Air Gauge Question on my 92 U300

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Air Gauge Question on my 92 U300

November 28, 2014, 02:29:20 pm

Good morning, I have a question on my air gauge on my u 300, I only hear my Air Dryer exhaust every so often, and I know I should be hearing it each time the pressure builds to 120lbs or so. I have a Bendix Ad 4 dryer, Not knowing how long ago it was serviced I decided to rebuild it. So I got all the OEM parts and finished the job today. Got it back in and fired the Coach up, it built air just fine, but as i watched the gauge build air the needles did not come up at the same time. I have two a white one and orange one. The white one lags behind about 3 to 5 lbs and never catches up to the orange one and my purge never goes off. Anybody have any thoughts. Do i have a bad valve someplace ? or is this normal for the Gauges to be off. Thanks for your help Andy

Re: Air Gauge Question on my 92 U300

Reply #1 – November 28, 2014, 02:38:20 pm

What is the condition of the airbags on your rear suspension, are any of them leaking?

Re: Air Gauge Question on my 92 U300

Reply #2 – November 28, 2014, 03:15:19 pm

Andy,

On page 20 of the Bendix AD-4 Air Dryer Service Data manual SD 08-2407 symptom 7 is air dryer does not purge or exhaust air. http://www.bendixvrc.com/itemDisplay.asp?documentID=2670

This should give you some guidelines as to what to do. Thank-you and Brett Wolfe for reminding me as I need to service mine also. It should be serviced at least every three years. I don't know when, if ever mine has been serviced. It's part of one of those vital systems on our coaches.

Jerry

Re: Air Gauge Question on my 92 U300

Reply #3 – November 28, 2014, 03:25:39 pm

And, a further suggestion.

If the dryer is over 10 years old, I would not get a kit and rebuild (which means replacing filters).

I would get a FACTORY (i.e. the manufacturer of your dryer, not Bubba's reman or overseas copy) reman or new. There are a lot of parts like check valves, heaters, etc that may or may not be working properly.

Re: Air Gauge Question on my 92 U300

Reply #4 – November 28, 2014, 04:02:13 pm

Ok the best I can tell is that I have no air leaks in the rear bags or any where. I did use all bendix replacement parts, at least said so on the box. The gov is a d 3 with limits at 100# to 150 # and it is older than 10 years . So Iam going to look at replacing the gov and see if that helps.

Re: Air Gauge Question on my 92 U300

Reply #5 – November 28, 2014, 04:56:08 pm

The white needle is the rear dry tank. The air dryer fills the wet tank, which in turn fills the two dry tanks. The wet tank is located in the back of my coach. The two dry tanks are located in the front, even though one of them is called the rear tank and supplies air to the rear suspension. I think you have a leak in the portion of the system that the rear dry tank fills. If you have the drawings of the air system you should be able to find the leak that way. My AD4 pops off at 110 psi. You should be able to hear an air leak that's bad enough to prevent the air dryer from popping off.

Re: Air Gauge Question on my 92 U300

Reply #6 – November 28, 2014, 05:06:48 pm

Other likely candidates for a leak in the rear air system include the drain valve in the bottom of the rear dry tank, the parking brake button, and the air brake treadle. On the 1992 and newer U300 you have an automatic leveling thingy that I don't have on my 1991 model. I've read on this forum that the six valves are troublesome and leak frequently, so you might want to investigate that. I don't think the 1992 U300 came with a hydraulic retarder unless it was special ordered, but if you do you could have an air leak in the electric air valve on the retarder. This happened to me a couple of years ago. Also, for the benefit of other readers who have the 1991 and older U300 with a rear radiator, there's a pneumatic temperature sensor on the back of the passenger side cylinder head that controls the radiator fan clutch and it can leak air, especially in cold weather. This happened to me last winter and it took some searching to find a new sensor.

Re: Air Gauge Question on my 92 U300

Reply #7 – November 28, 2014, 07:02:09 pm

Thanks Scott, Good information. So here's what I have discovered. I have the drawings and was not sure which tank was which. I do have the two tanks in the front and one in the back. On the drawing it shows the 1/4 inch line coming from the main tank all the way to the compressor, When I remove that line it does not empty the air system it runs air for maybe 3 to 5 min and then stops. So correct me if Aim wrong wouldn't all the air be dumped if it is coming from the main air supply? Or is the rear tank supply being supplied by the main tank up front. And so should the white needle be reading the same as the orange needle and if not. I really am stumped on this so if my post doesn't make any sense be easy on me lol.

