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Topic: frozen pressure switch? (Read 902 times) previous topic - next topic

frozen pressure switch?

I just noticed my auxiliary air compressor is short cycling, running for a moment and then stopping. We've had sub freezing temperatures here at Bandelier national monument for a while and I'm wondering if some moisture in the pressure switch or the pipe nipple leading to the pressure switch might have frozen and partially blocked the switch's pressure reading (?) I'm just taking wild guesses at this point. The compressor has never done this before. I suppose it could be a thermal overload in the motor but I don't know why cold weather would cause the motor to overheat. Anyone else ever had this happen and what did it turn out to be?
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)

Re: frozen pressure switch?

Reply #1 –
Scott the local NAPA store will have everything you need to put in the air pressure gauge in line near that box. It will help you a lot in diagnosis.
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: frozen pressure switch?

Reply #2 –
Scott,
Took me a while to remember, but yes, I think I have seen this short cycling on the compressor.  I think you're on the right track in thinking there is an ice dam somewhere in the output line and of coarse no surprise since the compressor doesn't have a dryer on it.  When it happened to me it was just a cold morning (probably single digits) and warmed up later in the day.  I just turned off the compressor at the breaker when it happened and later when I went to check it out it was working fine.  I think the reason I haven't had it happen again is because I have added two 300 watt heaters to the wet bay and the heat is enough to pour over into the bay with the compressor.  You might want to make sure your wet bay isn't getting too close to freezing.  If you're parked for a while there is the option of letting the coach sit on wood blocks next to the air bags and leaving the compressor off all together - will save some wear and tear on the compressor.
John Fitzgerald
1991 U300 (SAI) Side Aisle Island Bed 40'
Detroit 6V92 with Allison Retarder
Meridian (Boise), Idaho

Re: frozen pressure switch?

Reply #3 –
I opened up the cargo bay and it didn't feel like it was below freezing in there. I don't open it unless I have to in cold weather and I close it as quickly as possible. The compressor doesn't run at all now, making me think it's the motor. I already overhauled the compressor last summer but the motor was still okay so I left it alone. I think I'll use this excuse to purchase a 12 volt DC compressor tomorrow and I'll install it beside the 120 volt AC compressor. I've been meaning to do this for a long time. That will give me time to order the new motor. I like keeping my air brakes and suspension pumped up, oil pan heater on, and cranking batteries fully charged so I can bug out quickly in case of emergency. I've only had to do this once in over three years but it caught my attention... Does heat from back by the water tanks make it up front into the cargo bay? If I run my rear furnace I don't notice any warm air coming out in the cargo bay but I can definitely feel warm air blowing around the water tanks.
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)

Re: frozen pressure switch?

Reply #4 –
Scott yours may be different but my coach only heats the water compartment bay on both sides. I have the AH in the main bay and that keeps it warm and the front bay is cold and no heat. 
2025 Wanderbox Outpost 32 on F600 Expedition Motorhome
2015 Born Free Royal Splendor on Ford 550 nonslide version  for sale
Former Coaches  covering. 360,000 miles
1999 34 U270
2000 36 U320
2001 42' double slide U320
2018 Jeep Rubicon

Re: frozen pressure switch?

Reply #5 –
Quote from: RRadio –
... Does heat from back by the water tanks make it up front into the cargo bay?
Probably not much.  My statement in my prior post was purely speculation.  My furnace has one outlet into the water bay - that's it.  There is a hole in the bulkhead wall next to the water heater where air can flow into the storage compartment, but nothing is forcing it.
I can only suggest checking around the motor to see if there is an internal breaker button that might be popped.  I would also check the pressure switch to see if it failed.
John Fitzgerald
1991 U300 (SAI) Side Aisle Island Bed 40'
Detroit 6V92 with Allison Retarder
Meridian (Boise), Idaho

Re: frozen pressure switch?

Reply #6 –
I doubt the problem is frozen moisture because it warmed up to a little above freezing briefly today and the compressor still doesn't run at all. I'll try to diagnose the problem tomorrow. I've been real busy with snow removal and fixing broken heaters here at Bandelier national monument. Maybe I'll go home from work early tomorrow if I get everything in the park under control... not likely but possible. :)

I had to go to Santa Fe to look for an obsolete circuit breaker from a unit heater today so I went to Harbor Freight and bought an inexpensive little 12 volt DC compressor. I won't use it much, only for emergencies like this. I could also see myself using it when I come back to camp late and generator hours are over. That's happened to me a couple of times and I always wish I had a 12 volt DC compressor to level out my coach before I go to bed. The little compressor has an exploded diagram and maintenance instructions and everything. It looks like it might be rebuildable even. Harbor Freight is one of those stores that always surprises me with either quality or cheapness, if that makes sense. Their stuff is always low priced but sometimes the quality is surprising and other times I can't believe they even sell some of that junk. It's like low stakes gambling! haha
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)

Re: frozen pressure switch?

