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Topic: Air pressure while driving (Read 815 times) previous topic - next topic

Air pressure while driving

I have a quick question about air pressure.  I've done a forum search for "Air Pressure" and I'm not quite seeing what I'm looking for so here goes.

Yesterday while driving on the highway I was looking at the Cummins display with the tank pressure digitally displayed.  I have never really observed this measure beyond glancing at it from moment to moment.  I noticed that when the pressure is at 125psi it will drop down to 105psi over the course of three or four minutes.  Shortly after it hits the 105 mark it quickly (10 seconds or so) jumps up to 125psi and starts over.  This happens without touching the breaks.

We had just been in San Diego for a five day stay and I noticed the Aux compressor kicking on only 3 times.  We were parked on an uneven slab with the front at near full extension while the back was very low.  So only hearing the compressor 3 times did not seem unusual. 

This did not seem to cause a problem but I have not noticed this before.  Is this pressure fluctuation normal?
David
2004 U320
This is my good stuff

Re: Air pressure while driving

Reply #1 –
Yep, completely normal. The governor on your engine air compressor is what controls this....unloads the compressor at 125, kicks it back on at 105.
Ralph
96 U270

Re: Air pressure while driving

Reply #2 –
Correct.

But a drop from cut out PSI (125) to cut in PSI (105) in three minutes is unusual and may indicate an air system leak.

Would do the air system pressure test:

Air Brake System Pre-trip Check
By Brett Wolfe

1.   Be on reasonably level ground.  Block wheels so the coach can not move even with the parking brake off.

2.   Start engine and run until full air pressure is reached. Listen for the dryer to purge (about 120 PSI).
3.   Shut off the engine.

4.   Release the park brakes by pushing in the yellow button (make sure you don't roll, do NOT apply the brake pedal).

5.   After the initial pressure drop the system should not loose more than 2 PSI per minute.

6.   Apply the brake pedal firmly (still with the parking brake is OFF).

7.   After the initial pressure drop the system should not loose any more than 3 PSI per minute.

8.   With the engine off and parking brake off, rapidly pump the brake pedal to bleed down the air supply. During this you should watch for the warning light and buzzer at about 60 PSI and then the yellow button (parking brake) should pop out at about 30 PSI.

9.   Restart the engine and build up air again. While building up pressure check how long it takes for pressure to go from 85 to 100 PSI at "cruising RPM". It should be less than 45 seconds.


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 while driving

Reply #3 –
Thanks Brett, I will head to the storage lot and check this shortly.
David
2004 U320
This is my good stuff

Re: Air pressure while driving

Reply #4 –
I don't usually reply, but your system to me seems totally normal.  As I travel down the road based on how rough it is and any bumps or woop de doos the air system will continually cycle from 128-107 and back based on the coach changes to ride hight with tyhe air bags.  If on a perfectly flat top road this cycle can happen every 8-10 minutes on my terrible California roads every 3-4 minutes.  When parkedthe coach will hold 100+ pounds in both tanks for about six days.  Drops to 70-80 over the next week.  when I asked Triano about this he said the cycling was normal and was based on what I have just written.  This is not to say you don't have a leak but to me and I do bow to Bretts far superior knowledge he is terrific, I think your fine.

Re: Air pressure while driving

Reply #5 –
To add to the above comment, gusty winds (especially crosswinds) will also cause this type of fluctuation in air pressure.  The first time I saw it I also thought I had a leak but after watching it over a few short trips realized that it was using more air than usual to maintain proper ride height on a windy day.

Monti
2015 Born Free Triumph Royal 4x4
Previous:  2000 U320 3600

Re: Air pressure while driving

Reply #6 –
Indeed, there is no "every 5 minutes is OK, every 3 minutes is bad".  As mentioned, it depends on air consumption by suspension and brakes.  But every three minutes on smooth road/no brake use sounds excessive.  If on rough roads with brake applications-- may be normal.
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 while driving

Reply #7 –
Cycling of compressor that often on a coach would indicate to me, that I have a very bad air leak.  The easy test would be to stop at rest area or a easy non issue place, idle engine, watch gauge, if your cycling the compressor more than 1 time every half hour, I would look for a significant leak.  In my world, my old MCI Like Foretravel, air ride, air brakes, wipers, door lock, 3 belt tensioners, Point is it only cycled every 38 minutes on I-95, at 70 mph.  Air leaks are a serious concern to ME, so had it about as good as it gets.
Foretravel coach's are rather simple air pressure wise, makes easy find the leaks. Not counting the leveling and slide seal bags.
Dave M

Re: Air pressure while driving

Reply #8 –
When my compressor started cycling every couple of minutes, the first thing I checked was the airbags. Easy to check first with your hand or water/soap spray bottle. One air bag had started to leak. As Dave M's post from a few weeks ago noted, increased compressor on time cuts fuel mileage a bit. Pretty easy to crawl under and start spraying if the bags are not the problem.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Air pressure while driving

Reply #9 –
I just got back form doing the test as Brett suggested and I'm happy to report that I don't seem to have a leak.  After starting the engine and doing as described I saw no pressure loss after waiting 15 minutes.  I then pressed on the breaks once and the pressure dropped about 6psi and then no pressure loss after another 15 minutes.  I then pumped the breaks to bleed the pressure, at 60psi it started to ding at me and then at 30 the parking break came on.  After starting the engine the pressure went form 85 to 100 in about 6 seconds. 

The road we were on I-8 El Centro to Yuma is very bumpy with lots of wooptido's (bumps that make the coach want to porpoise).

I'm going to keep paying attention, but I think I'm good. 

Thanks for all the help.

David
David
2004 U320
This is my good stuff

Re: Air pressure while driving

Reply #10 –
Quote from: esaulten –
I don't usually reply, but your system to me seems totally normal.  As I travel down the road based on how rough it is and any bumps or woop de doos the air system will continually cycle from 128-107 and back based on the coach changes to ride hight with tyhe air bags.  If on a perfectly flat top road this cycle can happen every 8-10 minutes on my terrible California roads every 3-4 minutes.  When parkedthe coach will hold 100+ pounds in both tanks for about six days.  Drops to 70-80 over the next week.  when I asked Triano about this he said the cycling was normal and was based on what I have just written.  This is not to say you don't have a leak but to me and I do bow to Bretts far superior knowledge he is terrific, I think your fine.

Plus one.  Normal
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4