Dangers Associated With Freezing The Evaporator

[kings1527]

I have my R12 system adjusted coooollllld. I messed around with the low pressure switch the other day and I'm getting 26-28 degrees at the center vent at speed 2 with an ambient of about 65 but I was driving yesterday in about 105 degree ambient and had 43 at the center vent at speed 4. All of this was on MAX.

Today I was driving around with MAX at fan speed 1 or 2 with an ambient of maybe 85 and I started to get light fog coming out of the center vent so I thought things might've been freezing up so I dialed the temp dial away from max cold just a bit and it went away. I think I also switched over to NORM so I could mix in a little heat.

Can I cause damage by doing this? If anything, ill hook up my gauges and set the LP switch to its proper setting but man, I can hang meat in my car and its awesome. I'm just curious what the dangers are with freezing the evap if that's what I'm doing and if simply moving the temp dial away from the coldest setting will fix any issues.

Thanks!

[Ron]

It is a major cause of compressor damage.

[kings1527]

It is a major cause of compressor damage.

Thinking about it. I'm guessing it's sending liquid back to it?

[Ron]

Yep. ...and it can cause the oil to collect at the evaporator.

[WelmoedJ]

I agree with the other comments.

Usually temperatures between 41° and 43° F should be good (in a perfect system) at the vents.
However if you fiddle with the low pressure switch too much, it may destroy parts of the system (freezing up etc).

If you want to be on the safe side, up that vent temperature a bit.

[David T]

I agree with the other comments.

Usually temperatures between 41° and 43° F should be good (in a perfect system) at the vents.
However if you fiddle with the low pressure switch too much, it may destroy parts of the system (freezing up etc).

If you want to be on the safe side, up that vent temperature a bit.


The compressor needs to cycle for a couple of reasons. You don't want the cooling fans to run continuously. you do not want the evap coil to freeze. Running the cooling fans continuously will cause a melt-down in the electrical system. Freezing the evap coil will cause the high side pressures to skyrocket and between the liquid getting to the compressor and the excessive high side pressures you can damage the compressor and hoses. Hopefully the high pressure switch will shut things down before they get that bad. Readjust the low pressure switch so it cycles (this assumes you have the correct amount of refrigerant).
David Teitelbaum

[Bitsyncmaster]

It pretty normal that at idle the compressor stays on since it can't cool enough to switch the low pressure switch. So in my opinion running the fans running all the time is not what burns up some electical systems. Some people put 15 amp fuses in their fan fail jumpers with OEM fans. That will cause the fuses to get to hot, use 20 amp.

I don't think freezing the evap will cause damage but on a long trip you will not get much cooling and you will probably get water dripping out on you passengers feet. You do risk returning some liquid freon but I've seen many AC systems get that fog out the vents and never damage to the system.

[delornut]

The compressor needs to cycle for a couple of reasons. You don't want the cooling fans to run continuously. you do not want the evap coil to freeze. Running the cooling fans continuously will cause a melt-down in the electrical system. Freezing the evap coil will cause the high side pressures to skyrocket and between the liquid getting to the compressor and the excessive high side pressures you can damage the compressor and hoses. Hopefully the high pressure switch will shut things down before they get that bad. Readjust the low pressure switch so it cycles (this assumes you have the correct amount of refrigerant).
David Teitelbaum

I rewired my fan system using two relays, one for each fan. I also modified things so the fans run continuously while the air conditioning is on. I've had it like this for many years and miles and never had a melt down. I've never liked having the fans kick on and off with the compressor. I think continual running while having the load divided by two relays is easier on things than dumping a load on and off every time the compressor cycles.

Bruce Benson

[kings1527]

Thanks for all the input, everyone. I have a better understanding of this now.

Hi Bruce

So I was under the impression that on MAX AC, the fans will run continuously (to provide maximum heat exchange at the condenser, which makes sense) but on NORM the fans should cycle on/off with the compressor. Is that not right? My fans stay on all the time with MAX.