Reply #7 –
I finally got some time off from work during the daylight and took the compressor out. I checked the pressure switch and it was obviously closed. I checked the capacitor and it seemed okay. I plugged the compressor in and it started running normally, so I put it back in my coach and it's working fine again... Doncha just hate when that happens and you never figure out what the problem was? ...I used this excuse to install my new little 12 volt DC compressor and I tested it briefly to be sure it really works. It's super slow compared to the 120 volt AC compressor of course, but it'll only be used in emergencies and when I'm not allowed to start my genset. I think I'll order a spare capacitor for the 120 volt AC motor and carry it with me.
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)

Re: frozen pressure switch?

Reply #8 –
I don't know if anyone else has had this problem in cold weather, the 120 volt AC auxiliary compressor short cycling. I may have found the solution to the problem. I'm only talking about older coaches with the big 3/4 horsepower twin cylinder aux compressor, not the newer coaches with the little 12 volt DC compressor. The big compressors don't have a moisture separator, or at least mine didn't. I went to Harbor Freight and bought an inexpensive moisture separator, the kind with the transparent bowl with a condensate drain on the bottom. I installed the moisture separator on the aux compressor side of the check valve. This is important so make sure you understood what I just wrote. There's a check valve just downstream of the aux compressor and make certain that you install the moisture separator on the compressor side of that check valve, not the chassis side. If the bowl or gasket or any other part of the moisture separator blows out while you're driving you could lose air pressure to your brakes if you install the moisture separator on the chassis side of the check valve. While you're at it you might test the check valve to see if it still holds air pressure. If it leaks take it NAPA and they can order a new check valve for under $80. I replaced mine last summer. I think mine is a Bendix check valve. Some of the older ones can be disassembled and you might find debris in it which can be cleaned out and the valve will work again. One of my check valves had some teflon tape in it and worked fine after I cleaned it out. The new check valves are sealed unfortunately.  Anyway, back to the moisture separator cuz that's what I intended to write about. The moisture separator should remove any water the aux compressor is pumping into the wet tank. I believe the moisture introduced by the aux compressor was freezing in the line going to the wet tank, causing a partial blockage which made the aux compressor's pressure switch short cycle. After I installed the moisture separator I haven't experienced any short cycling even though the weather has been colder than it was before. I also bought a drop light with an incandescent 60 watt rough service bulb and put it under the moisture separator to keep the water in the bowl from freezing and breaking it. It's been several weeks since I did this and it seems to be working, so I'm gonna consider this mystery solved. Hope this helps someone who is banging their head over the same problem. :)

Also the moisture in the wet tank froze inside the blowdown line and damaged the blowdown valve to the point that it sometimes leaks a little... but that's an ideal place to have a small leak so I haven't replaced it yet.
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)
 

Re: frozen pressure switch?

Reply #9 –
I was thinking about stuff, which is always dangerous, and realized that the moisture separator verifies that the check valve is working because it dumps all the condensate out of the blowdown valve in the bottom of the bowl after the auxiliary compressor shuts off each time. If the check valve doesn't hold, the moisture separator's blowdown valve will stay closed and condensate will build up in the transparent bowl where I'll see it. I look at the bowl every time now, which is probably a good habit developing. Because the blowdown valve opens and dumps the condensate every time there's no chance of moisture or freezing anywhere in the system, so I stopped using the drop light to keep the condensate bowl warm. There's also no chance of water building up in the wet tank, which I replaced recently because it rusted out of course. This is another good reason to install the moisture separator on the aux compressor side of the check valve instead of the chassis side, so the blowdown valve will dump the condensate after the aux compressor shuts down. It takes a few minutes for the air pressure to leak out of the aux compressor after it shuts down, then the blowdown valve will open. This turned out to be an excellent and inexpensive solution to a bunch of annoying and potentially dangerous problems. I'm kinda surprised Foretravel didn't include a moisture separator on the coaches with the big 120 volt AC aux compressor but whatever, mine has one now and I'm glad I installed it.
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)