Re: Air Gauge Question on my 92 U300

Reply #8 – November 28, 2014, 07:32:16 pm

I'm not sure what line you are "removing". On my air diagram, there is a 1/4" line that runs from "Wet" air tank (which I think you are calling the "Main" tank) to the D2 governor. There is a 3/8" line that runs from Wet tank to the Air Dryer. The supply line from the Dryer to the Wet tank is a 5/8" braided hose. If you are removing one end of either of the smaller lines, they should indeed allow air to exhaust out of the Wet tank. If they don't, something is plugged up.

If you are removing the 5/8" hose end, that is a different story. There is a one-way check valve where the supply line enters the Wet tank (on the diagram it is a little box with a arrow inside - arrow indicates direction of flow). If you remove one end of that line (from the Dryer to the Wet tank), the pressure will bleed off inside the line, then the check valve should prevent air flow out of the wet tank. This sounds a little like what you describe, but it would not take "3 to 5 min" for the hose to bleed down.

Air goes from Wet tank to "Front" and "Rear" tanks. There is another one-way check valve at the inlet of each of these tanks. Red arrow on your dash gauge usually shows Front tank pressure - white arrow is usually Rear tank pressure. If you don't have any leaks, when the compressor "unloads", the pressure in all three tanks should be the same. Don't assume the dash gauge is perfectly accurate. You can connect a temporary pressure gauge to the drain valves from all 3 tanks to check the pressures, and see if your dash gauge is reading correctly.

As Scott says (above), if your compressor is never reaching the "unloading" pressure, then you have a pretty good leak somewhere...however, it can still be hard to hear it over the engine noise. Try letting the pressure build as high as it will go, then shutting engine off and immediately going back to the engine compartment to see if you can hear the leak.

Don't despair! The air system is hard to understand at first glance, but if you study the diagram and compare to the actual hardware on your coach, you will get the hang of it.

Re: Air Gauge Question on my 92 U300

Reply #9 – November 28, 2014, 09:23:44 pm

@Andy my white needle is always 3-5 lbs behind the red needle. The system does pop off about 120 lbs. My dryer is a Haldex. YMMV...

see ya
ken

Re: Air Gauge Question on my 92 U300

Reply #10 – November 29, 2014, 12:43:19 am

Chuck Thanks, I have not removed the 5/8 inch line at all. Just the 1/4 inch lines off the governor, the air only takes about 30 to 45 sec to drain the tank and if there is a one say check valve that would explain why the front tanks do not empty. Got that. So the whole reason i started this adventure was because I was not getting a pop off or purge out of my air dryer, that's why I rebuilt it. So is it possible my governor is out of wack ? They are pretty cheap . Iam really not sure i have a leak because I have had the engine hatch open in the passed and been all over this thing and never heard any massive leak. I wonder if i could put a gauge on the supply line coming from the wet tank and see what the pressure is ?
Ken thanks for that info as I first thought that might be the problem. I will start again tomorrow and see what I can find,I will go get a new governor and see if that make a difference. Thanks again for all the help, I will figure it out and probably laugh at what i fine

Re: Air Gauge Question on my 92 U300

Reply #11 – November 29, 2014, 08:49:46 am

Andy,

The new D2 is a REALLY good idea - just consider it cheap insurance against future problems. Try to get one that is adjustable - it will have a black plastic cap on the top that is removable. Under the cap is a threaded screw with a lock nut. Once you have the new D2 in place, if you are still having problems with your dryer not dumping, at least you have eliminated one possible cause.

I recently replaced what I think was the original factory installed D2 on my coach. The old one was still working, but when I took it apart (curiosity) I found the inside totally trashed out with gunk and corrosion. I was really surprised that it was still functioning.

While you have the new D2 on your bench, it is very easy to add a pressure gauge. This gauge will read your "WET" tank pressure, which I think is nice to know. Depending on where your D2 is mounted in your engine compartment, the gauge can mounted either directly on the D2, or remotely. My D2 is hidden behind a hydraulic tank, so I went with remote mounting. You will see, when you look at the D2, that there is a unused port on the side that reads RES (reservoir) pressure. I think it is 1/8" NPT thread. Take one of the supplied plugs that come with the D2 with you when you buy the gauge so you get the correct size connections. I used a short nipple, a collar, a needle valve, and a line to my remote gauge. Photos below of my install:

The needle valve came in VERY handy on my first trip with the new gauge setup: Driving Tip: Scan Your Gauges!
That plastic line has now been replaced with a braided stainless hose with threaded connections on both ends. Live and Learn!

