Switching AUX fan with Pressure switch, Why? (1 Viewer)

[KliersLC]

So,

Before I installed my aux fan, I did some research regarding switching, including reading the ABCs threads, one of which details hooking up the fan to be operated by the existing pressure switch. Here:https://forum.ih8mud.com/showthread.php?t=103530&highlight=abcs+cooling


So anyway, I wondered to myself, what do the existing two leads on the pressure switch do? And then I thought, well, they probably tell when the pressure is too low, and engage the compressor. So, then I wondered what the point was in hooking up a set of wires to a pressure switch that will turn the fan on when the compressor comes on, when you can just use the power wire from the compressor?

Same effect, same pressure switch.

One reason mentioned was that the fan could operate after the rig was turned off, but on really hot days in parking lots, it had the potential to run constant.


So, am I way off base here or is the new pressure switch wiring redundant?

Awaiting Enlightenment,

Dan

 

[Cruiser804]

https://forum.ih8mud.com/showthread.php?t=101703&highlight=abc's
https://forum.ih8mud.com/showthread.php?t=103188&highlight=abc's

Read the 2 earlier post by SUMOTOY. He runs down the pros and cons of 3 different set up.

 

[landtank]

I'm funny this way, but I'd never wire in an automatic circuit that had the potential of leaving me stranded. Especially when that situation would be a very hot one.

The general idea is a good one but you need a timer on there to ensure that the circuit shuts off at some point.

 

[KliersLC]

https://forum.ih8mud.com/showthread.php?t=101703&highlight=abc's
https://forum.ih8mud.com/showthread.php?t=103188&highlight=abc's

Read the 2 earlier post by SUMOTOY. He runs down the pros and cons of 3 different set up.

He doesn't explain how the one temp sensor activates the compressor and the fan at different times.

My set up is a combination of temp and compressor activation. I am just wondering if the operation would be any different if I wired it to the pressure switch.

 

[-Spike-]

It's a system that is above and beyond what was originally sufficient. IMHO, if the fan is wired to come on with the A/C, it will do exactly what you intended it to do- help cool you and the truck on hot days. It's fun to design a bunch of extra capabilities into something like this, but I've got better things to spend my efforts on.

-Spike

 

[97 FZJ80]

If you follow Scott's post and wired the aux fan to the med pressure switch, it will allow the fan to continue to run, even when the AC is cutoff due to overpressure (presumably from overheating) of the AC condensor. This is a very useful in the situation where the AC has been cutoff when sitting in traffic, due to the excessive heat from the weather, engine, and low vehicle speed.

If you do not want the fan to operate after the vehicle has been turned off, just wire the power relay to a ignition source (or triggered only by ignition source).

 

[SUMMIT CRUISERS]

If you follow Scott's post and wired the aux fan to the med pressure switch, it will allow the fan to continue to run, even when the AC is cutoff due to overpressure (presumably from overheating) of the AC condensor. This is a very useful in the situation where the AC has been cutoff when sitting in traffic, due to the excessive heat from the weather, engine, and low vehicle speed.

If you do not want the fan to operate after the vehicle has been turned off, just wire the power relay to a ignition source (or triggered only by ignition source).

Where is the med pressure switch located?

 

[97 FZJ80]

Where is the med pressure switch located?

It's in the first post referenced by KliersLC

 

[Cruiser804]

He doesn't explain how the one temp sensor activates the compressor and the fan at different times.

Not really sure I understand the question. From reading the post you reference the fan is triggered buy the line pressure at the switch he mentions.

If you are standing on the DS facing your rig look in the engine bay behind the DS headlight. You will see a AC tube fastened there with a inline switch. It is easily removed. You have to look back like you are trying to check your zipper.

 

[KliersLC]

If you follow Scott's post and wired the aux fan to the med pressure switch, it will allow the fan to continue to run, even when the AC is cutoff due to overpressure (presumably from overheating) of the AC condensor. This is a very useful in the situation where the AC has been cutoff when sitting in traffic, due to the excessive heat from the weather, engine, and low vehicle speed.

