RESOLVED: AC blowing weak mystery - RM auto climate control

I have a 96 RM wagon and I think this problem would be unique to the Roadmaster auto climate control (and maybe the Fleetwood would be the same). 7/14 update: was not related to auto climate control

Intermittently the HVAC (ac or heat) will start blowing weakly while driving. The air is still the same set temparature, it just is coming out of the vents at a trickle. The blower motor sounds like it is working fine. The system is always fine upon starting, but if you drive the car for very far (at least 15 minutes) it may do it. Or it may not, haven't seen any pattern. Changing settings or shutting it off and on does not correct it, but does adjust the temperature or blower speed or which vents the weak trickle of air comes out of. I disconnected the battery to reset the system, it worked fine for a while then started doing it again. It seems like there is some obstruction in the ducts blocking air from coming out, like a door that closes sometimes.

Any ideas?

7/14 update: problem was the condenser freezing up, the ice accumulation on the condenser would block airflow after about 15 minutes unless it was really hot out. The pressure cycling switch was not working, allowing too much pressure to build up, and the AC to get too cold.

5/16 update: Replacing the pressure cycling switch did seem to fix this problem, but I now think it may also have had to do with an unhealthy compressor. I had to replace my compressor recently. I have heard that a bad compressor can make the AC temps too low in the period before it gives out completely. This could have contributed to my problem of the evaporator freezing up.

Did you run the temp up as far as it goes, then down as far as it goes? You may also have a failing vacuum motor, failing manifold, or a loose/cracked vacuum line.

Does the compressor run steadily, or cycle on and off? Did you check for refrigerant level?

Fred Kiehl wrote:

Did you run the temp up as far as it goes, then down as far as it goes? You may also have a failing vacuum motor, failing manifold, or a loose/cracked vacuum line.

Does the compressor run steadily, or cycle on and off? Did you check for refrigerant level?

I have run the temp at all levels, makes no difference to the problem (but it does change the temp of the air trickling out). I did not think it is a vacuum leak because the the problem is on or off, no in between. It will work normally then suddenly go out. Then sometime later (in the same drive) just as suddenly may go back on. I don't know much about the system, thoug, so I am certainly open to possibilities.

There is a vacuum motor? Where is that? Sounds like a good possibility if that is on the fritz.

I haven't checked to see if the compressor cycles, but I have noticed that it is on even when the temp is set at 90. I thought that was odd. However, the air still is cold when the problem arises, it is just that the cold air is only coming out of the vents at a trickle.

Thanks for the suggestions.

There are a number of vacuum motors to operate the valves in the air box. They are located on the outside of the box. You really need an FSM to dig into it.

You mention changing the blower speed manually. Does it sound like the fan motor is actually changing speeds when you do? Or does it sound like the fan is still running at low speed, even though the display changes? It may be a problem with the fan speed control circuit.

The fan sounds just like it does normally, changes volume with fan speed adjustment, it is only the (slight) amount of airflow coming out of the vents that doesn't change.

This morning I drove to work and it did it, so I left the car on and got under the dash by the fan motor. I disconnected each of the connections for the fan one at a time. Each time the fan turned off, and turned back on when I reconnected, but the problem was still there. I also pulled the connector out for the vacuum manifold. When I reconnected that, I hoped it would trip the problem to resolve, but it didn't. A few hours later I drove the car briefly and everything worked fine.

I'll have to look into the vacuum motors. I figured it used vacuum off the engine.

The vacuum line goes through the firewall with the large connector on the passenger's side of the car. Disconnecting the vacuum line will not necessarily cause the motor to move. If it is in the "off" position when no vacuum is available it will not move when you pull the vacuum line. The motor that moves the vane for the heat, cold is on the front of the module (interior side). Check the heat vent at the bottom for air flow. If there is good airflow, the vane is either stuck, or the motor is not working.

If you want to check the motor, get a vacuum check tool at Harbor Freight, remove the line, and put vacuum on the motor. If it holds vacuum, and moves, you have a good motor. The vane could have something blocking it on the inside as well, so you need to check the movement of the vane itself. You can do this with the AC on as well to look for changes.

I can't think of any air valve in the system that could choke off air-flow completely. There's an inlet valve that selects between outside air or inside air inlet into the system. There's a temperature valve that blends the hot and cold air (the AC is on all the time to dehumidify the air. Unless set to "Vent"). There's a mode valve to divert some or all air to the dashboard vents and a defrost/heater valve to send air up or down. None of those valves shut the air flow off, they just control where it's coming from or going to.

I have to wonder if your system has sucked something like a piece of paper in, which is blocking one of the air flow paths intermittently.

buickwagon wrote:

I can't think of any air valve in the system that could choke off air-flow completely. There's an inlet valve that selects between outside air or inside air inlet into the system. There's a temperature valve that blends the hot and cold air (the AC is on all the time to dehumidify the air. Unless set to "Vent"). There's a mode valve to divert some or all air to the dashboard vents and a defrost/heater valve to send air up or down. None of those valves shut the air flow off, they just control where it's coming from or going to.

