Higher air pressure after air dryer replacement . Trouble ahead?

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Higher air pressure after air dryer replacement . Trouble ahead?

September 12, 2017, 06:14:11 pm

In a continuing effort to update or replace aged parts of the coach, replaced original OEM Bendix Air Dryer with AD9 which is touted as a direct replacement. Install went well but man oh man what a place to put a device which needs to be serviced. Here's my concern. Before air pressure seemed to purge and cut off at about 120 lbs. Both needles for front and back tank tracked pretty close together as well. Now red needle ( rear tank ) out runs white needle ( front tank ) and pressure tops out at right at 140. At that point front tank about 130. Know I am getting close to danger zone of 150 but cannot figure out what I could have done to change pressure. Just changed inlet and outlet of dryer and connected heater wires. Have considered new air governor but not sure this is the cause. Any thoughts?

Thanks in advance

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #1 – September 12, 2017, 07:37:29 pm

Steve, here is some basic info on the D2 governor: Haldex D2 Governor Service Data

Not sure why replacing the air dryer would affect the cutout pressure.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #2 – September 12, 2017, 09:17:40 pm

That's what is making me scratch my bald head. Appreciate the link, will check it out now.
Steve

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #3 – September 12, 2017, 09:38:19 pm

Sometimes messing with one component in the air system can cause other components to act up. No rhyme or reason to it.

As you see in the tech document linked above, the D2 is (usually) adjustable. If it was me, I'd try adjusting the D2 down a little, and see if the cut-in and cut-out pressures follow. If they do, get them set where you want them, and see if the setting holds.

If you can't get the pressures to change in a logical direction by adjusting the D2, then I'd replace it. They are cheap, and a new one should solve the problem. Or, it will at least eliminate that component as the trouble maker.

PS: Are you sure the red needle is the rear tank? I'm not saying you are wrong, but that is the exact opposite of our coach. I guess the air hoses on the back of the air pressure gauge could have been switched at some time on one of our coaches. No big deal - it's just curious.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #4 – September 13, 2017, 10:26:29 am

I don't have the schematic on front of me but I thought both the front and rear tanks ( not the wet tank ) were in parallel with the air source. If so tank pressures should be the same at rest. Can you switch the input to the gages to confirm that they are both good?
You need to confirm that both gages are good before chasing other possible problems.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #5 – September 13, 2017, 10:49:03 am

Agree with John - both front and rear air tanks should show exactly the same pressure at the instant of compressor cut-out. However, my 2 gauge needles sometimes seem to drift apart slightly, even when I know the pressure in the tanks is equal. I attribute that to 23-year-old "gauge error" and don't worry too much about it. As long as they both move consistently with increase and decrease in tank pressures, I'm happy.

Steve's primary concern seems to be the (now) higher cut-out pressure of his compressor. I think if he solves that "mystery", the slight off-set in his gauge needles may possibly correct itself. Or not...

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #6 – September 13, 2017, 02:34:44 pm

Need a little guidance here on "wet tank " or "rear tank" . There is one tank on the rear of the coach near the rear axle. There is a drain valve for that tank just in front of the left rear wheel. When I release pressure thru the drain valve red needle goes down. There are two tanks over the front axle that have independent drain valves. When I release that pressure white needle goes down. I am beginning to think after the comments the tank at the rear of the coach is known as the wet tank and that the two tanks up front are separate for front and rear brakes. is this correct?

Thanks, Steve

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #7 – September 13, 2017, 03:46:44 pm

The rear most tank is the wet tank. Differentiating between the front two may require the air schematic for your coach.

Quote from: steve31 – September 13, 2017, 02:34:44 pm

Need a little guidance here on "wet tank " or "rear tank" . There is one tank on the rear of the coach near the rear axle. There is a drain valve for that tank just in front of the left rear wheel. When I release pressure thru the drain valve red needle goes down. There are two tanks over the front axle that have independent drain valves. When I release that pressure white needle goes down. I am beginning to think after the comments the tank at the rear of the coach is known as the wet tank and that the two tanks up front are separate for front and rear brakes. is this correct?

Thanks, Steve

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #8 – September 13, 2017, 04:42:18 pm

Steve,

You are correct. Rearmost tank is the wet tank. The two tanks at the front end are the "front" and "rear" tanks, named according to which brake circuit is serviced by which tank. You can differentiate between the two front-mounted tanks, as Don stated, by comparing the numbered air hose connections shown on your air system diagram to the actual hose setup.

If you have all three tanks at the same pressure, like immediately after the engine air compressor cuts out, and then you open the drain valve for the wet tank (next to the rear wheel), you should not see either of the dash air pressure gauge needles move. The reason is there are one-way check valves between the wet tank and the two other tanks. If the check valves are working, you can bleed the wet tank down to zero and the other two tanks should not lose any pressure.

