I'm burying the needle on my air pressure, 170 psi

[lowrange]

Any ideas? It always used to go up to 125 psi and stop. Then, one time I noticed and it was 130 psi while cruising. Now, when I'm hill climbing, I bury the needle as I said. As soon as I stop climbing, it levels off around 135 psi.

How bad is the threat (how much air can the system handle)? What's likely the problem? Cost of repair/parts? TIA

[Rev.Vassago]

Sounds like the governor on the compressor is taking a dump.

[Kranky]

I sure hope the relief valve is working!

I agree, the air pressure governor has failed, or the air line between the wet tank and the governor, or the line between the governor and the compressor (if remotely mounted) is clogged or broken.

[Larry227]

I would think you also have a bad relief valve on the compressor. I think they should pop off around 150.

[lowrange]

Ok, thanks for the replies. I confess, I don't even know what the regulator looks like, and I didn't know there is a relief valve. The needle was buried, though, must have been somewhere around 170 psi. I see there's a place not too far from my pickup, Diamond State Truck Center in Elkton, MD. I'll give them a try (after I crawl under there and see if I can find anything obvious).

[Rev.Vassago]

and I didn't know there is a relief valve.

You know that "pop and hiss" that the compressor does when it kicks out? That's the relief valve.

[lowrange]

You know that "pop and hiss" that the compressor does when it kicks out? That's the relief valve.Honestly no, I'll have to listen for it. I've had this intermittent air leak behind the dash, I've been up in there myself before working on it. Anyway, the sound I'm always conscious of is the sound of leaking air behind the dash.

Thinking about it, I guess I do know that sound, but what does the part look like?

[Creek Jackson]

Honestly no, I'll have to listen for it. I've had this intermittent air leak behind the dash, I've been up in there myself before working on it. Anyway, the sound I'm always conscious of is the sound of leaking air behind the dash.

Thinking about it, I guess I do know that sound, but what does the part look like?

If you can hear air bleeding off behind the dash, you likely have an air leak some where on your tractor or trailer(s). This has happened to me. The air leaking out elsewhere creates a hissing or moaning sound near the valves on your dash. It could be leaking behind the dash, but I would guess it is somewhere else.

The pop-off valve or relief valve is generally located on the air dryer. A good service shop should be able to point right to it. With some investigation you can find it yourself, but in most cases, you will need to get under the truck.

[heavyhaulerss]

Honestly no, I'll have to listen for it. I've had this intermittent air leak behind the dash, I've been up in there myself before working on it. Anyway, the sound I'm always conscious of is the sound of leaking air behind the dash.

Thinking about it, I guess I do know that sound, but what does the part look like?

as far as air leak behind the dash.. it could be your park brake. the one with the yellow/red knobs. but you cannot detect leak by hearing it. you have to soap it, or put other source of leak detector on it. don't do like I did one time.. I clearly heard air when I put my ear to the dash valve. I replaced the valve. the leak was not in the valve. it was a leaking air bag!!! the air that was rushing to replace the air lost from bag could be heard from dash, but that was not where the leak was.

[lowrange]

Truck fixed. Bolt missing on the regulator (they call it a governor), said it was hanging. New governor installed, $125.

I've dealt with that dash leak before, I know exactly where it is. It's intermittent, and though it's too noisy, the rate of air loss isn't too bad. Anyway, unless something changes, I'm not ready to tear the dash apart again.

[GMAN]

If you have a leak in your dash then it is likely coming from the valves. Once you have the dash apart it isn't very difficult to change them out. I checked on one about a year ago and I believe they cost about $395. You may check to see if they have a rebuild kit for them. Of course, I would not replace anything until I found out where the air was leaking for sure.

