Originally Posted by Flyerpicker
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?
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.
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.