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Old 12-28-2015, 06:57 AM   #1
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Air Pressures

Requesting expert advice: 2005 DSDP air gauges read 125 rear, 75 front. It's been like this for 3 years/12000 miles. Never an issue, good air service. I always assumed it was just a defective gauge, and lived with it.

Yesterday We had a maximum braking event, which reduced front pressure below 65 and the alarm sounded. Now I'm concerned.

Never have replaced air dryers on my watch. Opinions.... Gauge? Sending unit? Dryers?
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Old 12-28-2015, 07:16 AM   #2
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Is it a mechanical gauge if so can you switch the two lines at the gauge? that would at least tell you if the gauge is reading okay.
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Old 12-28-2015, 08:09 AM   #3
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If your air dryer blows off after reaching 125 on 1 gauge, then I would suspect a bad guage.

If your air dryer never blows off, you have a leak in the front system.

In either case, 3 years is long enough to ignore a brake problem.
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Old 12-28-2015, 08:51 AM   #4
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I am surprise that after 3 years, you never took it in to have it checked out.
All Rv's need yearly maintenance. The Air dryer, requires service including replacing the regulator.

My one question is, how much value do we place on the unit and contents including the people that are in it?
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Old 12-28-2015, 08:52 AM   #5
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Probably not the air dryer if one of the two tanks is up to pressure, but you almost surely have a major air leak somewhere. Should not be difficult to find and fix, either yourself of any decent truck shop. And it is past time to change the cartridge in the air dryer as well, to prevent future problems.

What is the value of having gauges on the dash if they are ignored? Why assume the gauge is wrong when the readout shows an abnormal condition?
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Old 12-28-2015, 09:03 AM   #6
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I dont know if your gauges are electric or mechanical. Mine are electric and on a CAN bus.

When the compressor is filling the tanks, BOTH tanks receive air from the same supply.
there is no reason for them to be different.

Sounds like your alarm works correctly. You need to determine if the warning buzzer and the gauge run off of the same sensor. If yes, then the sensor is likely bad.

If the gauge has a air line to it, then switch gauges like the previous post suggested.

Bottom line, both gauges should show about 120lbs when the compressor burbs, and the warning should come on around 65 lbs.

Regards,

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Old 12-28-2015, 09:14 AM   #7
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75 lbs? and you never thought that was low? most alarms trigger between 60 to 70 lbs. DOT specifies 70 in their regs for air brake rigs. you shouldn't even move an air brake vehicle with less than 90 lbs pressure in both tanks. In my driving career I have done my own maintenance. I would be checking both of the tank sending units and both gauges. If air dryer is not purging every few minutes or not at all it is in need of cleaning or replacing depending on type you have. be safe
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Old 12-28-2015, 12:39 PM   #8
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Crawl under the MH and determine the make/model of air filter system. It will be either a Haldex or Meritor Wabco system. Replace the air dryer/desiccant filter! It should be <$50 at any HDT service center; or have them do the work. You've already received excellent advice about the air system. A plugged desiccant filter will do all sorts of strange things.
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Old 12-29-2015, 10:23 AM   #9
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one more thought...you mention front and rear. that confuses me a bit. air systems are primary and secondary; secondary being back up for primary system failure. either way though time for this system to have some diagnosis and maintenance...3 years overdue. be safe
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Old 12-29-2015, 10:35 AM   #10
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one more thought...you mention front and rear. that confuses me a bit. air systems are primary and secondary; secondary being back up for primary system failure. either way though time for this system to have some diagnosis and maintenance...3 years overdue. be safe
Front and rear because the front brakes work off the front tank and rear work off the rear tank.

There is a third tank, actually first in system, called the wet tank. It supplies both other tanks.

There are protection and shuttle valves between them all.

Download Air Brakes 2007 from the DOT web site.
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Old 12-29-2015, 12:47 PM   #11
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Quote:
Originally Posted by Flyerpicker View Post
Requesting expert advice: 2005 DSDP air gauges read 125 rear, 75 front. It's been like this for 3 years/12000 miles. Never an issue, good air service. I always assumed it was just a defective gauge, and lived with it.

Yesterday We had a maximum braking event, which reduced front pressure below 65 and the alarm sounded. Now I'm concerned.

Never have replaced air dryers on my watch. Opinions.... Gauge? Sending unit? Dryers?

Flyerpicker,
First off, you don't say what chassis you have. If it's a Freightliner, and, in the '05 era as you say, your gauges should be driven by what's called the MMDC or, Multi-Module-Data-Computer. Without going into a novel here, suffice to say that the MMDC receives data from all the important modules in the coaches operation systems.
1. Engine ECM
2. Transmission TCM
3. ABS Module
4. Air input, both Front/back-Primary/Secondary

The MMDC receives all this data and, interprets it, then sends it to the appropriate gauges, be them digital and or, analog. And that includes air pressure gauges in the dash. So, they are NOT MECHANICAL. The data link that carries all the information, to and from all the modules to the MMDC is called a J1939 Data link. But, the only "mechanical" input from any thing to that MMDC IS the air line inputs. There's a green and a red line into that MMDC.

