EvanJ

Thanks so much for the help Mac Pro! sounds like a lot of these euro trucks did not have unloader valves in the compressors and use a discharge line unloader system. I can source a AD-9 DLU locally and will give that a try. Thanks again everyone for the helpful comments and happy holidays! -Evan

Thanks again Mackpro your name says it all. I am curious about the pressure regulator valve (#3 on the diagram). In the table it shows that this valve vents to atmosphere when open and therefore would not send excess air to the dryer causing my issue. Offhand I am quite sure that there is no such valve plumbed into the system but I will take a good look tomorrow and take some pictures.

Thanks a lot for the helpful posts Mackpro. When I bought the truck it had a standard Bendix ad-8 air dryer so I replaced that as an attempt at trouble shooting. Sounds like the dryer original to the truck could have been a ad-2, ad-4 or ad-9 DLU unit that was designed to purge and also unload the supply line once pressure was reached. The truck appears to have been passed around around quite a few times so someone easily could have replaced the original euro dryer with a common and inexpensive ad-8. I should have said earlier but there is no discharge line unloader valve in the system. The compressor is plumbed directly to the dryer. I called around and the ad-2 and ad-4 are now obsolete but the ad-9 dlu could be ordered in for around $500 plus freight and tax. A bit steep for a reman dryer but at least its an option. I took the truck out for a good drive this afternoon and the constant purging sure gets annoying at higher rpm. In response to the earlier reply about spraying some light oil into the intake of the air compressor: I tried that today and the issue persisted. I am still not sure if there are valves in the top end of this compressor or not. Thanks for the advice though!

Thanks for the kind words Onyx610! Its my first post on BMT. Looking forward to see what others come up with.

Pictures of the air compressor:

Good day everyone, I am looking for some Information on the air compressor unloader system on a 1997 mack (Renault) cs300p with the e3 6.2L engine. Here is the summary of my trouble shooting so far: The truck was sitting for about 5 years. Started it up and once the compressor reached 125psi the air drier would purge. The purge valve would stay open and spit out air until the pressure dropped below 110psi, then it would seal and pressure would build again. I checked for air line leaks (nothing noteworthy), replaced the complete dryer (Bendix ad-8) (no difference), replaced the governor (no difference). So I am thinking that the unloader valve is stuck closed. The problem I am having is finding any information on how to service the compressor. I saw in another forum that heat from the exhaust (located right beside the compressor) can cause valve issues. Besides 4 bolts that hold the head on the compressor I don't see any access for servicing. finding parts, information or even a replacement compressor is next to impossible here in western Canada. If anyone has had similar issues I would greatly appreciate your input. Thanks for reading!