I'll quit messing around with my system and hook up my gauges and adjust my system to make sure my LP switch and compressor are in a healthy relationship.

[Ron]

I don't think freezing the evap will cause damage but on a long trip you will not get much cooling and you will probably get water dripping out on you passengers feet. You do risk returning some liquid freon but I've seen many AC systems get that fog out the vents and never damage to the system.Dave,
What you were seeing is probably where it is barely keeping up shedding the frost, so to speak. If the output was even colder, ambient goes low and/or humidity high and it freezes over, it will toast the compressor. It'a a very common problem (even with home and industrial systems. (Do a quick google)

[delornut]

Thanks for all the input, everyone. I have a better understanding of this now.

Hi Bruce

So I was under the impression that on MAX AC, the fans will run continuously (to provide maximum heat exchange at the condenser, which makes sense) but on NORM the fans should cycle on/off with the compressor. Is that not right? My fans stay on all the time with MAX.

I'll quit messing around with my system and hook up my gauges and adjust my system to make sure my LP switch and compressor are in a healthy relationship.

Hi Alex,

I'm by no means an expert on the AC but I've always been under the impression that the max setting bypassed the heater control valve and stopped hot water from moving through the heater core. I've always thought the fan cycling followed the compressor cycling no matter the mode setting. The mode switch is pneumatic and isn't in the fan electrical circuit. I've had my fans running steady with the compressor on for so long my memory will probably be proved wrong.

Bruce Benson

[David T]

Hi Alex,

I'm by no means an expert on the AC but I've always been under the impression that the max setting bypassed the heater control valve and stopped hot water from moving through the heater core. I've always thought the fan cycling followed the compressor cycling no matter the mode setting. The mode switch is pneumatic and isn't in the fan electrical circuit. I've had my fans running steady with the compressor on for so long my memory will probably be proved wrong.

Bruce Benson

In MAX the heater valve closes and the recirc flap moves to recirculate. The Mode switch is pneumatic AND electrical. It has not control over the cooling fans, that is off the compressor circuit. When the compressor clutch is energized the cooling fans come on (or when the otterstadt switch closes). The cycling is controlled by the high and low pressure switches. Cycling the compressor is essentially a "defrost" mode for the evap coil. Keeps the coil from freezing and thaws out any accumulated ice. Most mobile A/C systems do it with a complicated set of valves (only the Delorean has this cycling designed in). What makes it complicated as opposed to a system like a window unit is the varying speed of the compressor. You don't have to contend with that when the compressor is spun by an electric motor. With R-12 the A/C has enough excess capacity that they could do this cycling trick and still have enough cooling. One reason you can still get by with -134 even though it has 10% less capacity. On any other car if you see the compressor cycle it means you don't have enough refrigerant. Not so with a Delorean! Some newer cars will cycle the compressor on a thermostat but it is not common. The other time you might see a compressor cycle is on some cars with a WOT switch. To get a little more power they will shut the compressor when you floor the gas pedal (WOT).
David Teitlebaum

[David T]

Hi Alex,

I'm by no means an expert on the AC but I've always been under the impression that the max setting bypassed the heater control valve and stopped hot water from moving through the heater core. I've always thought the fan cycling followed the compressor cycling no matter the mode setting. The mode switch is pneumatic and isn't in the fan electrical circuit. I've had my fans running steady with the compressor on for so long my memory will probably be proved wrong.

Bruce Benson

Most A/C shops are not familiar with a Delorean and will charge the system till they see the compressor stop cycling. In most cases they are not far off since it shouldn't cycle all that much at idle anyway. This works as long as the low pressure switch is set correctly. Many shops do not measure in by weight. Most A/C systems can tolerate a wide range of fill, some can't.
David Teitelbaum

[David T]

Hi Alex,

I'm by no means an expert on the AC but I've always been under the impression that the max setting bypassed the heater control valve and stopped hot water from moving through the heater core. I've always thought the fan cycling followed the compressor cycling no matter the mode setting. The mode switch is pneumatic and isn't in the fan electrical circuit. I've had my fans running steady with the compressor on for so long my memory will probably be proved wrong.

