Air Compressor Issue

[Tikvah]

I started my bus, only had to move a few miles.  It aired up and I took off.  I had to stop on the way out of the parking lot to fill with water and I noticed that my air gauge was taking longer than normal to come up.
Then I drove about half a mile and got gas (for my generator).  When I left the gas station I used my brakes substantially (backing up, around a curve, traffic, etc.)
I got on the road and my warning lights were on, about 65psi.  Yikes!
I shifted to neutral and picked up the RPMs
Suddenly the gauge started to climb.

I got to my next parking area (using the brakes a lot) and noticed the pressure was down to about 90psi.  I let it idle and moved it to my parking place, and noticed it was down to about80psi.  I left it idle again for a long while and came back to 60psi. 
Now I can't gain any pressure.
Nothing leaking in the back, or anywhere else.  No sound of air leaks at all.

Ideas?

[eagle19952]

AIR DRYER.

[Tikvah]

AIR DRYER.

Wouldn't I hear air coming out?

[eagle19952]

No. often if it is plugged/needs service, it bulds against it and goes no further.

i would build a dryer bypass hose...just to have for road emergency...that's what i did.
are you in a freezing area ?

really the quickest way to find out.
it sounds like it to me.

[Tikvah]

AIR DRYER.

Actually, we're not in freezing area now.  But I did, in fact, have freezing trouble earlier this year.  Our air dryer froze and I struggled with it while driving.  It was building too much air then blowing off very slowly.  After the temperature came up the problem went away.  I assumed my problems were over.

So, are you suggesting that I'm actually building pressure at the air dryer, satisfying the compressor, but not supplying air to the gauge or brakes, ect.?

[gumpy]

Possibly stuck governor. Otherwise I'd say probably large air leak somewhere.

[Stormcloud]

I live in an area where it gets REAL cold in winter. I have travelled in the bus many times either escaping the cold, or returning (for work). I installed an AD-9 dryer, and STILL had a freeze-up. I installed a brake anti-freeze 'sniffer' after the dryer, and have had no further issues.    Kind of a belt AND suspenders thing.

[Jim Eh.]

Possibly stuck governor. Otherwise I'd say probably large air leak somewhere.

Ditto.

Step one
If the govenor is 5 yrs+ just change it for the minimal cost involved. If it does not solve your problem go to step two.

Step two
If you have a good shop air supply connect it to the compressor delivery line (the line going to the dryer) to see if you can detect a leak. Sometimes a very large leak is not detectable with the engine running.

[eagle19952]

a stuck governor usually makes no air..

or to much air.

[bevans6]

The air pressure sense line that controls the air governor on my MCI is taken from after the PPV (pressure protection valve), so basically the accessory air section.  If the PPV is not working, air pressure won't build past it, the governor won't control the compressor, pressure will build in the dry tank and wet tank side of the PPV and the emergency pressure release valve on the wet tank would pop.  Ditto if the air dryer was somehow clogged, I think it has an emergency release valve as well.  My first step would be to change the governor.  My air pressure gauge reads pressure in the dry tank, FWIW, before the PPV.  My two cents, anyway.

Brian

[Jim Eh.]

One other often overlooked item is if your air compressor has an air filter on the inlet port it could be clogged/iced.

[Tikvah]

Okay, here's the update.  I haven't done anything yet.  I decided to start the bus this morning and see what happens.

It started about about 50psi.   Climbed to 125 like normal.  However, I noticed that the "STARTER ALERT" and the "NOT GEN" lights stayed on.  I've seen those stay on a coupe times since I've owned the bus.  I always just shut it down and restart and they go away as long as the pressure is above 90psi.

So I shut down and they came back on.... every time.
I ignored them for now - but I expect they are a clue to somebody out there.

I pumped the brakes and brought the pressure down to about 60psi.  This time the pressure climbed very slowly at hi-idle.  But it did climb back up.  When it blew off up front again at about 125psi I pumped the brakes down to about 90psi.  This time it climbed even more slowly... almost not at all.  But after about 20 minutes it got back to 125 and blew off again.

One more time I pumped the brakes, slowly - more like city driving.  This time the pressure dropped below 80psi and no amount of RPMs would make the needle move again.  The pressure won't climb.

So - between the dash lights and the symptoms is there any more ideas?
I can pull the governor rather easily (It doesn't look old) but I don't want to pull it off until I'm somewhat comfortable that it's the problem.

[B_K]

Sounds to me like you need to service your air system! I'd start with servicing the air dryer and completely drain the tanks. (not just open the drains to see if anything comes out, but open them and leave them open for a long period of time to see if after a while sludge comes out, if it does then you got plenty of work ahead)
  BK 

[eagle19952]

So I shut down and they came back on.... every time.
I ignored them for now - but I expect they are a clue to somebody out there.

I pumped the brakes and brought the pressure down to about 60psi.  This time the pressure climbed very slowly at hi-idle.  But it did climb back up.  When it blew off up front again at about 125psi I pumped the brakes down to about 90psi.  This time it climbed even more slowly... almost not at all.  But after about 20 minutes it got back to 125 and blew off again.

One more time I pumped the brakes, slowly - more like city driving.  This time the pressure dropped below 80psi and no amount of RPMs would make the needle move again.  The pressure won't climb.

governors don't act like this when they go bad (in my experience)

plugged air dryers do.

[bevans6]

At this point I agree that it doesn't sound like a bad governor since it is cycling correctly when it gets to 125 psi the first times .  I'm skeptical of it being the air dryer if it purges healthily when it does, and based on the initial fairly normal initial pressure build and purge when cold.  I think that I would be pulling the air compressor and putting a rebuilt in - it seems to work well when cold but loses compression when it warms up.  A test I would do is to use an external compressor to pump up the bus through the air dryer (I have a fill port on the ping tank in the engine bay).  I would set the pressure on the external compressor so it can exceed the 125 psi governor setting and force the air dryer and governor to cycle a few times, and time how long it takes.  If things get blocked up over a few cycles, the external air compressor will take longer to fill to purge each time.  If not, then have basically proved the air dryer and governor are OK.  You need to turn the external air compressor off manually when the purge happens, it won't know to stop compressing, and reduce the air pressure below governor cut-in pressure with your brake fanning.  The no-gen light is fed from the accessory system (on my MCI anyway) so I might get pressure up to around 120 psi on the gauge (which reads the dry tank) and check for the same pressure in the accessory tank and the emergency tank (both of which are after the PPV and the E-filter).

Brian