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Ok so i am posting this in case anyone else around the country has this issue because it literally took me and my Honda dealership 3 days to properly diagnose and if it saves someone the hassle then i did my job.


We had a customer come in to the dealership with a 2009 ( my guess is it can be any 2nd gen Pilot with front & rear ac ) saying that the front vents blow cool but the rear vents blow cold when a/c is on.

I tested the a/c system by normal protocol standards by selecting a/c on, recirc. mode fan at the lowest setting & during this inspection i noticed that the compressor was staying on & not cycling on & off as it should, ok so i assume it has low refrigerant so i grab the ac machine & recover the freon & i noticed it is over charged ( .73 kgs came out of the system , spec is .65 kgs with front & rear ac ) so i think hhhmm strange it has too much freon but the compressor is staying on , ok so i evacuate & place the system in a deep vacuum to remove any moisture from the system & recharge to spec.



So after i recharge to spec i notice that the low side pressure begins at 10 psi ( too low in a normal operating ac system ) and begins to drop to vacuum ( negative pressure ) at idle, so in my experience this is a sign of restriction somewhere in the lines, so the usual suspect in this type of situation is the expansion valve is either clogged with debris or the desiccant bag has broken .



So i remove the desiccant bag from the condensor & i see it has not been torn but i replace anyway since i had one on hand, i reinstall & recharge to spec & still have the same problem, this time i notice at the Y pipe on the firewall side that the circuit that goes to the rear evaporator is freezing up or icing but the circuit that goes to the dash evaporator is still warm.



Ok now this is strange, so i replace the evaporator & expansion valve and recharge to spec & guess what ?


Low side pressure is still starting at 10 psi but drops to 0 now and vent temp is blowing cool but not cold while rear blows ice cold .


After several calls to Honda tech line going back and forth getting temp readings and HVAC electrical readings and pressure testing we found that the restriction is somewhere between the firewall and the rear evaporator, so once it was determined that the front evap temp sensor is not getting the cold temp reading thus not shutting off the compressor the freon is being stopped by the rear evaporator expansion valve .

So they have me replace the rear evap expansion valve and once i finished recharging to spec, low and behold the front vents start to blow ice cold air !!


After checking the low side pressure at idle after replacing the rear expansion valve it is now reading 40 psi at idle .


After recharging to spec i re tested the ac system and found now the compressor is cycling on & off and the front & rear are both blowing ice cold air .


How the system works is the freon was going through the system & being blocked by the rear expansion valve, so the front evap temp sensor was not getting the " cold " signal thus leaving the compressor on all the time to attempt to get the front air cold , this is why the rear a/c line was freezing because the front evap temp sensor was never getting the cold air because if it did the front evap temp sensor would send a signal to the PCM so it can shut off the compressor




Sharing this information to the 2nd gen Piloteers in case anyone with front & rear ac has a problem with their ac system not cooling in front but cooling in the rear .


BTW the rear expansion valve looks like it is made of brass connected to aluminum hoses
 

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Banned from wife’s 2005 Pilot LX
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Thanks for the detailed write up. I’m sure it will help someone down the line.
 

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Hi, I think I’m having the same problem. I have rear cold ac but not the front, just warm air. The pipes that go to the rear are cold but it’s weird that the line that branches off to the front is warm, I thought the expansion valve at the front evaporator might be clogged but from what you’re saying the rear expansion valve could be bad.
Is the rear valve hard to get to?
Thanks
 

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Brian, I had the same problem you did last year..... Exact same...... Expansion valve replacement did the trick, no problems since.

Don't forget the rear AC is pretty much completely separate from the the front AC it has it's own system contained under the console.
 

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137460
1593216988869.jpeg

System Description
The air conditioning system removes heat from the passenger compartment by transferring heat from the ambient air to the evaporator. The evaporator cools the air with the refrigerant that is circulating through the evaporator. The refrigerant expands in the evaporator, and the evaporator becomes very cold and absorbs the heat from the ambient air. The blower fan pushes air across the evaporator where the heat is absorbed, and then it blows the cool air into the passenger compartment.
• •

• •

Do not mix refrigerants CFC-12 (R-12) and HFC-134a (R-134a). They are not compatible.
Use only the recommended polyalkyleneglycol (PAG) refrigerant oil (DENSO ND-OIL 8) designed for the R-134a A/C compressor. Intermixing the recommended (PAG) refrigerant oil with any other refrigerant oil will result in A/C compressor failure.
All A/C system parts (A/C compressor, discharge line, suction line, evaporator, A/C condenser, receiver/dryer, expansion valve, O-rings for joints) are designed for refrigerant R-134a. Do not exchange with R-12 parts.
Use a halogen gas leak detector designed for refrigerant R-134a.
R-12 and R-134a refrigerant servicing equipment are not interchangeable. Use only a recovery/recycling/charging station that is U.L.-listed and is certified to meet the requirements of SAE J2210 to service the R-134a air conditioning systems.
Always recover refrigerant R-134a with an approved recovery/recycling/charging station before disconnecting any
A/C fitting.
EXPANSION VALVE
(Meters the required amount of refrigerant into the evaporator)
A/C CONDENSER (Radiation of heat)
This vehicle uses HFC-134a (R-134a) refrigerant, which does not contain chlorofluorocarbons. Pay attention to the following service items:
FRONT BLOWER FAN
FRONT EVAPORATOR (Absorption of heat)
FRONT EVAPORATOR TEMPERATURE SENSOR
137459
 

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Thanks! Unfortunately my wife’s Pilot broke down again. This time it's the head gasket, not sure what i;ll do with the truck yet.
 

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Sorry to hear that... Sometimes you have to call it quits on some vehicles when the costs to keep it going do not make sense. A gasket is cheap but depending on the damage or possible labor costs it may not be worth the time. How many miles on it? Good Luck..

 

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187, 000 on it. As long as I have somewhere to store it, I’ll keep it and fix someday, like a project. I’m thinking a used engine may be easier than messing with heads. My mom bought us the car so it has some sentimental value for me.
Thanks,
Brian
 
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