a close look at the discharge line from compressor to dryer. Some are steel pipe or copper tubing for the first few feet due to the heat the compressor makes, some are stainless with a Teflon liner.
I've seen several times where a clamp or something rubbed a hole on line, usually near compressor where it's almost impossible to hear leak....

You maybe able to put your hand over the "moisture ejection" hole with engine running, in bottom of dryer for leak, you have a major one. And since you can't hear it, leads me to believe it's only leaking when engine is running. Only a few places that could happen.

Some dryer don't have a threaded hole to plug off, If you have to you can disconnect the two 1/2" lines and plumb together with a couple of fittings to bypass the drier.
... just in case, also make sure the wet tank drain valve is closed, or the moisture ejector, if equiped, ain't leaking.
.....oh and post up some pics of your new whip!


EDIT: that twinboat must be a great thinker, I reread his post above, looking like I just copied it, ha-ha
....

Like I tell my wife, "smart and good looking, so few of us left"!
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Old 07-07-2020, 07:51 PM   #9
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Quote:
Originally Posted by 96 Wideglide View Post
Does the parking brake disengage? If so, you have more than 27psi, and it's a gauge problem.
He said the parking brake did not release in his opening post.
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Old 07-07-2020, 08:01 PM   #10
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Quote:
Originally Posted by twinboat View Post
He said the parking brake did not release in his opening post.
Missed it.

My bet is on a big hole in one of the lines.
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Old 07-07-2020, 08:14 PM   #11
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Just reviewing this before I look more tomorrow. My hope of course is that it is a line or dryer related. Those would be far less costly than a compressor failure.
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Old 07-08-2020, 07:01 AM   #12
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OK ---todays update
I started engine today -- same 27 PSI (was zero on gauges -- came ot 27 in under 30 sec)
crawled under rear --constant leak at air dryer from what I believe is purge valve.

photo attached (sorry sideways- right is down)
you can see on rail nearby where air is constantly blowing.
Now --- cause of constant purge?
Next steps?
Attached Thumbnails
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Old 07-08-2020, 07:03 AM   #13
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rereading mackwrench post again --- it appears this is "moisture ejector" which has constant leak -- -I assume fix is to replace it ?????
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Old 07-08-2020, 08:39 AM   #14
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OK --- continuing to research ===

Engine running -- air still 27

If I disconnect the 1/8 inch SILVER air line from side of DRYER, air comes out momentarily from Dryer side and stops === air leak from "moisture ejector?" stops

I reconnect that 1/8 inch line -- the system airs up to 130 PSI (no more leak at dryer) and I hear the PSSSHH of air release at max PSI

now --- if i hit brakes a few times and lower air pressure the air compressor does NOT recycle on.

If I then go back to line by air dryer -- remove it a few seconds (air from dryer side comes out a few seconds then stops) --- replace line

System airs back up to 130 PSI

Advice on where the problem is?
Inside dryer still??????

-- still researching ---
thanks in advance -- Dale
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