[Archived]

As a side note, my coach uses the same exact engine air compressor governor to control the on/off cycle of the auxiliary air compressor in the front compartment. A simple sealed miniature electric air pressure switch is installed into the governor which opens and closes the aux compressor relay.

Paul,

I don’t follow this. Are you saying the standard Bendix Air Governor normally mounted on your compressor is controlling your Aux Compressor?

[Archived]

Sorry for the confusion. I should have said same exact STYLE
engine air compressor governor.

There is a separate Bendix style governor mounted in the
front electrical compartment mounted just above the aux compressor storage
tank.

My aux compressor is three separate components, compressor,
remote mounted air tank, and remote mounted regulator/pressure switch.

An air line runs from the aux air tank to the Bendix
governor inlet. An Airtrol, low deadband, electric pressure switch is screwed
into one of the “out” ports of the governor and the exhaust port from the
governor then exits the compartment.

I have no idea why the converter set the aux compressor
control system up this way. Maybe it was to give the flexibility to use
different rated compressors or it was done for space considerations.

In any event, it does seem to be a complex control system
but it does work extremely well. I only mentioned it because I thought it was
an interesting use of the Bendix style governor.

[Archived]

Paul,

There is no comparison between the bus air system governor and the aux compressor control.

The aux compressor is on or off depending on the setting of an electric switch actuated with aux system air pressure. That switch could be a sealed, non-adjustable pressure switch with an “off” upper limit and a pressure differential to determine the “on” pressure or a pressure switch like a well pump control..

The engine driven compressor is controlled by a governor that is usually located at the compressor. I am not going to go into how it works or how to adjust it, but suffice it to say that it is part of a system that is more complex than an on-off switch. If you question your dash gauges or system pressure run your engine until the pressure reaches the maximum. You will hear the air dryer purge when this occurs. You can check the primary system pressure at the Schrader valve located at the RH rear inside the door, usually behind and low on the rear post. You can check aux system pressure in the front steer compartment and that Schrader valve is usually located on the rear steer compartment bulkhead.

Later model coaches have non-adjustable governors built into the air dryer, but I believe your vintage has the external adjustable governor.

Prevost calls for a system pressure of 120 to 125.

[Archived]

The schader valve in the engine compartment intended for tow operations does not have air in it when the dryer gets to operating pressure. On my coach this is how it should work. I can’t say how others work. The reason it does this is to ensure the air system is not subject to excess pressure.

I have a guage in the bay under the driver with a gauge that reads unregulated pressure. Sever other gauges read regulated pressure. So make sure you read the right one. If you have a CC there is always the digital dash display (Jon couldn’t resist).

[Archived]

Have you ever seen the primary and secondary gauges run 20 psi difference? As best I can tell the governor shuts down the compressor at around 125 psi on the primary gauge. But the secondary gauge only reads 105 psi. The other coach I had before this one, both gauges read the same pressure even while building up the air system. Haven’t checked the gauge, wouldn’t know where to begin to see if it is faulty. Any ideas?

[Jim]

Mike,

They should be the same. In fact when the compressor cycles the primary, secondary and aux air systems should all read the same and the compressor should shut off around 120 and cycle back on at a pressure drop of 20 PSI

[Larry & Diane]

Jon, we are just home from a 3500 mile trip. My gauges, primary and secondary, were the same, but they were cycling between 120 psi and 128 psi, not 20 pounds down. Didn’t get any bad symptoms, purge was normal. Intuitively the compressor kicks on and off more often but runs less in the on cycle than would with a 20 pound swing. What do you think?

[Jim]

Jon, So I’m thinking I either have a faulty gauge or something in line to the gauge that causes it to read the difference. Any ideas?
I am going to adjust the governor down 5 psi today so it will shut off at 120 psi on the primary. Then I should be at normal readings. But my concern is, if the secondary drops 20 psi down to 80 psi before the compressor comes back on I may be to low on the secondary. Worth a try!

Thanks!

[gary]

Dan, have you replaced the governor lately. Sounds like it may be contributing to the problem!

[Archived]

I agree with Mike, you may need a new governor if it’s cycling between 128 and 120. The governor won’t have any effect on the difference you are showing on your brake gauges. If there are no leaks, sounds like a gauge problem, but to be sure, you would need to put a gauge on the low air tank and see if it’s the same as your dash gauge, then you would know if the gauge was your problem.

[Larry & Diane]

In my six plus year relationship with Elegant Lady, I have never messed with her air compressor area. Maybe its time I did. I have always carried a new spare governor and have one now. Thought I would post to see if there was some other possibility causing the short cycles.

[Archived]

I put a Bendix rebuilt D-2 governor on the air compressor this spring, trying to correct the short cycling I had last year. No help.

Won’t be home til mid July. Everything works normally. My only symptom is and has been the compressor kicks on before I lose 15 or 20 pounds from its shut off set point.

Has anyone fixed this before?

Unloader valve? (two o-rings and a spring)

Dryer purge valve? (seems to function normal)

Dryer delivery check valve?

[Archived]

Dan

Switch the lines to the guages behind the dash and see what the pressures read, if the same guage reads low then it is a guage problem.

[Archived]

Greg, my gauges don’t read different, they show same pressure. My problem is the compressor is kicking on when my pressure has dropped only three to five pounds, then it puts those few pounds back in. I should lose at least 15 pounds before the compressor kicks in, right?

[Archived]

There should be a 30 psi cut-in/cut-out, usually 90 – 120.