140 PSI in the air tanks
 

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. :?: :?:

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.

thanks for the reply.

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

Re: 140 PSI in the air tanks
 

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.........

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

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...

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

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.

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.