[bob h]

Originally Posted by 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.

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]

Originally Posted by 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.

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]

Originally Posted by 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. :?: :?:

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...