If you do not want the fan to operate after the vehicle has been turned off, just wire the power relay to a ignition source (or triggered only by ignition source).

I understand the potential wiring options with the switched power Vs constant. I don't think my AC has ever been cut off due to overheating, but I suppose that would be an advantage over a simple compressor hookup. OTOH, my system has a temp sensor and a compressor hookup, so in any condition where the Compressor would get shut off due to heat, the temp sensor would be activated. I am just trying to figure out if there are any advantages to this when used with my current set up.

He doesn't explain how the one temp sensor activates the compressor and the fan at different times.

Not really sure I understand the question. From reading the post you reference the fan is triggered buy the line pressure at the switch he mentions.

If you are standing on the DS facing your rig look in the engine bay behind the DS headlight. You will see a AC tube fastened there with a inline switch. It is easily removed. You have to look back like you are trying to check your zipper.

I found the sensor. My question is regarding the fact that one switch is activating two different circuits at the same time, so how is that any different than tapping into the existing circuit? OR, Under normal operating conditions, would the AUX fan operation be any different when hooked up to the pressure switch itself, as opposed to being hooked up to the compressor which is also activated by the pressure switch?

 

[KliersLC]

If you follow Scott's post and wired the aux fan to the med pressure switch, it will allow the fan to continue to run, even when the AC is cutoff due to overpressure (presumably from overheating) of the AC condensor. This is a very useful in the situation where the AC has been cutoff when sitting in traffic, due to the excessive heat from the weather, engine, and low vehicle speed.

If you do not want the fan to operate after the vehicle has been turned off, just wire the power relay to a ignition source (or triggered only by ignition source).

I understand the potential wiring options with the switched power Vs constant. I don't think my AC has ever been cut off due to overheating, but I suppose that would be an advantage over a simple compressor hookup. OTOH, my system has a temp sensor and a compressor hookup, so in any condition where the Compressor would get shut off due to heat, the temp sensor would be activated. I am just trying to figure out if there are any advantages to this when used with my current set up.

He doesn't explain how the one temp sensor activates the compressor and the fan at different times.

Not really sure I understand the question. From reading the post you reference the fan is triggered buy the line pressure at the switch he mentions.

If you are standing on the DS facing your rig look in the engine bay behind the DS headlight. You will see a AC tube fastened there with a inline switch. It is easily removed. You have to look back like you are trying to check your zipper.

I found the sensor. My question is regarding the fact that one switch is activating two different circuits at the same time, so how is that any different than tapping into the existing circuit? OR, Under normal operating conditions, would the AUX fan operation be any different when hooked up to the pressure switch itself, as opposed to being hooked up to the compressor which is also activated by the pressure switch?

 

[landtank]

His big advantage is that if you were to shut off the vehicle and the condenser was above 156 the aux fan would continue to run and further cool the condenser until it was 156. That way when you come out of the store from buying beer on that hot day the AC would be colder when you start up the truck. But I'd imagine it would return to where it was when you shut off the truck rather quickly.

 

[stlcruiser]

Greetings:

Using the pressure switch activation for the aux fan does not turn on the fan with the compressor. It only turns on the fan when the pressure reaches a predetermined number due to heat and low airflow conditions. This predetermined number is different than the presure at which the compressor is set to kick on. Proper presure management equates to a properly running AC system.

I have been using this set up since Scott posted it, and it has run flawlessly. The fan turns on and off as needed. The afterrun feature never runs for more that 30-45 seconds as the presure falls. If you have an aux battery, than I would suggest the power for the fan to be on this battery.

Steve
94' 175,000

 

[KliersLC]

Greetings:

Using the pressure switch activation for the aux fan does not turn on the fan with the compressor. It only turns on the fan when the pressure reaches a predetermined number due to heat and low airflow conditions. This predetermined number is different than the presure at which the compressor is set to kick on. Proper presure management equates to a properly running AC system.

