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Air Pop Off


jeffl1107
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Hey all. I just bought a 1997 Mack RD688 tri axle dump truck. Been fixing some things on it and not being familiar where stuff is on this, have a question. The truck is, I am sure, building up to much pressure in the air system. The guage in the truck goes up to 150 psi. I have had it just past the 150, probably up to 160 and I hear not bleed off of the air to keep it from building up to much. Can someone help me with finding the bleed off valve because I would assume it needs replacing. I know on a Ford truck I had, that bleeder valve use to pop at around 130psi.

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Hey all. I just bought a 1997 Mack RD688 tri axle dump truck. Been fixing some things on it and not being familiar where stuff is on this, have a question. The truck is, I am sure, building up to much pressure in the air system. The guage in the truck goes up to 150 psi. I have had it just past the 150, probably up to 160 and I hear not bleed off of the air to keep it from building up to much. Can someone help me with finding the bleed off valve because I would assume it needs replacing. I know on a Ford truck I had, that bleeder valve use to pop at around 130psi.

That's the relief valve you're hearing and it's doing it's job by preventing over pressurization.

The real problem is that your air pressure governor is not kicking out the compressor at 120 psi the way it should be.

.

"If You Can't Shift It Smoothly, You Shouldn't Be Driving It"

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Is your air governor bolted directly to the air compressor? If so, sometimes whoever installed it may have neglected to remove a plug on the unloader port before they installed it. It happens. If the governor is remote mounted, the line back to the unloader port on the compressor is more obvious and harder to screw up.

Dave

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Thanks for the info. I am going to check it out today. Yeah, I hear no bleed off of air at any time. I have had it just past 150 psi and then I shut it down. I drove it home after I bought it around 10 miles and when I shut it down instantly heard an air leak. After fixing I built up air pressure and realized it was not being regulated. So I will check and see where the governer is.

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Because I am a rookie when it comes to working on Mack trucks, I cant seem to locate the pressure relief valve to see if it is faulty. There is somekind of valve where the line comes into the main tank. It has a small cylinder coming off it coming towards the ground with a hole in the bottom. I am wondering if that might be it. If anyone could lead me in the right direction I would appreciate it.

Jeff

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You are looking for one of these on one or more of your air tanks. But your real problem is still the air governor. Even if the relief valve is working, your air governor is not, you still should be carrying no more than 120-130# air pressure.

The pressure relief valve should only relieve the pressure when it gets into the 150 psi range, which it sounds like maybe it is working.

post-2383-12742907106.jpg

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Ok thanks for the info. Did not see it until I just came in from installing the relief valve which of course from your explanation did no good, which it didnt. I will look for the part that you have pictured, but where would the governer be. I am thinking I might see it under the cab, could that be where it is? If so, I dont know how I am going to get to it.

If so, looks like the cab will pivot up, but dont know what I have to go threw to get that up if that is what I have to do.

Thanks

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The air governor looks like this. It's about 4" long and 1 1/2" in dia. It is most likely mounted on the air compressor, or the firewall. If it's not mounted on the compressor, there should be a small line (trigger line)to an air reservoir, and another small line which goes to the air governor. The largest line also goes to an air reservoir.

post-2383-127430665629.jpg

See my Flickr photostream page

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Ok I found it. It is mounted right on the passenger side firewall. Should I attempt to adjust it. I found on Bendix site that after losening the lock nut, 1/4 turn clockwise is equal to lowering the pressure 4psi. I dont want to screw anything up. My guess is that it is probably bad, but I dont know. Sorry if this is an inexperienced question, but if I do attempt to adjust it, does the governer just not let anymore air build up after a certain point or am I suppose to hear the air discharge. And when I do hear most trucks discharge the air every once in a while, is that the valve you were telling me to look for on the reservoir? Just asking because my 94 Ford L9000 use to do that everytime it reached 125psi, but this Mack has not done it at all.

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Ok I just got back in from trying to adjust it. I went as much as 2 full turns which by my calculations was equal to decreasing the pressure by 32 psi. Started truck and still went over 150psi before I shut it down. I guess I will get a new one tomorrow.

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