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Old 11-08-2019, 06:53 PM   #1
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Purge valve going off every 90 seconds

Hello-

This past summer as I was driving down the highway, I would hear the purge valve going off about every 3 minutes. I took my rig to a truck repair facility and they suggested I replace the governor and the air dryer, which they did. After these repairs, operation of the air system was normal.

Today, I was driving my rig home from its chassis service, and I noticed that the purge valve was going off every 90 seconds or so. I was not applying the brakes or anything like that, just driving down a divided highway at a steady 45 mph or so. I did not see any drop in air pressure in either system on the gauges as I was driving along. They both sat at around 130psi or so.

If I sit in a parking lot not in motion, the purge valve does not seem to exhibit this behavior. It goes off once shortly after the application of brakes to stop the rig, but after that it does not go off, so it only seems to be happening when the rig is in motion.

Thoughts on what the problem might be? Troubleshooting? The only thing different than the last time I drove it was today it was quite cold (~30F) and the rig had sat outside for 3 days, cold soaked, before I drove it home today.

Thanks!
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Old 11-08-2019, 09:21 PM   #2
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I would suspect that somehow moisture has frozen in the desiccant filter/air dryer housing. There is a heating element in the housing to prevent this, but if it is not working it might be the cause. Then there is the possibility the purge valve needs replacing/rebuilding, the O-rings do go bad. When I replaced the desiccant filter I also replaced the purge valve. It's a dirty job and I only wanted to crawl underneath once.
BTW, when the air dryer/desiccant filter is replaced, the outside of the entire dryer unit must be cleaned thoroughly, if ANY dirt falls into the housing it can cause this problem, by interfering with proper purge valve operation.
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Old 11-08-2019, 09:41 PM   #3
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Could be a air compressor governor not stopping the compressor from pumping air after it reaches 120psi. But if it only does this when cold it could be the air dryer heater. The heater drys the air but also keeps the air dryer dump valve from freezing and not dumping, which also will keep the air compressor pumping. So you might check for fuses for air dryer heater. Easiest things first
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Old 11-09-2019, 05:57 AM   #4
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Quote:
Originally Posted by ualdriver View Post
Hello-

This past summer as I was driving down the highway, I would hear the purge valve going off about every 3 minutes. I took my rig to a truck repair facility and they suggested I replace the governor and the air dryer, which they did. After these repairs, operation of the air system was normal.

Today, I was driving my rig home from its chassis service, and I noticed that the purge valve was going off every 90 seconds or so. I was not applying the brakes or anything like that, just driving down a divided highway at a steady 45 mph or so. I did not see any drop in air pressure in either system on the gauges as I was driving along. They both sat at around 130psi or so.

If I sit in a parking lot not in motion, the purge valve does not seem to exhibit this behavior. It goes off once shortly after the application of brakes to stop the rig, but after that it does not go off, so it only seems to be happening when the rig is in motion.

Thoughts on what the problem might be? Troubleshooting? The only thing different than the last time I drove it was today it was quite cold (~30F) and the rig had sat outside for 3 days, cold soaked, before I drove it home today.

Thanks!
Quote:
Originally Posted by Cat View Post
Could be a air compressor governor not stopping the compressor from pumping air after it reaches 120psi. But if it only does this when cold it could be the air dryer heater. The heater drys the air but also keeps the air dryer dump valve from freezing and not dumping, which also will keep the air compressor pumping. So you might check for fuses for air dryer heater. Easiest things first
Ray and Cat, thanks.

If there was frozen water as you guys mention, why is the purge valve going off only while driving and not when the rig is stationary?

Will look for that fuse thanks!
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Old 11-09-2019, 01:02 PM   #5
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Ray and Cat, thanks.

If there was frozen water as you guys mention, why is the purge valve going off only while driving and not when the rig is stationary?

Will look for that fuse thanks!
Higher rpm when driving so you have a lot more air being pumped through the dryer, moving air can create a chilling effect. Its my best guess. I would again check fuses for the heater. And search the web for a ohm or amperage test for the heater. If you haven't serviced the air dryer for a while might be a good time to get it done, or buy the kits online and do it yourself. Its not a bad job, mine was easier if I just dropped the whole dryer and did it on a bench. Hope that helps.
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Old 11-09-2019, 03:24 PM   #6
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You most likely have a air leak in the wet tank. That tank doesn't have a gauge so your 2 dash gauges will stay steady.

Once the dryer purges, the compressor is off, not pumping.

If the pressure in the smallest wet tank drops, the compressor starts up again, until it reaches pressure and purges again.

Some MHs tap the wet tank to run the air suspension. If your ride height valves are leaking while traveling, that will cause more compressor cycling.

At 30 I don't think the purge valve heater is the issue. The purge valve is open when the compressor is not pumping.