I have to wonder if your system has sucked something like a piece of paper in, which is blocking one of the air flow paths intermittently.

The system can not suck anything big into it. The heat valve will direct the air to the floor vent, and will cut off 95% of the air to the dash vents, and the defrost valve will also cut 95% of the air to the dash vents as well. If they are not working the dash vents will not get any noticeable amount of air flow. The valve that closes the exterior air valve will not affect the airflow at the dash vents.

There's no cabin air filter on these, so who knows what could get in there. But you raise a good point: he says the air is trickling out of the vents -- I took that to include the floor or defroster vents, not just the dash vents. If the air is coming out somewhere else, then it could be the air valve for the dash vents (which is actually before the defrost/floor valve).

So the $64,000 question is: When it's trickling out of the vents, is it coming on strong out either the floor or defrost instead?

buickwagon wrote:

So the $64,000 question is: When it's trickling out of the vents, is it coming on strong out either the floor or defrost instead?

No, that's what's so confounding. The air doesn't come out strong anywhere. You can change the settings to floor and it will trickle out of the floor vents. You can change it to defrost and it will trickle out of the defrost vents. Then switch it back to middle and it will trickle out of the dash vents.

Something getting in and obstructing it is a good theory, except that it is intermittent. When the car sits for a good amount of time, it always works normally upon start up, then usually loses airflow at least 15 minutes into a drive. If driving for more than 45 minutes, it may spontaneously return to normal. If it is not working and the car is shut down for only a brief time, it will still not work upon start up.

[quote="jon7190"]

buickwagon wrote:

Something getting in and obstructing it is a good theory, except that it is intermittent. When the car sits for a good amount of time, it always works normally upon start up, then usually loses airflow at least 15 minutes into a drive. If driving for more than 45 minutes, it may spontaneously return to normal. If it is not working and the car is shut down for only a brief time, it will still not work upon start up.

That's why I wonder if it's something sucked (or constructed) in the intake -- say, in front of the evaporator. Eventually it can fall down and allow airflow for a period of time before it is sucked up again. You could try removing the passenger side wiper arm and intake cover to see if you can locate anything in there. I could be completely wrong, but it doesn't take long or cost anything to look.

buickwagon wrote:

jon7190 wrote:

buickwagon wrote:

Something getting in and obstructing it is a good theory, except that it is intermittent. When the car sits for a good amount of time, it always works normally upon start up, then usually loses airflow at least 15 minutes into a drive. If driving for more than 45 minutes, it may spontaneously return to normal. If it is not working and the car is shut down for only a brief time, it will still not work upon start up.

That's why I wonder if it's something sucked (or constructed) in the intake -- say, in front of the evaporator. Eventually it can fall down and allow airflow for a period of time before it is sucked up again. You could try removing the passenger side wiper arm and intake cover to see if you can locate anything in there. I could be completely wrong, but it doesn't take long or cost anything to look.

Thanks, I may try that.

Just to clarify: we are talking about a 96 model year, right? I got to thinking about this last night at work and realized that there IS a way to cut off the airflow with the air valves on the first generation models (91 - 93) only. The airflow was a bit different on the older models and they had a heater shut-off valve after the heater core to block off the heater core outflow for Max AC. If the heater shut-off valve is stuck closed AND the system is calling for max heat at the same time, airflow would be effectively blocked off.

In which case, Fred would be completely correct that the vacuum system may be at fault.

Its a 96.

So, I still have this problem and have been slowly trying things and thinking about it. I am sure it is not a fan problem and not a controller/valve problem. I have checked for obstructions as well as I can, including removing the fan, inspecting as far as I can from there, and removing the cowl screen and inspecting down that hole with a flashlight and mirror. The obstruction theory never was very satisfying anyway, due to the consistently intermittent nature of the problem.

I have a new theory that I think accounts for all the variables, I just haven't been able to confirm it. I think that it is temperature related, and something in the system expands and contracts allowing air to escape out before it can get to the proper outlets. I began noticing the problem last fall and noted that it happened whether you needed heat or AC (which both have the AC compressor running). If I set it on VENT(no AC running) and adjust the temp up for heat, no problem. The lack of airflow only happens when the AC is running. If the AC is on and the problem happens, setting it on VENT will generally correct it, but it takes a few minutes. Now that the hot weather is back, I am finding that the problem doesn't happen as much. If I drive early in the morning when it is 75-80 degrees out, it will happen, but I've found it is not always all or nothing. Sometimes the airflow just decreases without completely disappearing. If I drive in the heat of the day, it doesn't happen at all. So, I think that the cold of the AC coming into the system shrinks something enough to make a leak. In hot weather, whatever it is that shrinks doesn't get cool enough to shrink. In cooler weather, if the AC is off and just the VENT is running, it also doesn't get cool enough. It is the only theory that works, the only problem is that I haven't been able to find anywhere that feels like it has air blowing out of it that shouldn't!