Re: Air Gauge Question on my 92 U300

Reply #12 – November 29, 2014, 09:38:51 am

How long do the governors normally last? I haven't had any trouble with mine so I haven't messed with it and neither did the previous owner as far as I know. I looked at it and decided not to mess with stuff that's working perfectly unless I have a really good reason to. I know there's a manufacturer suggested overhaul schedule for the air dryer and mine was way overdue so I rebuilt that last year even though it was still working perfectly. Is there manufacturer scheduled maintenance for the governor?

Re: Air Gauge Question on my 92 U300

Reply #13 – November 29, 2014, 10:33:32 am

When I overhauled my AD4 a year ago the only difference I noticed was the popoff. It used to give a long slow blast of air when it released. After the overhaul it gives a very short, much louder burst of air... usually when my ear is near it so it scares the snot outta me!

Re: Air Gauge Question on my 92 U300

Reply #14 – November 29, 2014, 07:56:45 pm

A spare D2 governor is priced from somewhere from less than $20, to maybe $35, depending on where you buy one. Carry a spare, and know where your's is located. The truck parts guys have them under the counter or on a very near by shelf, as they sell lots of them.

Re: Air Gauge Question on my 92 U300

Reply #15 – December 04, 2014, 05:22:46 pm

Well just a update. It has been super cold here in Billings the last week so it shut me down as well as work lol. Today though I discovered a problem. I have been looking over my air diagrams and found a remote air drain for the wet tank or rear tank. So i opened the valve and zip nahda nothing, so i took the line off the 90 at the bottom of the tank. Nahda no air. So I took a screw driver and poked it in and big ice blockage. took me a few trys but i got it all out. When I get home tonite i will remove the service line to the governor and put some air to it and see if it has any restriction. It might be a case of not getting enough air to the governor to make the unloader work.

Re: Air Gauge Question on my 92 U300

Reply #16 – December 04, 2014, 05:54:11 pm

And water in the wet tank suggests that the air dryer has failed-- at least I assume yours has a dryer.

Re: Air Gauge Question on my 92 U300

Reply #17 – December 04, 2014, 10:00:41 pm

Yes is does have a dryer and that would be my guess as well. So looks like a new dryer.

Re: Air Gauge Question on my 92 U300

Reply #18 – December 04, 2014, 10:03:38 pm

Yup, one of the most neglected routine maintenance items.

In fact, I just replaced my Bendix AD-9 today as PM. As I have posted before, an air dryer is one place I would NOT use a "look alike" dryer of unknown quality. There are a lot of "Bendix style" and "Haldex style" dryers for sale. They are NOT the same.

Re: Air Gauge Question on my 92 U300

Reply #19 – December 04, 2014, 10:14:33 pm

Yes they are that's why owning one of these take attention to detail. I new I had a problem when it would not purge. I had no idea what had been done to the Coach and it sat in a shop for 3 years while Sam was sick. I have been going over all the systems the last couple of months to make sure. After driving in the oil patch and over the road for all these years i like to think i learned something. At least I can do the work myself and save the labor. Iam going to replace the AD4 with an AD 9 this week end. Thanks.

Re: Air Gauge Question on my 92 U300

Reply #20 – December 04, 2014, 10:15:56 pm

I did exactly the same AD-4 to AD-9 on our 1993 U240.

Re: Air Gauge Question on my 92 U300

Reply #21 – December 04, 2014, 10:23:29 pm

Bertt, Can i use the same mounting bracket ?

Re: Air Gauge Question on my 92 U300

Reply #22 – December 05, 2014, 08:25:17 am

Andy,

It has been several years since I did it, but as I recall it was an easy install.

Bendix has some pretty good PDF's of their dryers. Just Google AD 4 and AD 9 and you can see how both mount.

Re: Air Gauge Question on my 92 U300

Reply #23 – December 05, 2014, 09:48:31 am

Thanks

Re: Air Gauge Question on my 92 U300

Reply #24 – December 05, 2014, 11:06:52 am

Two needles means two different instruments.
Are you sure the instrument is reading correctly?
JD

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