I have been using this set up since Scott posted it, and it has run flawlessly. The fan turns on and off as needed. The afterrun feature never runs for more that 30-45 seconds as the presure falls. If you have an aux battery, than I would suggest the power for the fan to be on this battery.

Steve
94' 175,000

This makes no sense. Per Scott, the AC pressure switch is a ground circuit, which means that any system activated by said ground would activate at the same time. So, unless the compressor is not activated by the pressure switch, what you have described is impossible. The pressure switch closes the circuit at the same pressure for any circuit attached to it. You can't have one system operate at one pressure and the second operate at a different pressure if they are using the same switch.

Not really interested in the after run capability as I already have that with my temp sensor, and I don't want to risk the battery draw on a really hot day...

 

[stlcruiser]

Same presure switch has 3 circuits. Hi, Lo, and Medium. We are tapping into the unused one for aux fan activation. Maybe this is what is used for non US spec trucks with factory cooling fan. I don't know how it works, I just know it works. Maybe Sumo can chime in.

Steve

 

[SUMOTOY]

This makes no sense. Per Scott, the AC pressure switch is a ground circuit, which means that any system activated by said ground would activate at the same time. So, unless the compressor is not activated by the pressure switch, what you have described is impossible. The pressure switch closes the circuit at the same pressure for any circuit attached to it. You can't have one system operate at one pressure and the second operate at a different pressure if they are using the same switch.

Not really interested in the after run capability as I already have that with my temp sensor, and I don't want to risk the battery draw on a really hot day...

I thought that post was clear, and gave all the options and the benefits of using the pressure circuit

From memory... In my system design, the compressor can be on or off Kliers, and the aux fan could be running or not. The *key* element here is that if the compressor is shut off from A/C pump overheat (too high pressure in the system) OR from engine temp overheat, the fan will still run to cool down the condensor.

Overheat (press) the A/C circuit, or overheat engine, the compressor will shut down, as will the fan if you hook it up to the compressor. I can't think of a time when it's more important to have the fan *on* frankly. The med press switch runs the fan only and could care less what's happening to anything but condenser temp. This system has been used for years (with afterrun) in the audis.

The afterrun warnings are from those that haven't done the mod. You can put a timer on it if you so desire, but at 100 degrees in traffic and a hot shutdown, my fan ran ~3min max in afterrun. Rig it to the ignition if you are paranoid. This mod is better than a compressor on fan activation by definition. It runs simply based on A/C operating pressure, whatever the compressor decides to do or not do. Heat soaking a hot condensor on shutdown will and can spike A/C system temps and damage the compressor. I'd want afterrun however you hook it up.

The biggest benefit to this mod is that you will lengthen the service life of the A/C system. As another has pointed out, proper pressure = better A/C system. Dan, the medium circuit is the "normal' Pressure circuit, and is unused on 134a trucks. The high and the low are in series, and have nothing to do with the fan circuit. The biggest concept to keep in mind with A/C condensers Pressure is proportional to Temp (hi pressure = hi temp and vice versa).

I just ran to detroit over the weekend, and this mod worked flawlessly as it did when I put it in last year. I actually thought about adding a circuit that allows me to deactivate it during long highway trips (high airflow), but then still have the afterrun circuit on shutdown.

I have PM'd many on whatever specific questions they had on this mod (incl stlcruiser). I have heard of no issues with the mod itself, and from a design standpoint, it offers better protection than a compressor activated switch.

Edit: Kliers, reread the original posts. There are 3 circuits in the pressure switch 2 are in series for the compressor only, the medium you are hooking the aux fan to is alone and unused. There is high and low (interrupt on either too high or too low = compressor shut down) and medium (on above 193psi > off at 156psi regardless of the hi/lo compressor circuit operation). The low/high is a ground interrupt circuit - 2 pins. The medium (fan) circuit is a separate ground interrupt circuit - 2 pins.

HTH and my .o2

Scott Justusson
94 FZJ80 with Supercharger and Sumotoy Med Press Switch a/c fan mod

 

[KliersLC]

This mod is better than a compressor on fan activation by definition. It runs simply based on A/C operating pressure, whatever the compressor decides to do or not do. Heat soaking a hot condensor on shutdown will and can spike A/C system temps and damage the compressor. I'd want afterrun however you hook it up.