Bruce Benson

Most A/C shops are not familiar with a Delorean and will charge the system till they see the compressor stop cycling. In most cases they are not far off since it shouldn't cycle all that much at idle anyway. This works as long as the low pressure switch is set correctly. Many shops do not measure in by weight. Most A/C systems can tolerate a wide range of fill, some can't.
David Teitelbaum

[Bitsyncmaster]

If an AC shop is not charging by weight.....Find another shop. That is the only way to charge an empty system. Now if you just need a top up, then most experienced personnel can do that and get pretty accurate results.

[Farrar]

I rewired my fan system using two relays, one for each fan. I also modified things so the fans run continuously while the air conditioning is on.

Great idea! I've already done the first half of that... much more secure having each fan on its own relay & CB.

[dmc6960]

:frantic: I need to speak of a couple thing before my head explodes...

Turning up the the temperature knob on the HVAC controls will do NOTHING for preventing your evaporator from freezing over. The heater core is downstream of the evaporator. Also, "freezing" the evaporator does not actually freeze the inside of it, I was getting worried some people may have thought so. What happens is the moisture in the air passing over the evap will condense into water droplets, then if the evaporator is cold enough it will freeze to it. When this happens, the process quickly gets out of hand because for every bit of coil thats iced over, its not longer absorbing heat from the airflow, causing the rest of it to stay as cold or get even colder (depending on the low pressure cutout point) and quickly result in a completely iced over evaporator that cannot flow any air.

The low pressure cutoff performs several functions. First and foremost, is prevents the low pressure from dropping too low. It must be set based off of the low pressure gauge. Where it cycles on a cold day is NO DIFFERENT from where it will cycle on a hot day. That is a fixed PSI. What will vary between hot and cold days is if the system has enough capacity to drop the pressure low enough to cycle. On a HOT day, sitting at idle, with the blower on a high speed, the system is very unlikely to bring the pressure low enough to cycle.

Yes, setting the low pressure cutoff lower, will bring the evap temp down colder. But only when the system is able to lower the pressure that low. The low point is a delicate balance between achieving the coldest temp possible without causing freezing on the outside of the evaporator, and keeping a safe low return pressure for the compressor.

The factory designed wiring will cycle the cooling fans on and off WITH the compressor. THERE IS NOTHING WRONG with modifying that wiring to keep the fans on at all times when the mode switch is in an AC mode. Running continuous cooling fans WILL NOT MELT YOUR CAR. Make sure all the weak points are taken care of and you'll be fine.

"MAX" AC will engage the recirculation flap, and cutoff the hot water flow into the heater core.

The "Cycling Clutch Fixed Orifice Tube" automotive AC system is an extremely common system and any AC shop that knows anything about automotive AC will understand how it works. If you get someone who "charges until the compressor stops cycling" then they are a frakking idiot and should be punched.

Finally, there is nothing wrong when your getting fog coming out of your vents (unless of course your evaporator just blew out, then avoid it at all costs!). The fog is just the really cold vent air clashing with the hot air in the cabin causing its own weather. Fog or clouds anyone? Just watch out for an occasional thunderstorm or tornado.

[DMCMW Dave]

:frantic: I need to speak of a couple thing before my head explodes...

Turning up the the temperature knob on the HVAC controls will do NOTHING for preventing your evaporator from freezing over. The heater core is downstream of the evaporator. Also, "freezing" the evaporator does not actually freeze the inside of it, I was getting worried some people may have thought so. What happens is the moisture in the air passing over the evap will condense into water droplets, then if the evaporator is cold enough it will freeze to it. When this happens, the process quickly gets out of hand because for every bit of coil thats iced over, its not longer absorbing heat from the airflow, causing the rest of it to stay as cold or get even colder (depending on the low pressure cutout point) and quickly result in a completely iced over evaporator that cannot flow any air.

