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Daniel Wilson
Daniel Wilson,
Category: Medium and Heavy Trucks
Satisfied Customers: 5415
Experience:  Owner at DJ Wilson
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My '07 Kenworth W900 will not build air pressure. I had a similar

Customer Question

My '07 Kenworth W900 will not build air pressure. I had a similar problem last fall and replaced the air dryer with a reman air dryer (Bendix AD-IS) and new governor. That fixed the problem. A few weeks ago I installed a Right Weigh air scale system and new leveling valves on the truck and trailer (Haldex PR+ on truck and Haldex EGP on trailer). Since that time, I could hear a small hiss of air and I would lose pressure when setting and releasing the brakes within a short period of time (grain hauler). It ran at full pressure when driving and operated fine. The other day I went to start the truck and could not build pressure at all - not even 1 psi. I found a hole in the rubber line coming from the compressor to air dryer and replaced it. Didn't solve the problem. So I put on a new governor. Didn't solve the problem. I've pumped the system full of air with an air compressor and released the brakes to check for leaks and found none. I've sprayed soapy water over the steel braided line from the compressor to air dryer and found no leaks. The compressor pumps air, but I can put my finger over the outlet and block it off. Scratching my head to find an answer.
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Daniel Wilson replied 1 year ago.
Hi Neal, My name is***** will be glad to help. I see you already corrected the main discharge line and assuming you bypassed the air drier as well for test purposes. If so then the common cause of this is the unloader valves in the compressor. this means two options. Order an unloader kit for your compressor model and try that first to cut costs or replace the compressor. Let me know
Customer: replied 1 year ago.
How do you bypass the dryer for test purposes? I assume make some sort of tee to tap main line from compressor to the orange and green lines running back to the leveling valve?
Expert:  Daniel Wilson replied 1 year ago.
connect the main supply to the discharge using a union connector so it goes right to the primary tank bypassing the dryer.
Expert:  Daniel Wilson replied 1 year ago.
One other thing to check is the governor signal lines as they command the compressor to cut in and cut out. Not sure if any part of that circuit was disturbed during the install?
Customer: replied 1 year ago.
We didn't mess with the governor during install. I think this might just be a fluke occurrence relating to the scale install. Wondering if we damaged the compressor by running it with the hole in the main line (never allowing compressor to shut off?)Also, how much pressure should be coming out of the compressor? I can cover it with my finger and stop all air.
Expert:  Daniel Wilson replied 1 year ago.
You should not be able to do that.... So the governor is cutting it in
Expert:  Daniel Wilson replied 1 year ago.
That points right to a stuck unloader valve....
Expert:  Daniel Wilson replied 1 year ago.
What compressor model?
Customer: replied 1 year ago.
Bendix. I'll check model in a minute - working on another repair issue.
Customer: replied 1 year ago.
Bendix BA921
Expert:  Daniel Wilson replied 1 year ago.
Click on this link. There is some operation and troubleshooting... Also check the unloader.Troubleshooting
Customer: replied 1 year ago.
Thank you. I'll let you know how it progresses.
Expert:  Daniel Wilson replied 1 year ago.
The unloader kit part number is (5008557)
Expert:  Daniel Wilson replied 1 year ago.
Pleasure to help.