EvanJ Posted November 23, 2022 Share Posted November 23, 2022 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! 1 Quote Link to comment Share on other sites More sharing options...
EvanJ Posted November 23, 2022 Author Share Posted November 23, 2022 Pictures of the air compressor: Quote Link to comment Share on other sites More sharing options...
Onyx610 Posted November 23, 2022 Share Posted November 23, 2022 Sorry I’m of no help, but I’d just like to say I’m very impressed with the information/pictures given. You don’t see that too much on here. Someone will get you fixed up! Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted November 23, 2022 Share Posted November 23, 2022 did you check unloader line from compressor to air dryer. possible 1/4 in line; if it has a leak, system will act erratic. 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted November 23, 2022 Share Posted November 23, 2022 Since it seems to act right, for the most part... How about spraying a light 20W oil into the intake of the compressor to see if it loosens the unloaders? jojo Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted November 23, 2022 Share Posted November 23, 2022 update :: 1/4 in line from governor to dryer NOT ::repeat NOT from compressor to dryer Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted November 23, 2022 Share Posted November 23, 2022 changed the dryer AND the governor ;; did we check the line connecting the two . some systems will continue to cycle if air pressure is lost in leaking line. check complete line for chafing / hole etc. Quote Link to comment Share on other sites More sharing options...
Mackpro Posted November 25, 2022 Share Posted November 25, 2022 From what I remember from working on Midliners at the dealership in the late 80’s and into the 90’s was that the air compressor on most Midliners pumped all the time . They had a pop off valve down by the air tanks that would open at a set tank pressure and close when pressure dropped to a certain PSI. I remember it was a pain to install a air drier. I’ll see if I can find some info on it. Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted November 25, 2022 Share Posted November 25, 2022 basically your saying they used the "emergency" blow off valve to regulate pressure? i've heard trucks that constantly had the discharge blowing off, thought it was malfunctioning governor. interesting Quote Link to comment Share on other sites More sharing options...
Mackpro Posted November 25, 2022 Share Posted November 25, 2022 https://n0c357rmy1njbuit2friqwu.blob.core.windows.net/documents/3UaRo$KdI0EBhB_TCH-008-012_US_001.pdf Here is a bulletin on adding a Bendix air drier to to the Midliner air system. Also Robert Bosch built the air pressure regulator that controls the tank pressure. I think the larger and last Midliners used a traditional type air compressor and governor. Quote Link to comment Share on other sites More sharing options...
EvanJ Posted November 25, 2022 Author Share Posted November 25, 2022 On 11/23/2022 at 5:23 PM, Onyx610 said: Sorry I’m of no help, but I’d just like to say I’m very impressed with the information/pictures given. You don’t see that too much on here. Someone will get you fixed up! Thanks for the kind words Onyx610! Its my first post on BMT. Looking forward to see what others come up with. Quote Link to comment Share on other sites More sharing options...
Mackpro Posted November 25, 2022 Share Posted November 25, 2022 (edited) 31 minutes ago, mechohaulic said: basically your saying they used the "emergency" blow off valve to regulate pressure? i've heard trucks that constantly had the discharge blowing off, thought it was malfunctioning governor. interesting It was a weird set up compared to domestic trucks. The only downside is that the air compressor ran all the time. I guess it’s the European way of doing things. I still have lots of Midliner service manuals at work. Edited November 25, 2022 by Mackpro Quote Link to comment Share on other sites More sharing options...
EvanJ Posted November 25, 2022 Author Share Posted November 25, 2022 14 minutes ago, Mackpro said: https://n0c357rmy1njbuit2friqwu.blob.core.windows.net/documents/3UaRo$KdI0EBhB_TCH-008-012_US_001.pdf Here is a bulletin on adding a Bendix air drier to to the Midliner air system. Also Robert Bosch built the air pressure regulator that controls the tank pressure. I think the larger and last Midliners used a traditional type air compressor and governor. 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! Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted November 25, 2022 Share Posted November 25, 2022 I was just studying the pictures you posted. as i've stated numerous times here;; I'm always willing to learn (some )new things. Quote Link to comment Share on other sites More sharing options...
Mackpro Posted November 25, 2022 Share Posted November 25, 2022 I might have more info but it wil be Monday before I can get to my old paper copies of my Midliner service bulletins Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted November 25, 2022 Share Posted November 25, 2022 i think i lucked out when working for Mack, they were still an AMERICAN truck. MS and so on came later in time. B's/ R's/ MB/F's and of course the DM's ; the real Spirit of American !! what's this metric crap !!LOL 1 Quote Link to comment Share on other sites More sharing options...
EvanJ Posted November 25, 2022 Author Share Posted November 25, 2022 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. Quote Link to comment Share on other sites More sharing options...
Mackpro Posted November 30, 2022 Share Posted November 30, 2022 If you still need info . Message me your email and I can send you the complete bulletin 2 Quote Link to comment Share on other sites More sharing options...
EvanJ Posted December 14, 2022 Author Share Posted December 14, 2022 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 Quote Link to comment Share on other sites More sharing options...
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