Ive got an 87 MC9 (NJT) with some sort of air mystery ( I hate to say problem). I believe Ive got it narrowed down to the valve in the first bay up by the heater control. Its leaking pretty good. I had no air problem till I started messing with it...
The main issue now is that the blowoff dosent pop. The low air buzzer goes off at around 85 lbs now. I just put a new governer on today. I plan on changing the valve as it is leaking, but can anyone explain what the function of that valve is and where it sits in the system ? It airs at least to 125 with no blowoff at which point I eaither shut down or hit the breaks to lower the pressure.
Well, after Googling the part number, and doing a little research it would appear the leaking part is a pressure protection valve...
The function of the pressure protection valves is to protect each service reservoir from a
pressure loss in another service reservoir or air accessory reservoir. The AD-IS air
dryer is designed to receive compressed air from the vehicle compressor; clean and dry
the air; deliver air to the vehicle's primary reservoir, secondary reservoir, and accessory
reservoir (if so equipped); and control the compressor/dryer charge cycle. Additionally,
the dryer incorporates a "purge reservoir," that in most cases, replaces the "wet tank
reservoir."
if your system is building to much air pressure you need to adjust the new Governor that you installed, the Governor controls the reclying and the air dryer you should hear the dryer pop when the pressure is reached that the governor is set at
The pressure protection valve sticking shut will cause the brake air pressure to go through the roof - up to 150psi at which the safety valve pops -- while the general air supply - white needle -- will take forever to get up to the blow-off point. Happened to me and got worse and worse until I changed the valve.
Next to it on mine was a small filter/water trap and that needed a good clean out as well.
The valve is cheap.
As I understand the sequence, the two brake reservoirs come up first. Past 65psi the protection valve allows air through to the rest of the system until the dryer pops off and this diverts air to the governor to allow it to unload the compressor. (or something like that anyway)
Tony, when you changed out the valve, did it need to be "set" or adjusted ?
The valve is called a PR-2 valve. As Tony states, it allow the accessory tanks to fill after the main tank fills. That gives brakes priority. Once the main tank is up to 65 lbs, the rest of the system fills. Reckon the purge valve pressure on the dryer isn't present so the compressor keeps on pumping?
So if the unit was turned up (screwed in?), it wouldn't allow the accessory tank to fill, from which the air suspension, wipers, doors, and such are fed.
Wonder what would happen if the park brake was set when the pressure was up around 140 or so lbs? That may be bad.
When replacing the valve, set it at 65 lbs. Book says plus or minus 5 lbs.
I thought I had a schematic of the air system, but all I find is sections, none of which show the PR-2 valves plumbing.
I'll keep up the hunt for the MC9 air system schematic. Probably in the bus. I have several service manuals and I'll bet the page is in the wrong manual.
Cheers, JR
Quote from: NJT5047 on November 25, 2007, 05:32:05 PM
When replacing the valve, set it at 65 lbs. Book says plus or minus 5 lbs.
Any idea how I would go about this ?
Also, as a side note, anyone recomend a good place to order some manuals ?
For your NJT, Luke (US Coach) or coachinfo.com. You'll need an NJT specific service manual.
The overpressure problem is related to the governor. It needs to be adjusted. Use the dash gauge. The purge valve isn't purging because the there's no exhaust cycle from the gov.
I'm sitting here looking at the book...otherwise I wouldn't know such things! ???
To adjust the gov, remove the end cover, loosen the locknut, and screw the center screw clockwise. The books says that will lower the pressure.
The cutout pressure comes from the compressor main outlet.
I found the chart too....the compressor first fills the wet tank, then the dry tank, and from there to the PR-2 valve. Once the dry and wet tanks reach 65 lbs, the park brake tank and accessory tanks fill.
Check the PR-2 outlet pressure at the valve with a pressure gauge.
I'll see if the chart shows which line may be the pressure line. My guess is that the line with the "T" for the heater control is the outlet side. So you should be able to check the pressure at that "T".
Get the gov adjusted to cut out at 120 lbs and the purge cycle will work correctly.
Cheers, JR
PR-2 valve supply port is on the side, delivery port is on the bottom. So check for the valve opening and passing air pressure at approx 65 lbs. You could bench test the unit before installing by placing an air chuck in the side port and gradually applying air to the port with a regulator. The PR-2 should open when the input pressure reaches 65 lbs.
Or, take the pressure side up the 65 lbs (verify with a gauge), leave it and adjust the PR-2 valve knob until it passes air though.
It isn't a regulator per se...it's a set-pressure valve. Open when above 65 lbs, and closed below 65 lbs.
HTH, JR
Ive got a regulator and a guage, so I think bench set up will work for me....Ill be ordering the part in the am...
Thanks all...
The PR2 valve has at least three Bendix part numbers, each corresponding to different pressure settings. One is 65psi.
The PR2 should not leak externally if it is OK. If leaking you have either screwed the adjustment plunger right out, or one of the internal O-rings is bad.
I checked mine by connecting shop air via an adjustable regulator and guage and cranking the pressure up until the valve opened. This isn't exactly the same as the pressure it closes at (which is what counts) but the hysteresis is low and it will give you some idea if it is working.
"I thought I had a schematic of the air system, but all I find is sections, none of which show the PR-2 valves plumbing.
I'll keep up the hunt for the MC9 air system schematic."
Would be wonderful if there really was one single drawing that showed all these components.