Now, here's a suggestion. Find that MMDC in your coach, normally in the front area, maybe a left lower, front compartment or, any other massive electrical junction area(under dash) etc. and locate the air input lines to that MMDC.

The MMDC is a unit that's about 6"x6" x about 1" thick or so. It has at least one 21-pin plug and I think, a couple more 5 or 10 pin plugs, can't remember, it's been a while since I messed with one. But, you'll also see the two air lines leading into it. Inside that MMDC where the air lines enter, are two transducers. Those transducers change the air pressure mechanical input into electrical output, to be sent to those gauges.

Suggestion:
Reduce the air pressure in the total system to "0" psi. You can hit the brake pedal several times to bleed that system down to zero psi. Once that's done, then after you've located that MMDC, the air lines are held in by what's called "Push to release" type fittings. In that, there is a collar around each air line, right where they enter that MMDC.

The procedure for removing an air line that's equipped with push to release fittings is,
1. Push the air line into it's hole. It won't move very far at all. In fact, you may or may not actually feel it move.
2. While holding pressure inwards (towards the hole), push on the collar towards the hole.
3. When you do that, the collar will move a slight amount. What that does is, releases a "Barb" type effect and, with pressure on that collar, you can now pull on the air hose and it will slide out of it's hole.

Simply remove both hoses and, replace them in the opposite holes. That is, move the Green one to the place where the Red one was and, the same for the Red one.

Now, fire up the coach and let it build air to full pressure and, see how your gauges react to this potential analysis. If, you now see 75 psi in the rear gauge and 120 in the front, you have a bad transducer inside the MMDC. Hopefully that's not the problem. Those transducers are NOT REPLACEABLE!! You'll have do dig deep in the ole' pocket book 'cause the MMDC hovers around $450-$500 depending on who you get it from, which is severely limited due to it's nature.

If you do not see any difference in the two air pressure gauges after the change of the hoses, you quite possibly have a defective air pressure gauge in the dash. But, that's not explaining why you instantly dropped to 65 psi during a heavy brake application. My explanation of what's happening is merely trying to assist in analyzing what's what with your air gauges etc.

You state your unit (coach) is an '05 era coach. Well, again, if it's a Freightliner chassis, you more than likely have the Haldex air dryer on it. And, it's not just simply replace the "descant" air filter. There are two filters in that dryer, a pressure relief valve and, a purge cover that all need replacing/servicing. And, depending on how yours is mounted on your particular coach, it quite possibly will have to be removed to do all this work.

There are some on here that think all the work can be done on the coach, WRONG!!!!!!!!!!! Based on again, how yours is mounted, part of the components CANNOT be removed due to the close proximity they are to the mount plate. It's not really a big deal to remove that entire air dryer. You remove a couple of air hoses which, one of which may entail the use of some rather large wrenches.

The other is a push-to-release type attachment. And, there is a 12VDC heater on the bottom side, located inside the purge housing that is connected via an electrical connector that is easily disassembled. Once all those connections are disconnected, then break out the tools necessary to remove the air dryer/mount plate from the frame of the coach.

Now, before you get too carried away with all this, you'll need to find, order/purchase the correct service kit for your air dryer. If you have what I think you have, you'll need the DQ6026 kit. Better check on that to be sure.

But, if you're not up to doing this kind of work, well, then cruise on down to your local Freightliner/truck service center and let them take care of it. You'll be paying a bit of labor for something you're more than likely able to handle yourself.

Anyway, good luck on your trying to find out what's happening with your system. Pleas post on the results so that others may learn.
Scott
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Old 12-29-2015, 09:30 PM   #12
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Spartan chassis ?

I have a 2003 DSDP and had multiple false reading on air gauges with alarm's going off. It was a mess and "expensive" errors in finding the problem. If you have a Spartan chassis call them first instead of later (would have saved me a lot) and ask about the possibility of the "grounds" going bad on the gauges. I had the shop call them here in Kansas City....they ran new ground's for the gauges and no problems since. You will probably talk to Charlie at Spartan... Great guy!
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Old 01-01-2016, 07:26 AM   #13
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To all that posted: THANK YOU! I certainly have enough information to research, test, analyze and repair the problem.
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Old 01-01-2016, 09:19 AM   #14
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Curious why folks suspect the dryer.
The wet tank is in front of the dryer, and the pri & 2nd tank are fed by the wet tank.
If the pri tank shows 120psi, that proves the dryer is ok and the wet tank must have 120psi.

In thinking about this more, it is not likely to be a leak, if it was leaking bad enough to prevent reaching 120, it would certainly drop to zero right after the engine was turned off.

A restriction would not prevent reaching 120, it would just take longer than the other tank.

It really sounds more like a sensor problem which could affect both the gauge and the alarm.

The OP really needs to put another gauge on the system to confirm the actual pressure before buying parts and guessing.

Just the opinion of an arm chair quarterback

Dan
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