140 PSI in the air tanks

[Mackman]

I have been driving a 2001 mack CL 350. It builds a 140psi and the air dryer goes off once every 5-7 secs. I told the mech he said he would look at it and that was 2 weeks ago. Is this a major problem safety wise. :?: :?:

[special k]

140 is a liitle high but it's not going to kill you or anything. When the air governor starts to go they can cause the air dryer to purge a lot. Most of the time people start at the governor because it is usually the cheapest place to start.

[Mackman]

thanks for the reply.

[Birken Vogt]

Are you sure it is the air dryer and not the pressure relief valve popping off. That would be the case if the compressor was running continuously. In any case the governor is a good place to start. Put a pressure gauge on the unloader port of the governor. With low air pressure in the tanks there should be no pressur on that port, but by the time you have 130 psi, tank pressure should appear on that port as that is what is supposed to shut off the compressor. If you do have tank pressure the compressor is the problem, if you still don't, the governor is the problem (so long as tank pressure is reaching the governor's sense port)

Birken

[bob h]

I have been driving a 2001 mack CL 350. It builds a 140psi and the air dryer goes off once every 5-7 secs. I told the mech he said he would look at it and that was 2 weeks ago. Is this a major problem safety wise. :?: :?:

when the air dryer goes off, is it just a single blast of air? if so, I would guess that the compressor is cycling (loading and un-loading continuously). the reason the air dryer "goes off" is becuase the compressoor has been unloaded... that same signal pressure moves to the air dryer purge valve. when the pressure relief valve opens, it usually stays open for a number of seconds. pressure relief is usually set @ 150 psi.

the safety element is only due to the fact that the compressor will eventually fail from an excessive duty cycle. but..., in operation, as long as your primary and secondary pressures remain good.........

[SmoothCruizer]

hey mackman wanted to chime in on this on.......130-140 psi is normal but a normal air compressor should not release at will.... I had this problem with my 98 freightliner and needs attention cause your just wearing out your compressor faster and losing mpg... it has something to do with your pressure governer being off tell it to charge on and off / being confused

[JoeyB]

Oh I know what this is- The air governor is signaling the compressor to unload alright, but the compressor is not unloading. I think you'll find the problem to be in the head of the air compressor.

What happens is the governor signals the compressor to unload AND also signals the air dryer to purge. So the air dryer purges, the flex line up to the compressor, and the line back to the wet tank check valve all empty, but the compressor is still doing it's thing. You'll probably be able to hear and feel a gentle pulsating flow of air at the purge valve, which shouldn't be; it's from the still-running compressor. The purge valve is designed to "suck shut" when the compressor starts running- that's how it knows when to close. So that flow of air sucks the purge valve closed and presto!- In 5 to 7 seconds everything's up to cut-out pressure again and the gov signals the compressor AND the air dryer again and on and on.

Midland compressors are known for this...

[Mackman]

Thanks for eberyones input now i can go to work and talk to the mech. like i know what am talking about lol

[bob h]

Oh I know what this is- The air governor is signaling the compressor to unload alright, but the compressor is not unloading. I think you'll find the problem to be in the head of the air compressor.

What happens is the governor signals the compressor to unload AND also signals the air dryer to purge. So the air dryer purges, the flex line up to the compressor, and the line back to the wet tank check valve all empty, but the compressor is still doing it's thing. You'll probably be able to hear and feel a gentle pulsating flow of air at the purge valve, which shouldn't be; it's from the still-running compressor. The purge valve is designed to "suck shut" when the compressor starts running- that's how it knows when to close. So that flow of air sucks the purge valve closed and presto!- In 5 to 7 seconds everything's up to cut-out pressure again and the gov signals the compressor AND the air dryer again and on and on.

Midland compressors are known for this...

compressor unloading is usually not a problem if the governor sends air out the unloader port. if the compressor does not unload, the pressure relief valve will open and stay open for a long time (some compressors have them... all air reservoir systems have them). the compressor will quickly overheat if this happens.

i think the problem is compressor cycling; unloaded... loaded... unloaded... loaded. this can be caused by leakage in the wet tank or RES pressure line, effectively the compressor cycles on and off to keep the RES line/wet tank full, even though the PRI/SEC tanks stay full.

[JoeyB]

To get that thing to cycle every 5-7 seconds would take one hell of a leak. I've never taken one of these apart to see exactly what happens in there, but I'm told Midland compressors have some little pin that pops out of place or something that kills the unload function.

No Bob, I've seen this before. The governor is telling the compressor to knock off, but because of a defect in the unloader mechanism, it doesn't. If there were no dryer in the system, then the system pressure WOULD go way up and lift the safety valve as you describe. But since there is a dryer, it responds as I described earlier, and constantly pressurizes and purges, dumping the output of the compressor right then and there.

[bob h]

To get that thing to cycle every 5-7 seconds would take one hell of a leak. I've never taken one of these apart to see exactly what happens in there, but I'm told Midland compressors have some little pin that pops out of place or something that kills the unload function.

