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  #1  
Old July 9th, 2008, 12:32 AM
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Gary Young
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AC Icing up

My D90 AC unit appears to be icing up when run for an extended time period, say 30-60 mionutes. Anyone else have this problem? Is it just due to me running it on max cool temp setting with minimum fan setting? Or is there some other problem this is representing?
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  #2  
Old July 9th, 2008, 02:53 AM
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I'd say your lucky.
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Old July 9th, 2008, 07:11 AM
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I'm having the same issue. it's not supposed to do that. there's an electrical switch that senses refrigerent pressure that is supposed to cycle the compressor clutch on and off to keep it from over freezing. lift your bonnet, run the ac on max, and check to see if the compressor is clicking on and off. mine isn't cycling, it's just stays on, and overfreezes. likely the cycling switch needs replacement.
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Old July 9th, 2008, 08:44 AM
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max cool and min fan = ice up. You need to push more air through it to keep it from icing up.
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Old July 9th, 2008, 10:00 AM
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Mine ices on max fan and min cool. The compressor clutch is supposed to cycle for proper operation.period.
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  #6  
Old July 9th, 2008, 11:42 AM
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Thanks for the tip on the compressor cycling. I will run it today and see what happens. Anyone know where the mystery pressure switch is located?
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Old July 9th, 2008, 11:56 AM
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The mystery pressure switch is located on top of the drier, bolted to the chassis rail. It is called a trinary, or tri-function switch, because it not only switches the system off when the pressure is to high, it switches it off when pressure is to low. Probably first want to check and make sure the wires are connected, then see if they are corroded, and then lastly replace the trinary switch. Dont forget to evacuate the system first.
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  #8  
Old July 9th, 2008, 12:01 PM
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Great info, Thanks! I will check mine today and see what's up.
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Old July 9th, 2008, 12:49 PM
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me too. thanks! I haven't been able to source a schematic for this ac system yet, and was trying to figure that location out.

Follow-up Post:

Quote:
Originally Posted by JimC
The mystery pressure switch is located on top of the drier, bolted to the chassis rail. It is called a trinary, or tri-function switch, because it not only switches the system off when the pressure is to high, it switches it off when pressure is to low. Probably first want to check and make sure the wires are connected, then see if they are corroded, and then lastly replace the trinary switch. Dont forget to evacuate the system first.
Jim, Afterthought. Given that our AC is freezing up, which means that the compressor clutch is constantly on, NOT OFF, it would seem to me that there is no issue with connection from the switch to the clutch. If there as a bad connection, the compressor would not come on, due to a lack of power from the switch. What are the 3 wires for? I'm assuming #1 is power in from the AC controls, #2 is switched power to the compressor clutch, so what is #3? If the 3rd connection has anything to do with turning off power to #2 switch, then maybe that's it, but I was assuming the pressure switch is mechanically controlled by pressure between #1 and #2.
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Old July 9th, 2008, 01:02 PM
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I cant say really since thinking about wiring diagrams makes my head hurt and all my reference material is in the states whereas I am not. So I can't look it up for you. I've never though about it much, but what would be the result if the temperature probe into the core was broken or not lodged securely enough?
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  #11  
Old July 9th, 2008, 01:04 PM
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What does the probe do if cycling is controlled by pressure, not temp? My assumption was that the probe controls the AC supplemental electric fan on the radiator, but I might be wrong.

Anybody, anybody? Bueller?
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  #12  
Old July 9th, 2008, 04:17 PM
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Now I'm talking out of my ass, but I think its a primitive logic circuit. I think the trinary controls the fan withing a certain pressure range, and the compressor within a larger range. Perhaps the temp probe feeds into that as well.

Follow-up Post:

Mark - check your earthlink account, you've got mail.
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Old July 9th, 2008, 11:10 PM
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Temp Probe into core??? Please enlighten me.
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Old July 10th, 2008, 12:56 AM
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The temp probe is behind the fascia, it runs from the switch area down into the condensor core. Its just a piece of copper or aluminum.
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Old July 10th, 2008, 01:01 AM
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Oh...I should have know that was it. Once I figure out if my compressor is cycling or not, I may have to check the probe for proper operation.

