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Air tank differences

Our tanks used to always be in tandem but over the last week of travel they have developed an 8 pound difference.  The front tank reads higher than the rear tank.  Is this enough difference to delay travel?  We are still going to level on ride height.  Air systems are new to me so your insight is appreciated.  Thanks!
George and Steph
1997 U270 36 Build 5081 "Honu"
1180w Solar 400A lithium all Victron house system
Motorcade 17670, SKP 128300, FMCA F459019
73 VW Camper, 79 VW Camper, 2363 Sunline, and an Arctic Fox 25P

Re: Air tank differences

Reply #1 –
Not a problem as long as they read over 60 while driving. Could be one of the gauges is misbehaving or you have a decent leak on that tank lines etc. You need 60 to keep the rear brakes from going on from that particular tank feed.
JohnH
Coachless, now use aircraft. 2003 Ford Travelair TC280 class C. Super shape. Just for 1 yr .
1994 Ford E350 ClassC,total renovation inside and out. Now sold.
2000 U295  36' Cummins 350 c/w Banks Stinger, Resonator upgrade,Solar, LED lites.Residential fridge with slide out pantry. Build 5674. Sold
ex 92 GV 022C ored Cummins. Sold
ex 95 GV240 cat 3116. Sold
2017 Mini cooper s & 2016 land Rover LR2 HSE  LUX.
jhaygarth@aol.com    SKP #130098
treat everyone as you would like to be.

Re: Air tank differences

Reply #2 –
Thanks John...
George and Steph
1997 U270 36 Build 5081 "Honu"
1180w Solar 400A lithium all Victron house system
Motorcade 17670, SKP 128300, FMCA F459019
73 VW Camper, 79 VW Camper, 2363 Sunline, and an Arctic Fox 25P

Re: Air tank differences

Reply #3 –
I agree with John, could be either a leak or more likely a gauge issue.

But, disagree that all you need to safely drive is 60 PSI.  Anything under 100 indicates a significant issue with your air system (read that suspension AND BRAKES).  Yes, you need 60 PSI for the alarm to go off and slightly less than that to keep the emergency brake from setting.
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 tank differences

Reply #4 –
Quote from: George and Steph –
Is this enough difference to delay travel?

George, You are the person in the best position to answer this question!

Our coaches, with the standard factory gauges, don't provide you with a lot of information concerning the "health" of your air system.  The dash gauge (or gauges, depending on model year) show, at best, just a part of the big picture.  They are most valuable when viewed as "trend indicators", rather than precision measuring devices.  Watching them may give you a clue about what is happening but in many cases not much else.

Differential between the dash indicators, as mentioned above, can either be due to gauge error (malfunction), or to a leak somewhere in the air system.  If due to a leak, where is it?  Your best tool for finding the answer to this question is the Air System schematic.  It will require a series of logical experiments, and in some cases a LOT of time, to narrow down the possible location of the leak.

If you do have a leak, besides location, you have asked the more pressing question: "Is the leak a serious problem?"  To answer this question, it is helpful to have a good knowledge of your coaches normal behavior before you noticed the change in air gauges.

You should consider two factors when assessing a change in your air system behavior.  First, how fast does the air gauge fall?  Obviously, if the needle falls quickly = large leak.  Large leak requires more urgent attention.  Second, how rapidly does your engine air compressor cycle on and off?  Due to system check valves, you can have leaks in your air system that do not even register on your dash air gauges.  A more rapid than normal cycling of air compressor indicates a leak big enough that the compressor is working overtime to keep up.  In extreme cases, a leak can be large enough that the compressor cannot achieve "unload" pressure, and will run constantly.  Of course, before reaching this point, you would have (hopefully) taken some corrective action.

Use your knowledge of your coach, analyze what the air gauges are telling you, decide for yourself what you want to do.  To ease your mind, be aware that based on what I've read in this Forum, catastrophic failure of the air system is a relatively rare event.  From your description of your symptoms, I would guess you are nowhere near that point.  But in the end, you must do whatever makes you comfortable!
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Air tank differences

Reply #5 –
I agree with Brett"s W comment of mine on the 60lbs, and I should have added a "disclaimer" to it. Yes, you do not want to wait till it gets to that level and it should be investigated asap. If the pressure difference of gauges is always the same and as both tank pressures drop this difference stays as a percentage of the higher tank then I would definitely think it IS a gauge defect.
My other question would be "how long has it been showing this discrepancy. A long time or just started'?
That answer could give you some insight.
JohnH
Coachless, now use aircraft. 2003 Ford Travelair TC280 class C. Super shape. Just for 1 yr .
1994 Ford E350 ClassC,total renovation inside and out. Now sold.
2000 U295  36' Cummins 350 c/w Banks Stinger, Resonator upgrade,Solar, LED lites.Residential fridge with slide out pantry. Build 5674. Sold
ex 92 GV 022C ored Cummins. Sold
ex 95 GV240 cat 3116. Sold
2017 Mini cooper s & 2016 land Rover LR2 HSE  LUX.
jhaygarth@aol.com    SKP #130098
treat everyone as you would like to be.

