1999 M35A3 Air Dryer constantly blows off - Need Advice

[M35CAT]

I Have a 1999 M35A3 I rebuilt the Air Dryer and Purge Valve, bought the HALDEX rebuild kit from NAPA, put $183 in it. This was rebuilt last year because the truck sat all winter (Nov 2013 - Feb 2014) and the purge valve was stuck open and leaking air out the bottom. It fixed that issue and had no leaks out the bottom. It didn't seem to blow off much, I do not recall the specific PSI during this time.

Now the truck constantly blows off every 14-15 seconds at idle. It builds air up to 120 psi and blows off. Restarts at 100 psi and blows off again at 120 psi.

When driving (50 mph) it does it twice as fast it seems.

So far, I went over all my lines with soapy water and do not detect any leaks. I have heard of people getting dirt / rust in the "unloader valve" but I am not familiar with what this is or where it is located to check it.

Any advice would be appreciated.

 

[gimpyrobb]

Lots of info in this thread:

http://www.steelsoldiers.com/showth...2-Air-Dryer-rebuild/page2&highlight=air+dryer

 

[M35CAT]

Thanks for your reply. The upper and lower has been rebuilt. I just think it blows off too much. Is it normal?

 

[gimpyrobb]

I haven't got mine in yet to give you an honest answer.

The way its "supposed" to work is the blow off valve vents when ever the air governor kicks out the air compressor. That tells me your system might be cycling a lot or maybe the air gov is going bad. Do you know what the on/off pressures are set at?

 

[rustystud]

I haven't got mine in yet to give you an honest answer.

The way its "supposed" to work is the blow off valve vents when ever the air governor kicks out the air compressor. That tells me your system might be cycling a lot or maybe the air gov is going bad. Do you know what the on/off pressures are set at?

That what I'm thinking too. Your compressor unloaders are not working properly. More then likely there stuck closed.

 

[M35CAT]

So far I took off the governor, I adjusted the bottom screw by 2 turns out which increased the kickoff pressure. I let it built to 140 psi and shut it off because it hadn't blown off yet.

Took off the governor, screwed the screw back in 2 turns to the orig setting and then went an additional 2 turns. Put it back on and pressure kick off went to 90 psi, builds right back and kicks off again.

I feel the governor is working as it should like a regulator does and shows no sign of rust.

I took it back off and put it back to the way it was originally. It is back to blowing off at 120 psi, immediately starts building pressure at 100 psi and blows back off at 120 psi. About every 14-15 seconds. When driving the truck, it is blowing off every 5 seconds, builds to 120, blows off, restarts building at 100 and blows off again at 120.

So I took off the 1/4" line and replaced with all new. (truck only has 7000 miles on it and the line looked fine, replaced anyways). When you unscrew the 1/4" line off the air dryer and start the truck, a small amount of air comes out of that line, you put it back on to the air dryer and it blows off immediately. Then you can pull that 1/4" line off the air dryer and there is no air coming out of that line. As soon as you get close to 120 psi, air starts coming out of that line again. You can press your finger on the end of the 1/4" line but there is too much air to completely stop it. When you remove your finger real fast, it creates a blast of air from building it up in the line. Then there is no air for 3-4 seconds before you feel more air again. When you place your finger on the air dryer 1/4" fitting with the hose off, nothing happens, no air coming from it. If you don't compress your finger on the 1/4" line there is air leaking thru that line continuously and the truck stays at 120 psi and never blows off.

If you are standing at the engine compartment as the truck is running, when the system blows off at 120psi. There is a small burst of air coming from the exhaust port of the governor and then it stops until it blows off again.


The Central Tire Inflation System is deleted, unplugged and removed from the truck. I went over every air fitting on the whole truck. The only fitting that had a leak was the horn and it has been fixed. The only other part that I can think of is the Compressor Unloader valve. I have searched for hours for info on this. I have not been able to find a picture, a diagram or anything on how to locate this valve let alone check it. I think it is in the compressor, but have no clue. Like mentioned before, the upper and lower of the air dryer has been completely rebuilt with a new filter.

Any other thoughts? I hate to keep driving the truck in fear that it may burn up the compressor.

 

[quickfarms]

Replace the governor

 

[rustystud]

Like I said earlier check your compressor unloader valves. They are behind the intake filter assembly. They are poppet style valves. You can squirt some brake cleaner into them and use shop air to clean them up. Then use something along the lines of "Tri-Flow" (PTFE lubricant) to lube them. Carbon can get in there and build up so much that the valves will not close properly. Once the valves are freed up, adjust the governor again.

 

[gimpyrobb]

Isn't the cut in of 100psi kind of high? If it turns on at 100 and off at 120, I'd change the cut in pressure to 80.

 

[rustystud]

Actually that is perfect. 120 psi is where the pressure is suppose to be. It keeps building air though, which is why it constantly bleeds off. The compressor should not be building any air until it drops to 100 psi.

 

[M35CAT]

Like I said earlier check your compressor unloader valves. They are behind the intake filter assembly. They are poppet style valves. You can squirt some brake cleaner into them and use shop air to clean them up. Then use something along the lines of "Tri-Flow" (PTFE lubricant) to lube them. Carbon can get in there and build up so much that the valves will not close properly. Once the valves are freed up, adjust the governor again.

This is a 3116 CAT motor and I have looked all over and the only poppet valve that I can find is the one on the thermostat housing for the air clutch I believe and on the air dryer. The one on the air dryer blows off at 175. Cant find any others. Looked all over the CAT motor including around the air cleaner and intake plenum plate.

Are you able to describe this in more detail, where it is located ?

 

[gimpyrobb]

I think he is talking about the air gov. On the A3 it is mounted to the air compressor.

 

[M35CAT]

I think he is talking about the air gov. On the A3 it is mounted to the air compressor.

Ok thanks. I will buy a new governor. I may look into buying a new air dryer system as well.

 

[gimpyrobb]

Bring it to haspin this week and we can look it over!

 

[rustystud]

View attachment Scan0059.pdfView attachment Scan0060.pdf This is from an excellent site that explains the purpose of the unloader valves and what happens when they don't function properly.
You will also see that the correct pressure settings for a governor is to come on at 100 psi and off at 120 psi.

 

[rustystud]

This is a 3116 CAT motor and I have looked all over and the only poppet valve that I can find is the one on the thermostat housing for the air clutch I believe and on the air dryer. The one on the air dryer blows off at 175. Cant find any others. Looked all over the CAT motor including around the air cleaner and intake plenum plate.

Are you able to describe this in more detail, where it is located ?

These valves are IN the air-compressor.

 

[gimpyrobb]

So if it cuts in at 100 and out at 120, and his cuts in and out often, he probably has a leak?

 

[M35CAT]

Thanks guys for the info. I will let you know what I find. I really appreciate it.

 

[rustystud]

So if it cuts in at 100 and out at 120, and his cuts in and out often, he probably has a leak?

He could have a leak, but it sounds more like his compressor unloader valves are stuck. All air systems leak. Even our 1 million dollar "wonder" buses leak air (especially our buses since they have over 1/2 mile of air hose in them) . He's system sounds more like the unloaders since the compressor is never able to completely stop running. If it leaked down to 100 psi each time then I would say look for a large leak somewhere. Since this is not the case I feel it is more then likely the unloaders are stuck. Carbon gets up there all the time especially if the compressor is a little worn and passing some oil. Hot oil turns into baked on carbon. I have had to take compressors apart and sandblast the pieces to get the carbon out.