Ok, I think what you are dealing with is a frosted over AC evapourator. If the AC system is running too cold, the evap temp will drop below freezing. Moisture in the air (that normally condenses on the evap and drips down out of the system) will freeze to the fins. Eventually the ice builds up so high that the airflow is blocked off. Shutting off the AC (eg: "vent") allows the ice to thaw and air flow to return.

So, the answer is to make the AC less cold by adjusting the low pressure switch (AC temperature is directly proportional to AC pressure). You really should monitor the low pressure in the AC system for this, if possible. The adjustment is very sensitive -- tiny movements make a big difference.

The LP switch adjustment screw is hidden inside the connector -- you have to unplug the connector to find the screw between the two contacts. Clockwise raises the cut-out pressure, counter-clockwise lowers the cut-out pressure. Carefully adjust it, a tiny bit at a time, so the cut-out pressure is no less than 23 psi. Of course, you have to plug the wires back in every time you make an adjustment because the compressor won’t run with that switch disconnected!

Great! It sounds like I might actually be getting somewhere on this. I noticed recently that the underside of the HVAC assembly where the AC comes in has a lot of condensation on it, which would explain why the carpet is sometimes wet on the passenger side and why I found some rust on the floor pan under there when I first got the car. I also have noticed that the condenser under the hood will get a layer of ice over it when driving for a long time. And when I park it, there is a lot of water pooling under the car.

I have an AC pressure gauge which hooks up to the low pressure port where you add freon. Is that where it should read more than 23psi?

Yup, that's the one (although technically "Freon" is R-12. Your car should be charged with R-134a -- "Suva").

The low pressure switch (aka "compressor cycling switch") is also mounted the same can as the low pressure port. That can is called the "accumulator". The condenser is the finned tubing that lives in front of the radiator. My apologies if I sound picky, but it best to use the correct terminology when seeking help over the internet -- it minimizes misunderstandings.

The housing around the evaporator should have a drain tube through to the underside of the car. If your passenger floor is getting wet, the tube may be plugged with debris.

The switches are pretty cheap too so if you have trouble adjusting it, or find that it is faulty, you can easily replace it. It has a scrader valve behind it so removal will not open up your system, unless of course the valve has gone bad.

I am sure that you are right about the evaporator freezing up. All the symptoms fit it. I put my gauge on the low pressure port and it read 28-30psi. I backed the screw out to the left a little bit at a time and it didn't change the pressure. I eventually turned it about 2 rotations but it still didn't effect the pressure. So, I tried it out driving to work this morning about 30 minutes and it was still the same with airflow decreasing after about 15 min gradually to barely any airflow at about 25 min. There was no water dripping on the ground when I parked, but I went and checked it a little later and there was a huge puddle under there. Totally fits that it is iced up and melts. It was about 80 deg and humid. I assume the problem is worse with high humidity.

So is it normal to have to turn the screw that much? Is the valve bad?

I have had to deal with the clogged drain pipe and overflowing onto the passenger floor before with other b-bodies. Ichecked that on this one when I got it and put an extension tubing onto it. It drains well. I'm talking about the underside of the casing for the HVAC system where the evaporator is, it has condensation on there that drips off. I do have the cover panel off right now, so it is normally covered with the foam padding. I'm tiinking maybe with the evaporator all iced up, it is colder on that casing than normal and makes it wetter on the underside than normal.

I appreciate the tips, it is good to finally have a good diagnosis.

This is internet diagnosing so take it with a grain of salt, but based on your description, it seems to me that it's acting like the compressor is on all the time causing the system to freeze over. If the low pressure switch was working properly then it would have been configured to cut the compressor off when the pressure reaches 21-22 psi for an R134a system, or 28-30 psi for an R12 system. You tried adjusting the switch two full turns and you stated that the pressure didn't change. Do you know if the compressor disengaged at all during this time? It should have disengaged when you removed the electrical connector from the low pressure switch to make the adjustments.

If the compressor is constantly engaged even when you removed the connector for the low pressure switch, then the compressor has been jumped to run constantly. Find where this is at and remove it. Then go from there on figuring out why it was jumped in the first place, and what else might need to be fixed to get it to work correctly.

Now if the compressor did cut out when you disconnected the low pressure switch; then just making a 1/4 of a turn would have been a drastic change in its settings/operation. That being said I believe that if you have the switch adjusted to far to the low side (counter clock wise) then the system would never reach a low enough pressure and the compressor would still constantly run.

