M931a2 air pressure question

[Suprman]

My new 931a2 was delivered today. I have a 923a2 so I know pretty much what I am doing but am definitely no expert. The air pressure builds to around 115psi and it builds a little slower than my 923. I like to run at 120-125 the ctis seems to like this range it won't start filling till around 110psi. I uncapped the air regulator I couldn't remember which way was more and which was less so I loosened the nut and started backing it out no change though. I ran it in also no change. It did seem like there was a little oil at the bottom edge of the ring that holds the adjustment screw setup in place. I shut it off and checked all over for leaks no hissing and it sat for a good half hour and still held 80psi. The brake release is all the way out nothing leaking and the trailer brake control is out all the way. It only has 6 hours and 130 miles on it could the compressor be on its way out or am I overlooking something?
Will

 

[rrrr]

Did you bleed the system down a little to get the compressor to kick on ? Just pump the brakes a few times and listen for the compressor to kick on and see where it stops as it pumps the system back up.

 

[Suprman]

No i was standing on the tire adjusting and sticking my head in the window i will try it again later

 

[Suprman]

Well i adjusted and drained and adjusted again i get it up to 118psi thats the max. Does anyone know if there an adjustment on the compressor clutch? Something is def not right i can run my other truck up to 140 with the regulator adjustment. Thanks

 

[juanprado]

could be a bad governor?

The m939 uses a hd haldex model but you could get away with the regular one KN18530. They are cheap and easy to change.

 

[Suprman]

When i get around to it i am going to take it apart and see whats going on but i wanted to see if there was an adjustment on the clutch on the compressor.

 

[doghead]

There is no clutch on the compressor.

Might try reading the troubleshooting section of the -24-1TM.

 

[Suprman]

What is the difference between the hd haldex and the kn18530?

 

[Suprman]

Troubleshooting according to the tm. Since I can build pressure we can go right to the air governor. Step 1 replace governon with a known good one. If this fixes problem then governor was defective if not then compressor is defective. To the point but not exactly what I expected.

 

[juanprado]

Kn18532 is the OE unit which is water resistant. It is not a normally stocked item at hd stores or NAPA. If you get water that high on the firewall of a M939, then you have a lot more issues than the air governor!

Kn18530 or equivalent is a normally stocked item everywhere truck parts are sold. Cheaper and does exactly the same thing.

http://epi.hbsna.com/Products/Produ...C689158436EA1FE978E&pf_id=KN18532&dept_id=101

http://epi.hbsna.com/Products/Produ...27B824BFA91885EB210&pf_id=KN18530&dept_id=101

If you want the OE and can wait on shipping, epay selller checkerboardfarms sells the oe nos cheap.

 

[Suprman]

Put a new governor in same symptoms 118 psi. Could a bad compressor cause this? Anyone know if a compressor rebuild kit is available?

 

[Suprman]

M931A2 not building proper pressure after air dryer

I posted on this before but I have done further diagnosis. Yes I have read the TM up down and side to side. I have a M931A2 just out of rebuild I dont believe it has ever been to a unit just into storage and then auctioned off. It builds pressure slowly and only to around 118psi. It is reasonably quick to around 90psi then it takes a long time to get to 118 and it recovers very slowly. The brakes appear to function properly. I replaced the governor with no change in symptoms. It actually builds more pressure with the governor screwed out than in and the tm said turn the screw in for more pressure. I opened up the compressor air inlet and ran it no change so the problem is not caused by air input restriction. I disconnected the line coming out of the air dryer and attached an air guage. I checked the one way valve past it and it looks new. It built right up to 160psi and the air dryer relief valve opened so the compressor is working. With the line disconnected both tanks still held around 80psi and no leaks heard. The unloader port is connected to the control port on top of the compressor like it should be. There is a line coming off the other unloader connection on the governor it goes down over the trans and it looks like it goes to one of the air tanks. I am thinking somewhere a line was maybe improperly connected during rebuild. When I pump the brakes I can hear air discharge through the air stack when I lift off the pedal. Any input would be greatly appreciated.

 

[Suprman]

Can they be merged?

 

[swbradley1]

Done

 

[Suprman]

Does anyone know what tm shows the air system in an A2 with the ABS. All the TM's I found don't seem to show it or am I looking in the wrong place. I need to lay out air system, airbrake system, trailerbrake system and abs system hosing all at once so I can go thru the entire truck to figure out if anything is switched or misconnected.

 

[coecamo]

Logsa has a Jun 2012 Revision of TM's ABS is in this set Gary

 

[Suprman]

Fixed it the pressure protective valve was stuck open I don't see how it could cause all of the symptoms I had but I think it was stuck open but restricted. I noticed the wipers worked before they should have so I replaced the valve and new all appears well. The valve looked pretty old I had to replace the one in my 923 a while ago I think any more trucks I buy I will replace that valve from the get go.

 

[BKubu]

Glad you fixed it!

 

[Suprman]

Yer tellin me that one had me stumped for a few days

 

[Suprman]

So the air system works. At idle it takes some time but driving it goes right up to 130psi. Funny though I can hear each stroke of the air pump in the air system you can hear it in the air dryer it's like a tick almost. It sounds like a small exhaust leak on a gas engine. There are no leaks anywhere and the compressor can build plenty of pressure. None of the other trucks I have had or have heard sound like this. I don't see any check valves or anything else between the air pump and air dryer. Anyone else heard this from their truck?