The low pressure cutoff performs several functions. First and foremost, is prevents the low pressure from dropping too low. It must be set based off of the low pressure gauge. Where it cycles on a cold day is NO DIFFERENT from where it will cycle on a hot day. That is a fixed PSI. What will vary between hot and cold days is if the system has enough capacity to drop the pressure low enough to cycle. On a HOT day, sitting at idle, with the blower on a high speed, the system is very unlikely to bring the pressure low enough to cycle.

Yes, setting the low pressure cutoff lower, will bring the evap temp down colder. But only when the system is able to lower the pressure that low. The low point is a delicate balance between achieving the coldest temp possible without causing freezing on the outside of the evaporator, and keeping a safe low return pressure for the compressor.

The factory designed wiring will cycle the cooling fans on and off WITH the compressor. THERE IS NOTHING WRONG with modifying that wiring to keep the fans on at all times when the mode switch is in an AC mode. Running continuous cooling fans WILL NOT MELT YOUR CAR. Make sure all the weak points are taken care of and you'll be fine.

"MAX" AC will engage the recirculation flap, and cutoff the hot water flow into the heater core.

The "Cycling Clutch Fixed Orifice Tube" automotive AC system is an extremely common system and any AC shop that knows anything about automotive AC will understand how it works. If you get someone who "charges until the compressor stops cycling" then they are a frakking idiot and should be punched.

Finally, there is nothing wrong when your getting fog coming out of your vents (unless of course your evaporator just blew out, then avoid it at all costs!). The fog is just the really cold vent air clashing with the hot air in the cabin causing its own weather. Fog or clouds anyone? Just watch out for an occasional thunderstorm or tornado.

Best set of explanations so far. Thank you Jim, my head was close to exploding as well and you saved me a lot of typing. Like Bruce I also have my fans wired to run ALL THE TIME with the mode switch in A/C and nothing has melted or otherwise caught fire. If the electrics are maintained and not corroded, they will handle the current just fine.

[kings1527]

Hi Alex,

I'm by no means an expert on the AC but I've always been under the impression that the max setting bypassed the heater control valve and stopped hot water from moving through the heater core. I've always thought the fan cycling followed the compressor cycling no matter the mode setting. The mode switch is pneumatic and isn't in the fan electrical circuit. I've had my fans running steady with the compressor on for so long my memory will probably be proved wrong.

Bruce Benson

Thanks Bruce. My fans cycle with the compressor on NORM but run continuously as soon as I switch on MAX. I'm guessing my system was modified somewhere down the line. I'll figure it out and switch it back so my system won't melt. Just kidding, Jim. :wink:


If an AC shop is not charging by weight.....Find another shop. That is the only way to charge an empty system. Now if you just need a top up, then most experienced personnel can do that and get pretty accurate results.

I completely agree. If the system is specified for a particular weight, it seems to me that the most accurate way of filling to fill by weight. Shocking concept.


:frantic: I need to speak of a couple thing before my head explodes...

Turning up the the temperature knob on the HVAC controls will do NOTHING for preventing your evaporator from freezing over. The heater core is downstream of the evaporator. Also, "freezing" the evaporator does not actually freeze the inside of it, I was getting worried some people may have thought so. What happens is the moisture in the air passing over the evap will condense into water droplets, then if the evaporator is cold enough it will freeze to it. When this happens, the process quickly gets out of hand because for every bit of coil thats iced over, its not longer absorbing heat from the airflow, causing the rest of it to stay as cold or get even colder (depending on the low pressure cutout point) and quickly result in a completely iced over evaporator that cannot flow any air.

Ok, thanks Jim. I thought introducing warmer air would reach the evaporator but you're right. Anatomically, they're apart from each other when I think about it.



Yes, setting the low pressure cutoff lower, will bring the evap temp down colder. But only when the system is able to lower the pressure that low. The low point is a delicate balance between achieving the coldest temp possible without causing freezing on the outside of the evaporator, and keeping a safe low return pressure for the compressor.