Its closed when the compressor is pumping and it wouldn't build air if leaking. Plus you should hear that.
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Old 11-09-2019, 09:47 PM   #7
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Originally Posted by Cat View Post
Higher rpm when driving so you have a lot more air being pumped through the dryer, moving air can create a chilling effect. Its my best guess. I would again check fuses for the heater. And search the web for a ohm or amperage test for the heater. If you haven't serviced the air dryer for a while might be a good time to get it done, or buy the kits online and do it yourself. Its not a bad job, mine was easier if I just dropped the whole dryer and did it on a bench. Hope that helps.
Thanks. I'll go looking for that fuse. The air dryer was serviced this past summer when I had a similar problem with the purge valve going off every 3 minutes as mentioned in the original post. Now it's going off more often : (

Quote:
Originally Posted by twinboat View Post
You most likely have a air leak in the wet tank. That tank doesn't have a gauge so your 2 dash gauges will stay steady.

Once the dryer purges, the compressor is off, not pumping.

If the pressure in the smallest wet tank drops, the compressor starts up again, until it reaches pressure and purges again.

Some MHs tap the wet tank to run the air suspension. If your ride height valves are leaking while traveling, that will cause more compressor cycling.

At 30 I don't think the purge valve heater is the issue. The purge valve is open when the compressor is not pumping.

Its closed when the compressor is pumping and it wouldn't build air if leaking. Plus you should hear that.
Cool thanks. I understand that the air suspension "consumes" air pressure as one is driving down the road. I am not familiar with the wet tank, though. Could you give me a brief explanation as to what it does so I can talk intelligently to the diesel shop when I take it in?

Also, is this type of problem that I am currently having difficult to diagnose/fix? Or should a decent mechanic be able to get to the source of the issue?
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Old 11-10-2019, 06:20 AM   #8
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The compressor sends its air thru the air dryer to the wet tank. Before air dryers, wet tanks were the catch all for water and oil and were to be drained daily.

From the wet tank, the air is distributed to the primary and secondary tanks.

If you look at your tanks, one may look larger then the other. That tank will often be a double chamber tank, with a weld seam around it. The smaller section will be the wet tank.

The govorner signal line comes from that tank so it controls the compressor. The governor controls the compressor and purge valve at the same time. ( stop sending air and purge the dryer ).

If there is a leak in any of the 1/4 control lines or excessive air is being drawn from the wet tank, the PSI will drop and start the compressor cycle over and over, with no indication on the dash gauges.
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Old 11-10-2019, 08:45 AM   #9
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If there is a leak in any of the 1/4 control lines or excessive air is being drawn from the wet tank, the PSI will drop and start the compressor cycle over and over, with no indication on the dash gauges.
Thanks very much twinboat. Now that you describe it that way, I remember that in the Freightliner class I took.

If this is the problem, is it difficult to diagnose? I'm at a new diesel repair shop. They're very nice and RV friendly which is unusual, but not sure if they are good technicians. Is it easy to determine if the ride height valves are leaking, especially if they only leak while I'm driving?
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Old 11-10-2019, 09:07 AM   #10
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Great discussion, especially for an air system noobie like me!



Obviously, 90 s purge valve cycle time is a problem. However, what is a reasonable cycle time (given "normal" demands from brakes, suspension, air ride seats, etc)???


Thanks!
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Old 11-10-2019, 10:08 AM   #11
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Air compressors are designed for a 25% duty cycle. Meaning they should only be filling the tanks 1/4 of the time the engine is running.

Compressing every minute and a half seems excessive, depending on how long it runs. That causes more moisture thru the dryer and then oil past the rings, into the system, from overheated compressor.

If there is a leak, you will need to wait for the compressor to purge and quickly shut down and listen for it. Once the wet tank drops to 90 psi, the governor switchs to compress again and the leak may stop.

Tapping in a test gauge to the wet tank is a good way to see what's going on.
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Old 11-10-2019, 11:07 AM   #12
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Given what's been discussed so far, the fact that it doesn't purge when parked seems to point to the ride height valves, IMO.
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Old 11-10-2019, 11:48 AM   #13
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Thanks very much twinboat. Now that you describe it that way, I remember that in the Freightliner class I took.

If this is the problem, is it difficult to diagnose? I'm at a new diesel repair shop. They're very nice and RV friendly which is unusual, but not sure if they are good technicians. Is it easy to determine if the ride height valves are leaking, especially if they only leak while I'm driving?
Any HDT repair shop can diagnosis what is happening and perform the repairs, you must call first as some will not touch a MH.
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Old 11-10-2019, 11:04 PM   #14
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Given what's been discussed so far, the fact that it doesn't purge when parked seems to point to the ride height valves, IMO.
Today, to add another diagnosis data point, I took the RV out for a drive and the purge valve goes off every 90 seconds or so whether the RV is moving or stationary. I put the RV back in storage and crawled around looking/listening for a leak and heard nothing.

Regardless, it's going to a mechanic to have it looked at. I'll report back as to what the problem was when I get a diagnosis.

Thanks everybody for the advice and information.
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