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

[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

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. Tons of Ipod stuff though.

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.

[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..

[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.