The biggest benefit to this mod is that you will lengthen the service life of the A/C system. As another has pointed out, proper pressure = better A/C system. Dan, the medium circuit is the "normal' Pressure circuit, and is unused on 134a trucks. The high and the low are in series, and have nothing to do with the fan circuit. The biggest concept to keep in mind with A/C condensers Pressure is proportional to Temp (hi pressure = hi temp and vice versa).

I just ran to detroit over the weekend, and this mod worked flawlessly as it did when I put it in last year. I actually thought about adding a circuit that allows me to deactivate it during long highway trips (high airflow), but then still have the afterrun circuit on shutdown.



94 FZJ80 with Supercharger and Sumotoy Med Press Switch a/c fan mod

OK,

So the pressure switch will activate the fan and the compressor at different times right?

If so, please explain.


I understand the AC cutoff and the after run. Not an issue as my system runs also off of a temp sensor.

My understanding (perhaps flawed) of a pressure switch, is that it has a little ball and a little spring that get depressed by the pressure in the system, and when depressed close or open contacts close in this case. How does this switch activate this at different times?


As far as the afterrun, you said that if the ambient temp was 116 deg F, the fan would run constant. Tools measure parking lot temps in AZ in the 120 range due to reflection from the pavement and high temps. Sounds like a recipe for a long hot walk home in that case.

You said above that

"It runs simply based on A/C operating pressure, whatever the compressor decides to do or not do."

Does the Compressor not run based on A/C Operating pressure?

Here is a link to my install so maybe you can understand where I am coming from.

https://forum.ih8mud.com/showthread.php?t=159699&highlight=aux+fan

Thanks,
Dan

 

[SUMOTOY]

OK,

So the pressure switch will activate the fan and the compressor at different times right?

If so, please explain.

No. The A/C compressor needs a non interupted ground from the hi/lo pressure switch, it doesn't activate anything, it deactivates the compressor under certain conditions. Low pressure means that the A/C compressor will overheat, high pressure means the A/C compressor will overheat. If either condition is present the compressor doesn't work. The compressor also shuts off if the engine temps exceed 226F. In *all* these conditions (whatever is the cause of them), the aux fan will continue to run to help cool the condenser if condenser temps exceed 115F.

I understand the AC cutoff and the after run. Not an issue as my system runs also off of a temp sensor.

In my post, I'm not convinced the aux fan does a lot for the radiator (maybe on shutdown). The aux fan is really to keep the condensor from overheat = overpressure.

My understanding (perhaps flawed) of a pressure switch, is that it has a little ball and a little spring that get depressed by the pressure in the system, and when depressed close or open contacts close in this case. How does this switch activate this at different times?

According to FSM, the the little toyota man in the med press swtich circuit hits 193psi>on and 156psi off. If you look in my original posts, 134a and r12 temps and pressures are proportionally related. That's how you can test a pressure switch by temp, it's Mr. T's diagnostic for that pressure switch.

As far as the afterrun, you said that if the ambient temp was 116 deg F, the fan would run constant. Tools measure parking lot temps in AZ in the 120 range due to reflection from the pavement and high temps. Sounds like a recipe for a long hot walk home in that case.

In the posts and the FSM, pressure switch activation is 193psi on, 156psi off. At 120F CONDENSOR TEMPS, the fan would be running.

You said above that

"It runs simply based on A/C operating pressure, whatever the compressor decides to do or not do."

Does the Compressor not run based on A/C Operating pressure?

Here is a link to my install so maybe you can understand where I am coming from.

https://forum.ih8mud.com/showthread.php?t=159699&highlight=aux+fan

Dan, I can only repeat what's in the post and the above. Pressure and Temp of 134a are directly related. If you read my original post, I give the 134a pressure at a given 134a temp. They are directly related, and IS the reason you have a hi pressure cutout, it means the temp *and* pressure of the 134a is getting dangerously high.

