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1997 cs300p midliner constantly purging


EvanJ
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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!  

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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. 

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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. 

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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.  

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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. 

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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! 

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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. 

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