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air pressure relief

hello, on U240 cat at idle she releases the burst of excess air pressure every 12 to 13 seconds...i'm thinking thats to frequent..if so,what steps do i take to address...thanks
Dale Gass
1993 U240 36                                                             

cat 3116 allison 3060

Re: air pressure relief

Reply #1 –
Could be the air governor ?
John Duld
1995 U320C SE 40'

Re: air pressure relief

Reply #2 –
When was the last time the air dryer was serviced?  My suggestion is to upgrade to a reman Bendix AD-IP. Much easier to service than the AD4 or earlier Bendix models and is an easy installation.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: air pressure relief

Reply #3 –
thank you...
Dale Gass
1993 U240 36                                                             

cat 3116 allison 3060

Re: air pressure relief

Reply #4 –
My dryer was doing the same thing, popping off every minute.  I did as Brett suggested and installed a reman. dryer.  Figured by the time I remove and clean the old unit, buy the rebuild kit to install, my time would be about the same.

Dave Cobb
Buckhorn Lake Resort The Club, #6202, Kerrville TX
check the map.  I do rent it out when I am traveling!
2001, U320, 36' #5887, in Kerrville, FT Club #17006, (7/23 to present)
2024 Jeep Grand Cherokee L, Summit, white
EX: 98 U295, 36' #5219, (mid door), (4/13-10/23)
EX: 93 U225, 36' (4/11 to 4/13)

Re: air pressure relief

Reply #5 –
Dale, Sounds like the air dryer needs service.  IF it relieves and will not shut off you will barely have enough air pressure to keep moving and when you lose all pressure the brakes will set.
EMERGENCY cure is to carry pipe fittings with you and if this happens disconnect the dryer and bypass it. You can then proceed to a repair site.  Even if you do not have the tools or cannot do it yourself you can always get a road service to bypass it for you, this will keep you from being stranded.
Gary B

Re: air pressure relief

Reply #6 –
Dale,
 It could be one or more of several things.  It's not possible to tell without more information/instrumentation. 
The questions regarding when your air dryer was last PM'd/replaced are good ones.  It could be the air dryer column, but if you want to approach from a most likely, least expensive parts replacement approach, a reasonable candidate is, as John has already pointed out, the Bendix D2 Governor valve.  Is yours separate and external to the air compressor?

The D2 governor shifts the air compressor from sending its output to the wet tank (when wet tank pressure is low), to sending its output back to the engine intake (when wet tank air pressure is high/satisfactory).
Your engine air compressor seems to be running under pressure constantly (not good over the long term).  It sounds like it is never shifting from wet tank to engine input air system and it's pressure is staying high. 
A likely culprit in that scenario is the Bendix D2 governor which has a propensity, with age, to not shift properly. If the D2 internal components and/or internal porting become defective (or sluggish with cold temperature) the D2 may not be developing any (or less than full) output pressure, therefore the air compressor output valve doesn't shift (redirect) air from a port to the wet tank to a port that returns air compressor (idling) output air to the engine air intake system.  It sounds like the D2 may be shifting enough to cause blowdown of the air dryer column but not enough pressure to cause a shift in the engine driven air compressor outlet valve.  I've had this happen recently and replacing the D2 restored everything to normal.  I've also had a D2 fail to shift properly and cause the air dryer to hang-up in constant blowdown mode.  Lots of low air pressure alarms going off and a quick exit to the side of the road ........another story, another time!

If it isn't a defective or sluggish D2, another likely component would be the above mentioned air compressor outlet valve.  My son has CATs in his Peterbilts, and has had to replace them.  That component is a precision and considerably more expensive part.  Due to the cost of the part, it would warrant installing temporary instrumentation prior to  "Easter-egging" the problem with replacement parts.
NAPA (and any truck parts supply store) has a ready supply of the D2's in stock and the rebuilt ones are inexpensive, usually available in several ranges of operating pressures.  They are usually $20 or less.  I use the 105/125 psig Bendix D2 and like the higher pressure.
Neal
 
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Neal (& Brenda) Pillsbury
'02 U320 SPEC, 4200, DGFE, Build #5984
'04 Gold Wing
'07 Featherlite 24'
'14 Jeep Grand Cherokee Summit
MC #14494
Exeter, NH & LaBelle FL
Quality makes the Heart Soar long after Price is Forgotten

Re: air pressure relief

Reply #7 –
It's always good to know what the leakage spec. is for your air sys.  On our 91 ORED the spec is 2psi per minute.  I use that as a guide as to whether or not I need to take corrective action.  Have a great day  ----  Fritz
Fritz & Kathy Johnson
1991 36'

Re: air pressure relief

Reply #8 –

We carry a rebuilt spare D2 Governor.

http://www.amazon.com/Bendix-OR275491X-D2-Governor-Remanufactured/dp/B0045V6 U4A/ref=sr_1_6?ie=UTF8&qid=1386866920&sr=8-6&keywords=d2+governor

Fig. 2 Sectional View of Type D-2 Governor

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