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Mattdiem · 05-18-2025, 03:50 PM

2008 Fleetwood Excursion on an 07' Freightliner chassis. I think the air dryer is a haldex. Is it normal for the pressure relief on the air dryer to keep purging air while accelerating?

If my system is trying to build air, the pressure relief on the side of the dryer will either blast off over and over again, or constantly, until the motor rpms drop. When the purge valve finally purges and system stops building air its quiet. If idling or low cruise I never hear the pressure relief. Its sort of always done this the past 6 years I've owned it, but it got worse last year so I started trying to fix it. I rebuilt the air dryer. New desiccant, turbo saver, purge valve, and check valve. Problem was the same and I finally realized the different sound I was hearing was the pressure relief blasting off and not the purge valve. Replaced pressure relief and didn't go away. I then replaced the governor and the compressor discharge to dryer. (braided line with flare fittings). I also found and replaced a bunch of small leaks on the rest of the system. I thought maybe bad replacement pressure relief valve so I tried another. Was slightly better but still happened and now happens all the time. Coach makes air and brakes work fine but this cant be normal. I also put a tee inline with the purge valve and ran a line from the dryer, in through a back window, all the way to the cab. I could watch the valve on the gauge opening and closing all the time and never saw pressure above 120 psi. If I remember correctly, the valve is supposed to be 175psi. Why would 3 different pressure relief valves be opening way below the 175psi they were set for. I did notice while watching the gauge that was connected to the dryer right at the same port as the pressure relief that the needle was violently shaking on the air gauge while compressor was working. You could even feel it in the line. The needle was like a blur, but you could see the center point and read the gauge as the pressure built. Is that an issue?

Skip426 · 05-18-2025, 07:04 PM

Following ; because you've already done everything I would have suggested.

Only thought is an internal issue in the air compressor that won't allow the governor to put the compressor into it's " Unloaded cycle " .

Is the governor attached to the compressor , or remote with a tube connecting it to the compressor ?

EDIT: Governor is connected by a 1/4" air line to the dryer ; on my coach .
Any damage to that line , that would stop pressure from reaching the governor , that would cause the compressor to run continually at full pressure, like the situation you have.

J_C · 05-18-2025, 11:11 PM

Are you sure the air is coming from the safety valve? Don't mean to question this but after you have replaced the safety valve more than once with a 175 psi and don't see that high of pressure on the line, it doesn't make sense it is coming from the safety valve. Is the air spitting from the air dryer when the engine rpms are raised and it gets faster the engine rpms are at? If you have a Cummins engine, the unloader plate in the air compressor head will get stuck and cause this issue.

shootist · 05-19-2025, 01:22 AM

Another vote for the governor

Mattdiem · 05-19-2025, 10:40 AM

Yes, governor is attached directly to the compressor and yes, there is a 1/4 line running from the governor to the dryer that makes the purge valve open at the end of a cycle. I'm assuming it's working because when the high pressure relief holds, (let's say at low rpm or idle) the compressor unloads and also pops off the purge in the dryer just as it should at 130 or 135, can't remember. It seems to be working but that could be my next experiment. A new line from governor to dryer and see if it changes anything.

What doesn't make sense is if the governor malfunctions, or the compressor valves stick you would think the pressure would go past the normal cut out and then pop the pressure relief. That would make sense but when I checked it with a gauge it never even got that high. This is all happening and below normal cut out pressure.

As far at it being sure it's the pressure relief and not the purge... At one point it was way worse and it was blowing off air three times or so even at idle and then finally the pressure would make it to the cut out pressure and dump from the purge. Then everything would stop as it should until we hit the service brakes a bunch and made it start over. The wife hit the brakes so I could go listen. I was laying there underneath the coach and could watch it happen. That's when I figured out it was the pressure relief. They sound different and now I can tell which one is doing what.

Also... my wife ran the above post through some ai chat posing as a diesel mechanic and it said that pulsations in the compressor discharge are not normal and that they could cause premature opening of a pressure relief valve. It mentioned oil build up or damage of the valve in the compressor head. Maybe it's on to something but I've seen videos with people working on compressor discharges and they seem to flutter the same way.

twinboat · 05-19-2025, 12:40 PM

The small governor line probable has a hole in it. That line needs to hold air otherwise the governor kicks the compressor back on.

You can remove that line and plug the unloader port of the governor . The dryer won't purge but the air pressure should stabilize at the set point.

J_C · 05-19-2025, 06:19 PM

A leaking 1/4” governor signal line will cause the compressor to continually cycle. I don’t think that is the issue. You stated you rebuilt the air dryer already but a safety valve popping off is an indication of high pressure in the air circuit or a faulty safety valve. Did the air dryer get rebuilt properly?

Mattdiem · 05-19-2025, 07:19 PM

What has be baffled is that if it is high pressure, why cant I see it on a gauge connected right at the same port as the pressure relief? That's what had me convinced it was a faulty pressure relief and tried another new one. As far as the air dryer rebuild... it seemed pretty straight forward but could be wrong. That was the first and last haldex purest I've ever rebuilt. Something could have not gone together correctly. What part of it could cause this? The turbo saver? The check valve? I was starting to wonder about the check valve because I remember it not seating properly at first and then once it was correct all seemed to function normal until on the road under acceleration. I thought the check valve only held air in the lines after the dryer but I'm wondering if its sticking or fluttering it could cause issues.

J_C · 05-19-2025, 09:02 PM

Possible the check valve is not seating or was installed improperly. I myself and have installed pieces upside down or rolled orings during installation that will cause leaks. It might be worth to pull the check valve back out and reinspect. I have not rebuilt this Haldex air dryer but have rebuilt plenty of Wabco versions. Is it possible your gauge that you were using for testing is faulty? It doesn't make any sense to me either that the safety valve opens at 175 psi and you are only reading 120 on the gauge. Just wondering if you exceeded the pressure on the gauge and it got damaged.

Mattdiem · 05-21-2025, 05:25 PM

I was looking into another check valve kit but now I have a whole new air dryer on the way. The haldex PURest DA34100 units aren't too bad at around $600, but there are military surplus versions for around $300 shipped. They are made by Haldex and are supposedly the same except you'd have to swap your old heater over. They have 24volt heaters instead of the 12volt. That's said to be the only difference, but I noticed the lower unit with all the fittings is clocked differently as well. I think my line configuration will reach the fittings in the new locations though.