I read your install. I would be interested in how much the temp circuit in the radiator is really helping you. As I noted several times, I see this mod only as a way to help cool the condensor, not the radiator. Maybe on hot shutdown, but I doubt it cuz I have never seen the fan come back on after it cooled the condensor to 115f.

I also have hood vents installed, and although you can feel slight movement of air thru them on shutdown (aux fan running in afterrun), I doubt it's doing much with the radiator cooling.

The pressure switch mod allows progressive/independent startup and shutdown of the aux fan without the additional load of A/C compressor startup and shutdown cycles. I have never seen an aux fan hooked up to the compressor circuit. Audi/bmw/mercedes all use supplementary fan activation for A/C operation. They all are run off relays hooked to either the climate control (AC on, not 'compressor on') or to a pressure switch.

Your rig requires a manual override of the temp probe to assure no aux fan, my hookup only requires turning off the a/c. Your mod will run longer in 'after run' than a condenser cooling fan because the hookup I have only uses condenser temps with a shutoff temp of 115F.

Not sure we agree that the aux fan is 'helping' the radiator cooling, that was never my target. My target with the aux fan mod is *only* that the aux fan address the condenser temps.

HTH and my .02

Scott Justusson

 

[KliersLC]

Overheat (press) the A/C circuit, or overheat engine, the compressor will shut down, as will the fan if you hook it up to the compressor. I can't think of a time when it's more important to have the fan *on* frankly. The med press switch runs the fan only and could care less what's happening to anything but condenser temp. This system has been used for years (with afterrun) in the audis.

Here you say that the fan operation is desirable when the engine overheats, but below you say it won't do much for cooling, so what's the point. You cool your condenser after AC cut off, but when the condenser is cool, you lose the extra airflow for cooling the Rad.

No. The A/C compressor needs a non interupted ground from the hi/lo pressure switch, it doesn't activate anything, it deactivates the compressor under certain conditions. Low pressure means that the A/C compressor will overheat, high pressure means the A/C compressor will overheat. If either condition is present the compressor doesn't work. The compressor also shuts off if the engine temps exceed 226F. In *all* these conditions (whatever is the cause of them), the aux fan will continue to run to help cool the condenser if condenser temps exceed 156F.




In my post, I'm not convinced the aux fan does a lot for the radiator (maybe on shutdown). The aux fan is really to keep the condensor from overheat = overpressure.




In the posts and the FSM, pressure switch activation is 193F on, 156F off. At 120F CONDENSOR TEMPS, the fan would not be running.

Medtro:
The only way I could see this mod running forever is if the ambient temp is 116F.

I'm on several weeks with this mod in chicago, and even on the 105F days, it ran maybe 3 minutes max. What I've been seeing recently is about 30 second afterrun with temps 85-95F.

HTH

Scott Justusson
94 FZJ 80 Supercharged aux cooled/after run/ pressure switch mod

So, you said that 116 deg ambient would cause the fan to run constant. I noted that ambient in a parking lot in AZ is 120, and then you said that CONDENSER temps of 120 don't cause activation. I understand this, but the question was about ambient temps.
Source:
https://forum.ih8mud.com/showthread.php?t=103530&highlight=abcs+cooling

I read your install. I would be interested in how much the temp circuit in the radiator is really helping you. As I noted several times, I see this mod only as a way to help cool the condensor, not the radiator. Maybe on hot shutdown, but I doubt it cuz I have never seen the fan come back on after it cooled the condensor to 156F.

I also have hood vents installed, and although you can feel slight movement of air thru them on shutdown (aux fan running in afterrun), I doubt it's doing much with the radiator cooling.

The temp circuit serves the same purpose as the pressure circuit in your install, although the fan would be on constant long before the AC cut off temp was attained.

It is also useful for extra cooling on long pulls when I turn the AC off. I can adjust to activate at any pressure between 160 and 240, so length of after run is dependent on what I set it at + radiator temps.