That's the point I'm trying to get to. And like you said, I have to go off of the low pressure gauge. The DeLorean Tech Manual shows the low pressure switch should cut out at 20-28psi and cut in at 41-51psi.


The factory designed wiring will cycle the cooling fans on and off WITH the compressor. THERE IS NOTHING WRONG with modifying that wiring to keep the fans on at all times when the mode switch is in an AC mode. Running continuous cooling fans WILL NOT MELT YOUR CAR. Make sure all the weak points are taken care of and you'll be fine.

It looks like mine's been modified to run fans constantly on MAX. It makes sense that they would only need to go on when the compressor cycles since that's when refrigerant will be moving. But I'll keep mine they way it is, even if that means my system might melt down. :deviltail:




The "Cycling Clutch Fixed Orifice Tube" automotive AC system is an extremely common system and any AC shop that knows anything about automotive AC will understand how it works. If you get someone who "charges until the compressor stops cycling" then they are a frakking idiot and should be punched.

Agree with that.



Finally, there is nothing wrong when your getting fog coming out of your vents (unless of course your evaporator just blew out, then avoid it at all costs!). The fog is just the really cold vent air clashing with the hot air in the cabin causing its own weather. Fog or clouds anyone? Just watch out for an occasional thunderstorm or tornado.

That's what I was thinking. It's hot/cold air mixing and causing fog. Unremarkable. And thanks for clarifying exactly what a 'freezing' evaporator means. I was under the impression that there was internal freezing occurring. And I'm guessing the only real reliable ways of telling if your evap is freezing is by reduced airflow and warmer temp? Coupled by things going back to normal if you shut it down.

I'll hook my gauge set up and make sure my LP switch is set between the specs listed in the Tech Manual. And just so everyone knows, I came across some incorrect info out there but I can't remember where I read it. As far as adjusting the LP switch goes, counterclockwise will dial the low side down to a lower/colder pressure. Not clockwise.

Thanks again! And no exploding heads allowed!

[Ron]

:thumbup: Jim

[kings1527]

So I adjusted my LP switch today and it was very easy. I actually video recorded everything and I'll include it in my write up, which I'm almost done with. In short, turning my adjustment screw about 3/4 of a turn clockwise increased the cut out pressure to where it needed to be.

Originally, my compressor was cutting out at about 13-14 psi if I remember right. I'll have to look at the video but it was somewhere right around there. Now I have it adjusted right on the low side of the spectrum, per the technical manual. My low side cuts out at 19 psi (manual states 20-28 psi) and cuts in at 41 psi (manual states 41-51 psi). With an ambient of about 75-80 while driving home tonight, I was getting vent temps of 39-40 degrees on MAX AC with fan speed 4. Of course, I could get that a bit lower if I drop the fan speed.

So I think that's about it with that. I'm sure this setting is much better for my system's health. Thanks again for all the help!

[Farrar]

I've had my fans running steady with the compressor on


I also have my fans wired to run ALL THE TIME with the mode switch in A/C

What's the secret, gentlemen?

[Ron]

Move the Pink Wire going from the Low Pressure Switch to the Diodes over to the Pink/Orange Wire on the other side of the Low Pressure Switch.

[Farrar]

Move the Pink Wire going from the Low Pressure Switch to the Diodes over to the Pink/Orange Wire on the other side of the Low Pressure Switch.

Thanks, Ron! I'm still teaching myself electronics... so I appreciate the help. :) So in other words, you're bypassing the low pressure switch?

[Ron]

Exactly (WRT to the fans ;-)

[DMC5180]

Or you could get Dave M's Digital FAN FAIL Module and FAN RELAY. It has a built in Fan sequencer and shut down timer set at 20 seconds (after the A/C clutch powers off) which means under normal A/C cycling the fans never shut down unless you turn the A/C off or turn the ignition off. If the A/C is on at the time of shutting the car off, the Fans WILL continue running 20 seconds until they time out.

[Farrar]

Did ANYONE ever TELL you that you put CAPITALS in the ODDEST of PLACES?