Re: Air tank differences

Reply #6 –
Ran a hundred miles today watching the gauges closely.  Caught the movement cycle.  Top gauge would run up to 125 and stop as it should.  Lower gauge would run up to 115 and stop when the other hit 125.  The lower would slowly wind its way down to 99 or so and then the cycle would repeat. 

I ran the system before getting underway this morning.  The bleed down on both gauges was 5 lbs in an hour. 

Stopped in truck service and we raised the coach.  No leaks.  Drained all three tanks to try to get equilibrium. But gauges read same 115 over 125.  Traced the schematic and believe it may be the regulator cutting out as nearest tank hits 125.  I don't think it is a check valve but...

One positive note the tech noticed the trans harness chaffing through and we wrapped it.

Waiting for part under a tree in Gallop beats a corner in Winslow.
George and Steph
1997 U270 36 Build 5081 "Honu"
1180w Solar 400A lithium all Victron house system
Motorcade 17670, SKP 128300, FMCA F459019
73 VW Camper, 79 VW Camper, 2363 Sunline, and an Arctic Fox 25P

Re: Air tank differences

Reply #7 –
If you want to find out if you have a bad gauge,drain the air system,open the dash, remove the hose on each gauge and swap them,
refill the air system and note any change.
The selected media item is not currently available.Bill&Doris 97 U270 36'
University of Parris Island Class of 66
Semper Fi  Build# 5174 MC#17094

Re: Air tank differences

Reply #8 –
I experienced the same problem with one gauge reading 60lbs follow Brett advice of switching gauge lines and it was the gauge. Bought the exact replacement off eBay installed and solved that air problem.
John
1998 U270 34'

Re: Air tank differences

Reply #9 –
Quote from: George and Steph –
Traced the schematic and believe it may be the regulator cutting out as nearest tank hits 125.
I'm not sure what you mean by "nearest tank".

You have three tanks on your coach: wet tank, front tank, rear tank.  The D2 governor only sees the pressure in the wet tank.  When wet tank pressure reaches the "unload" setting, the D2 tells the compressor to stop compressing.

When the compressor is compressing, the pressure readings in all three tanks should increase simultaneously.  At the moment the compressor "unloads", the pressure in all three tanks should be exactly the same.  If, at the moment the compressor unloads, you are reading 115 on one pressure gauge, and 125 on the other pressure gauge, then I would say this has to be a bad pressure gauge.  I can't think of any type of leak or valve malfunction that would consistently maintain a constant pressure differential of 10 psi between front and rear tanks in all circumstances.

The suggestion of swapping gauge air lines is a good one, and should be pretty simple to do.  With the lines swapped, if the "suspect" gauge still reads 10 psi lower than the other, you have your answer: bad gauge.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Air tank differences

Reply #10 –
John/Pat,

It could be a bad sender on one tank. Try switching wires on the two sensors to see if the problem goes with the swap.
Jim Frerichs
2002 U320 42'

Re: Air tank differences

Reply #11 –
Our experience today confirms your explanation Chuck.  Thank you.  We changed out the regulator to no effect.  Most fittings etc were changed out in January and none showed leakage.  Same for brakes.  I will swap the two when we are able to layover in Ouray.  Neither gauge fell below 100 during our run from Gallop to Cortez.

JR truck service in Gallop were really good to work with.  In the end, we had the schematic tracing it under the coach.  Four hours cost me 200 and they apologized...learned a lot about the system. 

Most importantly, I now feel safe moving on with the kids.  Tomorrow the Pueblos...thanks all!
George and Steph
1997 U270 36 Build 5081 "Honu"
1180w Solar 400A lithium all Victron house system
Motorcade 17670, SKP 128300, FMCA F459019
73 VW Camper, 79 VW Camper, 2363 Sunline, and an Arctic Fox 25P
 

Re: Air tank differences

Reply #12 –
I had this problem recently, and it was the gauge.  Replaced with new gauge and solved the problem.
Mike
Mike Brady
'97 U320 SE. #5137
'13 Honda Fit
Willow Springs, MO