The pressure switch mod allows progressive/independent startup and shutdown of the aux fan without the additional load of A/C compressor startup and shutdown cycles. I have never seen an aux fan hooked up to the compressor circuit.

Your rig requires a manual override of the temp probe to assure no aux fan, my hookup only requires turning off the a/c. Your mod will run longer in 'after run' than a condenser cooling fan because the hookup I have only uses condenser temps with a shutoff temp of 156F. Most radiator temp probes are still in after run at that temp

Not sure we agree that the aux fan is 'helping' the radiator cooling, that was never my target. My target with the aux fan mod is *only* that the aux fan address the condenser temps.

HTH and my .02

Scott Justusson

The AC performance was my target as well, but there are also fringe benefits to recognize. I *think* that I understand how the fan cycles on during normal operation at different times than the compressor, but still kinda foggy.

In the second quote you explain the function of the AC pressure switch in relation to the compressor. If that is the case, then when sitting at idle or going down the road, the compressor does not run constant--it cycles on and off. Is it being deactivated by the pressure switch or by something else?

Thanks again for taking the time to explain this,
Dan

 

[SUMOTOY]

Here you say that the fan operation is desirable when the engine overheats, but below you say it won't do much for cooling, so what's the point. You cool your condenser after AC cut off, but when the condenser is cool, you lose the extra airflow for cooling the Rad.

I don't think there is any/much extra Dan. A target of cooling the condenser is the goal of all factory and aftermarket installs IME. In the audi/bmw/merc/most usa, the aux cooling circuit is running *independent* of the radiator fan or thermostat or A/C compressor. It's just on when the A/C snowflake is pressed and/or the A/C refrig pressure is normal.

So, you said that 116 deg ambient would cause the fan to run constant. I noted that ambient in a parking lot in AZ is 120, and then you said that CONDENSER temps of 120 don't cause activation. I understand this, but the question was about ambient temps.
Source:
https://forum.ih8mud.com/showthread.php?t=103530&highlight=abcs+cooling

156psi condensor the fan is off. Ambient temps only play a role in the length of time in afterrun. I'll look thru the thread and see what you are referencing. Edit: Got it, yup you are correct, the 193psi and the 156psi are pressure readings, sorry (corrected posts above too). So it's on at 134a temps of 136F, off at 115F. I would test a 120ambient before calling this a constant run. Or, just use a ignition switch activation to the relay, or a time out after run relay. In fact Dan, you can use your temp probe in the condenser to do this. Set a probe to turn off at 136F and you are in the normal range of the pressure circuit.

The temp circuit serves the same purpose as the pressure circuit in your install, although the fan would be on constant long before the AC cut off temp was attained.

It would be on before and after longer. The temp circuit you put in is radiator temps. You can imply that rad temps will increase with condenser temps. I don't believe that's necessarily a given. You can have a faulty A/C compressor when it's 32degrees outside (defrost with A/C), and the condenser is overheating but the radiator has enough extra capacity that the aux fan isn't running.

It is also useful for extra cooling on long pulls when I turn the AC off. I can adjust to activate at any pressure between 160 and 240, so length of after run is dependent on what I set it at + radiator temps.

I don't think or believe that fan (and I have the exact same one you do by the looks of it) does much in 'extra' cooling of the radiator. Again, if that's your target, you won't get much agreement that it does much on this forum. If you have the ability to set it to 160, I'd put the probe in the condenser, but my mod simplifies those 2 circuits you made to be just 1. I don't care whether the A/C is on or off, I only care if the condenser is hot.

In the second quote you explain the function of the AC pressure switch in relation to the compressor. If that is the case, then when sitting at idle or going down the road, the compressor does not run constant--it cycles on and off. Is it being deactivated by the pressure switch or by something else?

That's the primary job of the thermistor and relay in the exchanger. It cycles to prevent snow buildup on the exchanger (you can hear the relay clicking below the glove box). If the exchanger gets snowed up, your pressure will rise, and the compressor will start to squeal (btdt), then the hi pressure cutout hits and the compressor will turn off.

HTH

Scott J