SO. . . . . .

The way I see it is, you can take the long accurate approach to diagnosing the low pressure switch; The long not so accurate approach at diagnosing the low pressure switch; OR you could just replace the darn thing and be done with it.

Long accurate approach: Take the switch off and test the variable resistance to see it it still functions. If so then reconfigure it to measure in the resistance value that would trigger the switch when the system gets down to 21-22 psi (you would have to find the specs online because I don't know them). Then install it back on the system and see if the compressor does in fact cut out when the low pressure side reaches 21-22 psi.

Long not so accurate approach: Take the two full counter clock wise turn corrections you applied back out, and then work your way to the high side (clock wise turns) little by little until you actually get the compressor to disengage when a certain pressure is reached ( I think you stated its reading around 28-30 psi). Then you could re-install the switch and slowly make the corrections until the compressor disengages when the low pressure side reaches 21-22 psi.

Me? I'm not usually one to just throw parts at a problem but. . .I would just replace the switch and be done with it. At this point there's just to much hassle to get that switch to work correctly, if it is in deed not faulty, than what the cost of a new switch would set me back.

On a 1993 the R12 switch can be found for less than $6.00, and the R134a switch can be found for under $15.00. If your car is 1994 or newer I think the low pressure switch can be found for less than $20.00.

Hope my long winded response help you out. but like I said Internet diagnosis, take it with a grain of salt.   

The compressor disengages when I pull off the switch, but it never cycled on and off when the car was idling there for 15 minutes or so. Which is odd, since now that I think about it, this car and others usually cycle on and off constantly.

I think I would be inclined to just replace the switch unless there is something else that could be at fault. Is that difficult to do? Is it something you can pick up at a regular auto parts store or dealer only?

The function of the pressure switch is to shut off the compressor when the evaporator nears the freezing point. On cooler days, the compressor will cycle very frequently. On very hot, humid days, the compressor may not cycle at all because the air flowing over the evaporator is keeping it above freezing. Humid air holds more heat than dry air, so it actually keeps the evaporator warmer than dry air of the same temperature. There's a chart in the FSM that show the relationship between relative humidity and the ability of the system to cool the air. At 80° the compressor should be running almost constantly. I would suggest checking in the evening, after things have cooled down below 70°F. The cooler the better.

In cool ambient air, 1/16 of a turn should make a big difference. 2 full turns and nothing changes? Compressor cuts out when switch is disconnected? If everything else is in spec, and it won't cycle in sub-70°F ambient air, it sounds like time for a new switch. They should be readily available through an auto parts store. You should not even have to recover the refrigerant. As Scoffman already pointed out, there is a Schrader valve between the accumulator and the pressure switch so the gas can't escape when you unscrew the switch.

That said, there are other possibilities that come to mind. The system could be undercharged. Ideally the refrigerant should be almost completely vapourized in the evaporator and just a small remnant of liquid reaches the accumulator. However, if the refrigerant is completely vapourized before exiting the evaporator, then engine heat may be warming the vapour in the accumulator enough to keep the pressure up. Check the temperature of the tubes at the firewall -- both tubes should be about the same temperature (cold!). If the exit tube is warmer than the inlet, the system is most likely undercharged.

Of course, if you are using one of those cheap gauges that comes with a corner store recharge kit (aka: "death kit") it may not be accurate.

I am pronouncing this problem solved! I replaced the pressure cycling switch and it appears to be functioning correctly now. I was waiting til I had an opportunity to drive to work in the morning, since that is the coolest weather that the freezeup is likely to occur in. It had been happening about 15 minutes into the drive, but after the 30 minute trip today, it was still blowing fine. The drain pipe was dripping when I idled the car in the parking spot, before it would not drip until the car had been turned off for a little while. Also, the accumulator wasn't caked in ice the way it had been before. I have not heard the compressor cycling at idle, but given everything else, it seems to be working normally.

I also had to get a second cycling switch, because the first one I put on, I screwed it on too tight and the base cracked, which is no good since it was obviously leaking refrigerant. Don't use a wrench!

So after months of head scratching, tearing into the dash looking for obstructions and buying a fan motor and fan module, the cost for the actual repair was $10.99! But I learned a good amount about my HVAC system and AC in general, so that's worth something. Thanks to everyone with tips and especially BUICKWAGON for his correct diagnosis.

5/16 update: Replacing the pressure cycling switch did seem to fix this problem, but I now think it may also have had to do with an unhealthy compressor. I had to replace my compressor recently. I have heard that a bad compressor can make the AC temps too low in the period before it gives out completely. This could have contributed to my problem of the evaporator freezing up.

jon7190 wrote:

Thanks to everyone with tips and especially BUICKWAGON for his correct diagnosis.

Even a blind squirrel occasionally finds a nut...   

Seriously, glad you got it fixed!