I have a Ford L9000 dump truck that I recently replace the pop off valve on bottom of air dryer since it wasn't completely closing and leaking air constantly and causing loose of air pressure. Now I have a new problem with it. When going down road and I'm in the accelerator it is constantly popping off like every 10 seconds (sometimes a slow spit/hiss unlike when it was working normally with one hard release of air pressure), but I still maintain air pressure around 123psi the whole time. When I let off accelerator the air slows down or stops. When I'm sitting still at idle it will sometimes still be slowly letting air out but when I first start up truck in the morning it will not let any air out at all. Like I said the pop off valve on the bottom of the air dryer is maybe 3 months old. Any ideas?
You have a leak on the wet tank side of the system , the tank the drier goes into is where you should be looking . The govenor also runs of this tank which also operates the compressor unloaders .
What model/type of air dryer do you have, AD-9 maybe? If it is popping off faster with rpm, the possibility of unloader/governor issue. But when was the last time this air dryer was removed and completely cleaned and serviced with a new cartridge?
More than likely you have leaking unloader valves in your compressor. They are signalled off the same line that controls your drier. What compressor do you have?
This is the type of case that is better diagnosed by connecting shop air to the compressor discharge line. Charge the system using a regulator with an accurate gauge, then you can actually hear what's happening. Went through this same sort of thing with a truck a while back, traced the problem down to a small leak in the governor mounting gasket keeping the unloaders from working. It was popping the tank relief valve not the air dryer purge valve.
And if it's a AD-9 the large o-ring at base of cartridge has air by-passing it will do the same thing.
I will have to look to see what compressor I have when I get to the shop and also what air dryer I have. I have had the truck 2 years, I have only replaced the popoff valve at bottom of dryer and put in new cartridge about 3 months ago.
ok, here is what I have, holset a/c, haldex dryer. I hit the road the other morning and a/c couldn't keep air pressure up so i found myself in parking lot trying to fix this issue (my own fault). What I did, 1. replaced governor since I had one at shop already, 2. new signal line (black air line) from governor to dryer, 3. then i just bypassed dryer to get home with some fittings from napa. So bypassing the dryer allowed the truck to build pressure, but going down road it would just keep building (I assumed there was blow off/pop-off valve on the tanks but guess not), I never let it get over 140psi. Also, side not, I was draining both air tanks regularly but then when I wasn't seeing anything I got out of the habit, so in this parking lot I decided to drain tanks and one had fair amount of water and oil in it. So once back to shop I went ahead and replaced air dryer with AD9 unit and also replace the braided line from compressor to dryer. Afterwards, I still have an air leak that keeps it from building past 90psi at idle, but when running down the road it will air up to 130psi and then I hear the dryer pop off. I know I need to fix the leak, but my question is, since I had just replaced cartridge to haldex dryer, is there something else going on that would have caused it to fail or is a dryer just something that needs to be replaced from time to time?
When you by passed dryer to get home you lost the signal to governor. The Haldex brand dryer never was a good unit/JMHO. But I have had trouble with AD-IP model dryers and never could find the problem with them.
So if you apply 125-140 psi to the compressor unloader, is it quiet? Does it leak, if so where? Can you hear it at the hose, the intake or nowhere? Now apply the same air to the dryer unloader. Any leaks? Shouldn't hear anything but the valve opening, no air flow. Charge wet tank, that would be the hose from the dryer. Watch gauges. Between 125-140 you should receive air from governor to both the dryer and compressor unloader. Now back in the 90s holset compressors usually required a #4 hose jumper from compressor inlet to isolation valve at outlet, IIRC. Hope this helps. I'm also posting a link to a site with some real good info. BTW, wet tank or compressor head MUST have a safety valve. If it isn't there, put one there. No one wants to be close to a rusty grenade. http://beamalarm.com/Documents/haldex_isolation_valve.html Good luck
Like you found its better to replace the drier assembly every few years or if they are giving you any trouble in my opinion at least its not worth the time and money cleaning them up and putting parts in Other then that an air leak that significant will be easy to find its certainly not uncommon to have more than one thing going wrong at once
You have two main problems a tired compressor/pushing oil and a lack of maintenance. Air dryers should be serviced once a year. And either you have one hell of a audible air leak or the compressor is real tired at idle speed. Perform a simple 0-100 psi test at 1000 rpm and time it.
Ad9 drier with that set up had what they called a soft seat purge valve ,mack used to have that setup and if i remember correctly it was to keep head pressure on the compressor to help stop them from passing oil by the piston rings .
And almost always on a HOLSET unless someone changed the dryer and didn't know what it was for.
Not near as many Holsets had that as one would think, Kenworth yes, Pete and Freightliner not so much, but Pete liked to install Wabco dryers mainly.