No Bob, I've seen this before. The governor is telling the compressor to knock off, but because of a defect in the unloader mechanism, it doesn't. If there were no dryer in the system, then the system pressure WOULD go way up and lift the safety valve as you describe. But since there is a dryer, it responds as I described earlier, and constantly pressurizes and purges, dumping the output of the compressor right then and there.

speaking of cycling every 5 to 7 seconds... how fast is this engine running if the compressor discharges only once every 5 to 7 secs?... that's a slow air compressor rpm ;0)

i think that the discharge air cannot be pumped out through the purge valve because the cut-off valve blocks that opening (compressor discharge to purge valve opening). if that connection were left open, your 30 psi of manifold boost pressure would move down the discharge line and out the purge valve as well.

and the cut-off valve is closed by UNL (unloader air) from the governor. and, seeing as how there's 140 psi in the tanks, i'll assume it is in the "unloaded" cycle.

[JoeyB]

Bob, the air dryer is cycling every 5-7 seconds, not the compressor. That's just about right, no? 5 to 7 seconds to pressurize the dryer and the lines? Your manifold boost is on the inlet side of the compressor, not the discharge side. And when the dryer purges, the dryer, the line to the compressor, and the line to the wet tank check valve should all be empty, and the purge valve open. In another thread you stated that the check valve is in the dryer on some, (I think that was you that said that) OK. But the idea is the same, the discharge side of the compressor should be open when the unit is unloaded.

There's an easy way to check this to end the controversy- stick your hand under the purge valve and see if you can feel a few seconds of pulsating air immediately after the thing purges. If you do, the compressor is still running. We know the governor is OK because dryer has received and responded to it's signal to purge, but the compressor has failed to respond to the same signal.

[bob h]

Bob, the air dryer is cycling every 5-7 seconds, not the compressor. That's just about right, no? 5 to 7 seconds to pressurize the dryer and the lines? Your manifold boost is on the inlet side of the compressor, not the discharge side. And when the dryer purges, the dryer, the line to the compressor, and the line to the wet tank check valve should all be empty, and the purge valve open. In another thread you stated that the check valve is in the dryer on some, (I think that was you that said that) OK. But the idea is the same, the discharge side of the compressor should be open when the unit is unloaded.

There's an easy way to check this to end the controversy- stick your hand under the purge valve and see if you can feel a few seconds of pulsating air immediately after the thing purges. If you do, the compressor is still running. We know the governor is OK because dryer has received and responded to it's signal to purge, but the compressor has failed to respond to the same signal.

the air dryer and the compressor cycle together - they bothe use the same UNL signal from the governor I.e - if the dryer is in purge, the compressor is unloaded as well.

manifold boost can move in the inlet, and out the discharge of the compressor, that's why the air dryer has a cut-off valve. the discharge valve in the compressor is only held closed by a light spring pressure.

when the dryer purges, the inlet and delivery (outlet) of the dryer are both blocked; the inlet is blocked by the cut-off valve (which is closed as long as the purge valve is open). the delivery is blocked by a check valve, which stops back-flow from the wet tank.

the air that is purged from the dryer comes from an internal "purge volume", which is a small storage of compressed air used to blast out the contaminants --- regenerating the dessicant beads, and cleaning the oil separator. you can prove this to yourself by loosening the discharge line (discharge is the large line between the compressor and the dryer inlet) while the system is unloaded, and see if there's air there --- there should be.

the discharge line, the unloader line, and the purge valve line ALL have air pressure when the compressor is unloaded... this is why i say it is possible that the leak can be caused by the purge or cut-off valve, you seem to be telling us there is only one possible problem --- I disagree.

my only point behind mentioning possible location of the check valve is to exploit the fact that replacing the complete air dryer would repair this "leak-back" if that check valve had failed, same as if the purge/cut-off valves failed.

[JoeyB]

I think we have to know what dryer is on there, and I'm not familiar with all of them. I do know that it's not uncommon for Midland compressors to fail to unload, I've experienced it myself.

Will you agree with me that if Mackman can feel a stream of slightly pulsating air exiting the purge valve immediately after the purge "burst" then the compressor is still running?

Regardless, we'll have to wait until Mackman tells us how this thing gets resolved.

[Birken Vogt]

I've seen Bendix and Meritor/WABCO compressors do it too.

Birken

[Mackman]

I will look wed. to see what dryer it is.

[RockyMtnProDriver]

I have been driving a 2001 mack CL 350. It builds a 140psi and the air dryer goes off once every 5-7 secs. I told the mech he said he would look at it and that was 2 weeks ago. Is this a major problem safety wise. :?: :?:

Ya, that is getting kind of high. It should really not be much higher than 125 psi, and a maximum of 135 psi.

You are getting close to the Safety Valve maximum limit of 150 psi before it starts blowing.

What you probably need is a new governor. Don't try and adjust it, it is cheaper to replace than fix, and many are not adjustable at all.

Also, that it is cycling every 5 to 7 seconds tells me that the Governor is not working on the bottom either.

A Governor should be set about 125 maximum (it can be anywhere from 105 to 135), then shut down the air flow to the tanks and then start to send air again about 20 below maximum, or ab out 105 (if you have a 125 system.

[JoeyB]

We should have an answer to this by now. They had all of Christmas day to work on it...