SO, you apparently have one or more check valves leaking. Not unusual at all - they are easy to replace. I would suggest you get that done ASAP, because they serve a important safety function by conserving air pressure to operate your brakes in the event of a catastrophic failure of your air dryer, or the engine driven air compressor, or any air hose upstream of the check valves.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #9 – September 13, 2017, 05:12:32 pm

As Don says, it's time to study a schematic. I don't think either pressure gage should move when you drain the wet tank. I think there is a check valve in there to prevent the front or rear tank from loosing pressure back to the wet tank.
Something is going on that doesn't seem right. I wonder if draining all air pressure to change the dryer caused a check valve to get stuck?

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #10 – September 13, 2017, 05:14:36 pm

Oops, I agree with Chuck!

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #11 – September 13, 2017, 06:34:09 pm

Depending on the make / model of your check valves, you may be able to clean them and install a rebuild kit.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #12 – September 23, 2017, 04:47:57 pm

Finally got a chance to get back online. As always this forum a huge database of knowledge. Been studying schematic, searching the Forum ( found some pictures! ) and been under the coach multiple times. Bottom line is air system to important to fool around with . Replacing D2 air governor and almost all check valves and will test the rest. Parts are cheap and labor ( mine ) is as well. Won't get a chance to do the work until next weekend. Will let you know how it turns out.

Again thanks to all for the help.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #13 – October 05, 2017, 01:20:59 am

SUCCESS !!! Replaced D2 governor , double check valves and single check valves. Governor was the culprit however check valves needed to be upgraded. Going to rebuild old unit and keep it in the coach. One never knows. Less than $150 plus my time so will feel a lot better rolling down the road. Unloads and dryer purges right at 125 lbs, can't ask for more than that.
My heartfelt thanks to all.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #14 – October 05, 2017, 05:59:49 am

Why rebuild a 15 dollar D2 governor?

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #15 – October 05, 2017, 11:00:35 am

White Pages Technical Information (Valuable. Thank you Chuck):

Quote from: Chuck & Jeannie – September 13, 2017, 04:42:18 pm

Rearmost tank is the wet tank. The two tanks at the front end are the "front" and "rear" tanks, named according to which brake circuit is serviced by which tank. You can differentiate between the two front-mounted tanks, by comparing the numbered air hose connections shown on your air system diagram to the actual hose setup.

If you have all three tanks at the same pressure, like immediately after the engine air compressor cuts out, and then you open the drain valve for the wet tank (next to the rear wheel), you should not see either of the dash air pressure gauge needles move. The reason is there are one-way check valves between the wet tank and the two other tanks. If the check valves are working, you can bleed the wet tank down to zero and the other two tanks should not lose any pressure.

SO, you apparently have one or more check valves leaking. Not unusual at all - they are easy to replace. I would suggest you get that done ASAP, because they serve a important safety function by conserving air pressure to operate your brakes in the event of a catastrophic failure of your air dryer, or the engine driven air compressor, or any air hose upstream of the check valves.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #16 – October 05, 2017, 02:12:24 pm

Stump: What I really want to do is pull the old one apart and see why it quit. Might as well rebuild it as long as it's apart. Might learn something.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #17 – October 05, 2017, 04:05:32 pm

So after reading this thread I decided to take a look at my airtanks..2 located near front axle (cannot tell which services the front or rear brakes). Red needle White needle???.
Another question is on the rear wet tank...it has no drain valve but does have a line attached to the lowest part of the tank..?? can not trace the line......so is this a auto drain system.??

Thx

Hans

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #18 – October 05, 2017, 04:36:12 pm

My wet tank drain is in front of left rear tire on bulkhead little brass valve

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #19 – October 05, 2017, 04:42:45 pm

Hans,

Check your air system schematic - compare the diagram to the actual numbered air lines attached to each of the front tanks. That will allow you to figure out which is "front" and which is "rear".

On our '93 U280 tank on passenger side is "front", tank on driver side tank is "rear".

On our '93 U280 dash air pressure gauge, red needle is "front" tank, white needle is "rear" tank.

Look just in front of the driver side rear tires, along the bottom outside edge of the wheel well...usual location of "wet tank" manual drain valve.

Re: Higher air pressure after air dryer replacement . Trouble ahead?

Reply #20 – October 06, 2017, 11:09:48 am

Thanks all ....found the drain valve and figured the Front/Rear tank layout from the schematic..One question tho, on the front tank is this regulator set @ about 50lbs...what is its function??

Hans