M931A2 Air Dryer purging problem

[Redneck92]

Hello Guys,

I can´t seem to figure this out by myself. I had read a bunch in the forum here and in the TM´s but im lost now.

Problem:

Truck airs up fine and quick. Horn, Wiper and so on are working. Brakes are working, Handbrake is working. Truck running fine.
It airs up to about 125 psi, the air dryer purges for about 3 4 seconds but the compressor is not shutting off after that and the cycle repeats itself.
So the air dryer purges at idle every 30 40 seconds or so.

When the air dryer purges, the compressor seems to shut off for as long as the purge goes, then the little exhaust port on the governor purges and then the compressor seems to start again.

What I have done so far:
- I replaced the 3 checkvalves that are prone to corrosion on the two passenger side tanks and the tank on the drivers side with stainless steel checkvalves.
( today I noticed another checkvalve at the back of the wet tank... didn´t see that before... will replace this aswell with a stainless steel one )
- I replaced the governor from the compressor and checked its lines
- I dissassembled and cleaned the PPV ( had crud and dirt in it, rubber nose cone gasket thingy was still fine )
- checked the gladhands and drain valves for leaks

Overall truck status:
- I cant hear any air leaks on the brakes or on any stuff in the cab.
- I can hear something leaking in the stack when the truck is off. This leak goes away as soon as I press the break pedal a slight bit.

Can somebody help me fix this darn problem? Been reading and trying stuff for quite a while now.

Thank you very much in advance

 

[fuzzytoaster]

The unloader valve on the compressor may be sticking causing this cycle. When it happens it can either cause air pressure to not build or continuously overbuild because it's not accurately giving feedback to the air governor. I just replaced the air compressor head gasket one one and the gasket has deteriorated to the point it was dust and caused the unloader to get stuck.

 

[charlesmann]

I cant remember which youtubers it was, but they were having a very similar issue. One ended up being, as fuzzytoaster said, unloader valve, and the other guy, it was a check valve in the dryer.

One of the guys started at the dryer, pressure checking from the dryer inlet port, working his way through the system to the unloader valve.
They other guy started at the purge valve and ended up replacing his dryer and solving the issue.

 

[Redneck92]

Today I disassembled and cleaned the air dryer as much as I could. I got the old style dryer with the aluminium cooling fins body.

I'll replace it with a Wabco 1200 series dryer soon. Mesured the fitting sizes and made a list with stuff that I need for the Upgrade.

I also will replace the last checkvalve going into the wet tank with a stainless steel one this week.


Can I disassemble the unloader valve on the head of the compressor without taking the head off? By removing the two bolts with the washers which hold that Part on which the unloader line goes to? ( See pic below )


View attachment 865266

 

[simp5782]

Today I disassembled and cleaned the air dryer as much as I could. I got the old style dryer with the aluminium cooling fins body.

I'll replace it with a Wabco 1200 series dryer soon. Mesured the fitting sizes and made a list with stuff that I need for the Upgrade.

I also will replace the last checkvalve going into the wet tank with a stainless steel one this week.


Can I disassemble the unloader valve on the head of the compressor without taking the head off? By removing the two bolts with the washers which hold that Part on which the unloader line goes to? ( See pic below )


View attachment 865266

Most wabco units require a purge tank most of the time. Their purges will act strange at times

Stick with a AD9 found at your local truck parts place about 230$

 

[Redneck92]

Most wabco units require a purge tank most of the time. Their purges will act strange at times

Stick with a AD9 found at your local truck parts place about 230$

Problem is that the AD9 air dryers are as good as unobtainable here in Germany. And when I need spare parts and stuff I will not get anything quick.

Do you have any other more available solution maybe?

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[Redneck92]

Problem for the Constant purging Was the checkvalve going into the wet tank. It was rusted and stuck open.

Never the less, I threw out the original air dryer and replaced it with a wapco unit.
The wapco unit has a built in governor.

When the governor Kicks in at 125 psi, the dryer is purged and air that is coming from the compressor is vented to the atmosphair.
So the compressor is not Ranking against the tank pressure anymore and has no load on it anymore.

So I bypassed the original governor circuit. the compressor is only building Pressure when the dryer senses 105 to 110 psi and the switches internally to filling the tanks again.

You could also send a Signal to purge and Start the unloaded pump cycle from the original governor on the truck. Since I hat no fitting metric to npt Adapter I went on my rout as described.

It has also the 24v Heater für the purge valve that I will wire in next week.

There are so many different styles of the wabco units which All Look the same but have different functions and pressures so you need to select for your needs.

Thanks for your help guys.


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