Quote:
Originally Posted by JimC
The temp probe is behind the fascia, it runs from the switch area down into the condensor core. Its just a piece of copper or aluminum.
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Old July 10th, 2008, 09:25 AM
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On my '93 D110 - if the AC is on then the condensor fans run also. The condensor fans can also run with the AC off but when the engine temp goes up and senses additional fan cooling is needed.

The AC circuit has two temp probes that I can see in the schematics. One is for the thermostat (to regulate your cooling based on what setting you are using) and the other is the Eng Temp 'probe' which will shut down your AC if it determines the engine is overheating.

I don't know of any other temp 'probes' related to the AC system but I am no expert here so.......

Gary - where do you see the icing (what pipes or components)?


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  #17  
Old July 10th, 2008, 05:09 PM
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The supply hose will ice up under the hood and the inside heat exchanger will ice over. I get a ton of water under the truck after I turn it off and the ice melts.
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Old July 10th, 2008, 05:26 PM
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In general AC coils (not just the ones in cars) ice up from one or more of the following reasons: 1.) insufficient airflow, 2.) low refrigerant pressure, 3.) low outside air temperature.

BTW, I've never had the coils on my D90 ice up.
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Old July 11th, 2008, 11:34 AM
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Talking

BIG UPDATE FOR ALL OF YOU! Before this first post, I emailed Reg Ford at CoolAir UK, LTD on this issue, asking for guidance. He's been out but finally got back and replied. For those of you who don't know, CoolAir UK is the manufacturer of the OEM underdash systems that Land Rover installs in their Defenders, to include the units that Rovers North sells now. It seems that Pendy was likely correct based on Reg's feedback, and I thought it would be good for posterity to post his diagnosis. Reg states:

"The problem you have is with the thermostat (could be called a frost
switch) not the pressure switch. There are really only two possibilities, the first being that the thermocouple (probe) may have dislodged from the evaporator coil which means it's not sensing air temperature and will allow the compressor to run continually and allow any water on the evaporator to freeze and effectively block airflow.
(It's the evaporator that freezes not the condenser)

The other possibility is that the electronic box has failed. This is a rare occurrence but possible.

Question for you, has the evaporator been removed or has anyone tampered with the electrics? The reason for asking, is that a feed in the wrong place may cause the control unit to fail.

My suggestion to correct the problem, is remove the evaporator assembly, check the probe position and if it's correct change the control box.
I'll e-mail a photograph to you so you can see what's involved.

A word of caution. If you run the ac without free airflow, the blower will over speed and may burn out. The compressor will be protected from overpressure by the pressure switch. However, if it's allowed to run continually the expansion valve will close and starve the compressor of oil and there is a risk of seizing. Until the problem is resolved, I recommend you turn on when required and leave running for no more than ten minutes before turning off and leaving it for a few minutes."

I'll post up the picture as well as my findings after finishing this issue, but glad I don't have to replace that $143 pressure switch!

Follow-up Post:

UPDATE. Reg followed up with pics and further guidance. Here it is.

"Further to my previous e-mail. I've attached two images. One showing the control box and the other the connector to the probe. Removing the probe is quite difficult as the unit requires dismantling. Therefore to test the probe, disconnect as shown and have your service shop check the resistance. This will vary dependent upon the temperature but with an ambient around 68 F you should have a reading circa 3.6 ohms with the meter on the 20k setting. (Your service tech should understand this) If you are able to pass warm air into the blower the reading will change in a uniform manner but I can't say if it will be a higher or lower reading unless we test one here. The important thing is a changing reading and not an erratic movement.
This will indicate if the probe is okay but not necessarily if in the right position.

Thinking about it, it might just be worthwhile changing the box and seeing if this solves the problem to avoid two visits to the repair shop. I would say though, that if any of the ac electrics have been touched you need to exercise caution as it will blow the new unit.

Rovers North will be able to supply you with a box.

Please let me know how it goes.

Thanks again

Reg Ford



PS The parts are visible when the evaporator has been dropped down."

Reg has been fantastic help and very responsive.
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  #20  
Old July 12th, 2008, 01:14 AM
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Thanks for the info! I will be testing mine as soon as I get my radiator back and installed.
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