I lost my cool yesterday and started throwing parts at a problem. Started out with truck wouldn't build air above 80psi, found leaks in compressor discharge line (not the easy braided steel line off the head but the pia line from it to dryer mounted behind cab). Replace line and building air like crazy c13 with tu-flo 750 fill to 130psi in just 2 or 3 minutes at 900 rpm. Now the compressor is only cutting out during the purge cycle of ad-ip bendix dryer (purging every 15 to 20 seconds). Here is were I lost my cool; I changed unloader valves then govenor then purge valve still not keeping compressor unloaded till cut in pressure! No bad leaks in air system it only drop 10 or 15psi over night. What am I missing?
Had two AD-IP's do that same thing. Went through every thing like you did changed out air dryer's problem stopped never really did know what was wrong with those dryers. Truck Shop
Same here mine would blow off every 30sec or so changed dryer and all is well
Thanks, I think I have a spare to swap out. But thinking about it I wonder if it is a leaking control line?
Always found the dryer purge heads were fouled, corroded or mechanisms failed in almost all of the driers I worked over. Hated most of them when trouble but when working are the Bee's Knees!
Put a gauge on the wet tank and see how fast the pressure drops when it unloads. Had the same problem on a C-15 with the one cylinder Cat/Bendix compressor, turned out to be the unloader valve O-rings. Pressurize the unloader bore on the compressor head and see if they hold. I think you said you have replaced them, but its an easy check.
The "wet" tank is not dropping pressure that fast (1 or 2 psi a minute max) edit: the primary and secondary tanks are only leaking 1 or 2 psi a hour. Is the volume of air in the control line the only air keeping the unloader valves in or is the pressure from the wet tank open to control line until it falls to cut in pressure? Also shouldn't the purge valve stay open till the cut in pressure is reached?
The unloader should read from the Primary tank, have not seen one sensed from the wet tank in a long time. The Unloader should not cycle due to the drier cycles just allow the discharge line to reach zero unless there is a blown o-ring/seal from the sensed air pressure to the discharge line pressure.
Usually the purge valve "pops" for a second when you hit cutout pressure. It shouldn't stay open long.
Unless the drier system purge unit dumps it all off.
FWIW. I have seen a bunch of newer Paccar trucks with the Unitized air dryer/wet tank/ air compressor governor things that do read governor pressure from the wet tank. That was the issue with my example above.
That could cause a problem if the discharge line or tank inlet check does not hold.
you missed the outlet check valve in the drier , you have to take a plug out to get to it it is a little junk peice of plastic with an o-ring on it in the adip driers ,posted a picture below i have had most of my problems with the new style in the right of below picture, sometimes the run so long with no o ring left that it wears into the aluminum and no kit will fix it , thus new drier fixxed it not sure why comes up. my service truck is currently doing this it starts out slow and not really noticable untill it gets worse and worse. mine is purging about every minute right now when i shut my truck of i can here air coming out the purge valve after it stops leaking i checked my wet tank and it was empty .
walkerv , You got it right, thank you! I had swapped the drier out to get the truck going. The other day I pulled the outlet check valve and sure enough the o-ring was gone so I repaired it and checked it out and now I have a good spare again. Thanks again!
The newer driers are seemingly more problematic than the ones we started with in the 70's/80's/90's. Getting cheaper as to construction is not helping the systems. UNLESS there is a recommended overhaul the drier time frame they are not telling of. How often you guys change desiccant cartridges? Is that even a recommended action now?
I normally change them in the fall as well as check the rest of air system before the cold weather hits. On low mile units (less than 25k a year) every other year.
Everyone does something different especially when it comes to fleet maintenance , personally i dont touch them untill water starts to show up in the air tanks im not sayyen alot just a little mist and i know something is up . Anymore there is usually a drier problem long before that happens unless its an over the road truck doing nothing but highway .
Bendix used to indicate a service interval, by hours IF I remember correctly on the old styles, to keep moisture control at a maximum. Compressors always throw a taste of oil into the system where I would check for the gray goo at the drier discharge port on the earlier ones as excess oil tended to corrupt the desiccant more quickly than just moisture. Do not know if the newer systems protect from that any better.