[Windwalker]

If you have a leak in your dash then it is likely coming from the valves. Once you have the dash apart it isn't very difficult to change them out. I checked on one about a year ago and I believe they cost about $395. You may check to see if they have a rebuild kit for them. Of course, I would not replace anything until I found out where the air was leaking for sure.
Back when I had my own truck, I had a leak that I traced back to the parking brakes too. Sure did not cost me $395 unless someone else replaced it. I think mine was only about $170 or so. But, don't expect to get in to the air-line connections with any big wrenches. It's a bit cramped to get into a couple of them.

On the side of your air governor, there is a line that controls the "kick-out". If something has happened so that it does not get the pressure back to the governor, it will not kick out. Things like getting blocked with ice will do it. If that line is going through somewhere that it is getting "pinched off", it would do it. Take that line off the governor and the compressor won''t quit. Building up 170 psi stands to blow out lines and components that would go on for a few more years under normal conditions.

[heavyhaulerss]

if it's the dash park brake it's called the mp3 valve. bought 1, 1 month ago from truck pro. 138.00 same price more or less at nearby truck parts stores.

[Jackrabbit379]

You know that "pop and hiss" that the compressor does when it kicks out? That's the relief valve.

I thought that was the air dryer. Or can I not hear the relief valve, because of the air dryer...

[Bull]

Last time I heard air leaking from my dash it was from a leaking brake pot.

[GMAN]

if it's the dash park brake it's called the mp3 valve. bought 1, 1 month ago from truck pro. 138.00 same price more or less at nearby truck parts stores.


Was that for the entire valves or just the rebuild kit? One of my trucks is also losing air. I am having difficulty finding the leak. I can't hear air leaking but the air drops fairly quickly when the engine is off. When the engine is running the air is where it is supposed to be. It has to be something simple. It is too difficult to find.

[Kranky]

Was that for the entire valves or just the rebuild kit? One of my trucks is also losing air. I am having difficulty finding the leak. I can't hear air leaking but the air drops fairly quickly when the engine is off. When the engine is running the air is where it is supposed to be. It has to be something simple. It is too difficult to find.

Did you check the air operated fan clutch and associated lines and control valve?

[GMAN]

I haven't noticed anything from around the fan clutch. I am thinking it may be coming from somewhere under the cab since it is so difficult to hear or find. I need to get underneath the truck to see if I can find it. My other truck had a leaking air bag under the cab last year. It was very difficult to find until the leak got worse.

[lowrange]

There is a plastic T to my air lines inside the dash. I had the dash apart before, I could feel the rush of air coming out of one of the lines. I also managed to find a replacement T, but I went to a number of places looking for it.

I think, in the end, all I did was jam air line harder back into the existing T and if I remember correctly, the new one I bought is laying on the floor of my storage compartment.

Yesterday I couldn't hear the leaking sound and the pressure held pretty well. You know, I don't plan to mess with it like it is.

[matcat]

Gman I have an air leak similar to yours, coming from passenger side towards the engine, but I can't find it exactly. I am thinking its the air line to the differential, since the differential don't work! But turn the truck off and poof, air goes down to 55psi in a matter of a minute, but stays there.

As far as the relief valve going off, you cannot miss it, you will hear it every now and again when it goes 'psssssffffft' :) One truck I drove used to sound like it was farting when the relief went off.

[Rev.Vassago]

As far as the relief valve going off, you cannot miss it, you will hear it every now and again when it goes 'psssssffffft' :) One truck I drove used to sound like it was farting when the relief went off.

I hate when I'm at a truck stop parked next to a truck that the governor isn't working properly. That relief valve will go off every minute or so. Friggin annoying.

[stonefly]

I hate when I'm at a truck stop parked next to a truck that the governor isn't working properly. That relief valve will go off every minute or so. Friggin annoying.

It's not always a relief valve that's going off.

I recently had a problem on an old CAT 3406b with a Bendix AD4 dryer.

The trouble was in a faulty check valve at the discharge port of the dryer. The discharge port connects directly to the wet tank. The pressure in the wet tank is signaled to the governor by a small air line. When the wet tank gets up to pressure, which should be between 120 and 130 psi, it signals the governor to unload the compressor. The same air pressure that unloads the compressor is fed down another small airline to the dryer purge valve.

If the check valve in the dryer is malfunctioning, then the air in the wet tank surges back into the air dryer when the dryer purges.

Because the air in the wet tank surges back into the air dryer, as it should not be doing, the wet tank pressure can drop as much as 40 psi in less than a second. The drop in pressure is signaled back to the governor, which responds by reloading the compressor to pump air and closing the purge valve on the dryer for another charge cycle. Depending on the output of the compressor and the size of the wet tank, it may take anywhere from 15 seconds to perhaps almost a minute to get the wet tank back up to cut off pressure. When cut off pressure is reached, and the governor gets the signal, the compressor kicks off, the dryer blasts off, the wet tank leaks back, and on and on and on...

Until the problem is corrected, the compressor never shuts off. Running the truck in this condition can quickly ruin a good compressor.

A faulty check valve is a common cause of constant purging. (Usually it's every 15 or 20 seconds.) The drop in wet tank pressure is not seen on the gauges in the cab of the truck because those gauges monitor the primary and secondary tanks, which are protected by one-way check valves. It is only wet tank pressure that escapes back into the air dryer.

The problem can be easily and inexpensively solved without disassembling the air dryer to replace the internal check valve. (The AD9 has an external check valve.) Instead of replacing the check valve in the dryer, a universal type check valve can be installed at the input of the wet tank. Northern Tool sells check valves for about $22. It's about ten minutes work.



stonefly

[heavyhaulerss]

Was that for the entire valves or just the rebuild kit? One of my trucks is also losing air. I am having difficulty finding the leak. I can't hear air leaking but the air drops fairly quickly when the engine is off. When the engine is running the air is where it is supposed to be. It has to be something simple. It is too difficult to find.

it was for the entire valve. I took apart the old one. it looks easy to rebuild. nothing but -o-o rings to replace.

[heavyhaulerss]

it was for the entire valve. I took apart the old one. it looks easy to rebuild. nothing but -o-o rings to replace.

I bought mine at a truck store named kurts in decatur AL. Kurts Truck Parts (http://www.kurtstruckparts.com/)

these are small town folks who know there stuff. Ben Crider is the most informative person there.

[GMAN]

it was for the entire valve. I took apart the old one. it looks easy to rebuild. nothing but -o-o rings to replace.


That sounds like a good price. It doesn't look like it would be difficult to replace those o rings.

[blanco]

if it's the dash park brake it's called the mp3 valve. bought 1, 1 month ago from truck pro. 138.00 same price more or less at nearby truck parts stores.

I tried a google search for MP3 valve and came up with nothing. :confused: Tons of Ipod stuff though. :lol:

Mine is leaking as well and I can hear the hiss from the air line that is providing the exhaust (?) to the parking valve. When you push the yellow knob, this line hisses for a min, then silence.

I'm home for a couple of days and would certainly look into this. Truck Pro is around the corner.

This is on a 1999 Volvo. :eek1:

[heavyhaulerss]

I am sorry. it is a mv3 valve. just looked at receipt.

[Bigmon]

How long does it take to replace the MV3 valve. It looks like you have to take the dash apart.

[heavyhaulerss]

How long does it take to replace the MV3 valve. It looks like you have to take the dash apart.
took me 30 min to replace. 4 screws to get access to the valve, then remove all the brass fittings on back of valve & replace i'm new valve, along with all the air lines going to it. then scew it all back together. my dash is easy to change out valve. others may be difficult

[blanco]

thanks for the info.. :thumbsup:

[YerDaddy]

Sounds like the governor on the compressor is taking a dump.

I thought that was the air dryer. Or can I not hear the relief valve, because of the air dryer...

Yeh that's the air dryer spitting out the water and oil at the end of the cycle.The relief valve is on the primary tank. 150 psi. for when the compressor won't kick off. It looks just like the one on your home water heater except a bit smaller.