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2003 MDX Touring w Nav. 245000 miles. Replaced the A/C compressor in engine compartment in May 2018 to fix whole car A/C, also replaced blown rear blower motor resistor to get rear air working again. Compressor was an OEM, rear resistor was a cheaper aftermarket Amazon purchase.

All A/C both front and rear was working fine including rear in manual mode, until week ago. It didnt blow super icy cold in front or rear before but was definitely well conditioned air and very acceptable. Now suddenly rear A/C is blowing only warm air. Front vents still blow perfectly cold air - now actually almost colder than when rear was also working. The rear vents will operate fine in manual mode - will increase fan speed or switch from top to floor vents etc. But I can only get warm air on coldest settings calling for A/C. Oddly, the rear vents will increase temp and blow hot air properly when temp is moved up to heat setting or the manual temp setting in rear is moved to red/hot. Again front A/C and everything seems to be fine otherwise - just now the rear A/C suddenly stopped blowing cold conditioned air.

Any thoughts on the issue? What should I check? What is likely culprit?

Do the 2003 models have separate compressor for the rear system? Or one compressor for whole system but separate evaporators?

Please help! We are in NC and the heat in back is causing all sorts of complaints from the family...thanks in advance!
 

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One compressor, two evaporators. I don't know if low refrigerant levels will cause one system to stop cooling first, but it would kind of make sense. The fact that the air wasn't all that cold after the repair might indicate low gas (or too much, for that matter). I'd say it's worth a quick check if you've got a set of A/C gauges (or even one of the cheapo auto parts store gauges that screw onto a can of R-134 - I've found that it's usually pretty obvious when the gas is low in the system using one of those, though I wouldn't trust them for any kind of serious troubleshooting effort.
 

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You could also check the ac clutch relay. With the engine running, ac off, then turn on the ac and check for a audible clunk and the engine slowing for a bit.

If there no noticeable change in engine speed or no clunk when the ac button is pressed on, the clutch relay is in the main fuse box inside the engine compartment and it’s labeled “mag clutch” or something like that. With engine off, key out, yiu can swap it with the same style one - usually the horn and headlamp uses the same relay. If it needs to be replaced, pep boys or rockauto has best price.

It’s probably Freon or relay. A long shot is that that is also a heater control valve in the engine compartment that directs hot coolant to the heater cores when that is engaged. The control wire can sometimes break or valve get stuck. This forum has more discussion about that and some pics.

Good luck!
 

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We know the A/C compressor clutch is good (since the front cools). Also, if the heater valve isn't closing properly, the front will get plenty warm (ask me how I know...). ;-)
 

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Thanks guys for the responses so far.

Yes, I belive the clutch relay is working. At least, I know that when I have the car running with AC off, and the AC gets turned on (front dash control), yes there is a noticable change in pitch and RPMs and I can hear the compressor (?) engaging. Also when doing that, I can see with flashlight the belt/pulley for AC clutch on the left side of the engine (facing from car front, in the bottom) engage and begin spinning. As habbyguy noted - I am getting cool conditioned air out of front vents fine.

— Where is the rear evaporator coil unit located? // I am thinking if that has gone bad or there is a problem there, how/where to replace it.

— And are you certain there is not a second compressor specifically for the rear AC system/evaporator?

As counter point self-analysis: if the rear AC evaporator is bad, meaning it is leaking refrigerant (?) would that not basically take down the whole AC system including the front vents? If its a single closed loop system with one compressor and one line/circuit of refrigerant being used (by both evaporators), I would think any leakage in that line would also impact the front AC just the same as in the rear.

— Is there any risk of air mix motor issue causing this lack of cooled conditioned air coming out of the back? I dont think so, given that in the rear in manual mode the air can be properly turned fully shut off, or can be redirected from top to floor vents on switch command. Also when in manual mode, the rear can put out noticably very hot air if the temp dial is turned towards red heat instead of blue cool AC. All indicating to me the mix motor is working fine..?

— Any chance the rear blower motor resistor is causing this somehow, causing the rear evaporator to not be engaged? I would not think so, if the resistor plug was broken/blown again somehow it would impact the fan operation, not air temp...eg the fan would blow out of rear vents only on one speed, or continuous, or not at all.

TIA guys...
 

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Yes, there is absolutely, positively only one A/C compressor. If you find a second one, let us know... ;-)

The rear evap is located in the big lump between the front seats. It wouldn't be a lot of fun to replace, but I doubt that's necessary. If it was leaking, you'd lose ALL A/C, not just rear.

It IS possible that an air mix motor would cause the air to not pass over the evap unit.

I'd recommend running this procedure - likely to tell you what you need to know:
 

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...I don't know if low refrigerant levels will cause one system to stop cooling first, but it would kind of make sense...
Just wanted to chime in and say that low refrigerant levels definitely can cause this. I had the same symptoms (cold in front, warm in back) and it turned out to be a leaky schrader valve. Replaced that and recharged the refrigerant and the A/C has been solid since. Start with the easy stuff.
 

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If its not Low Refrigerant, I say its the Blend Door Actuator. It is responsible on Air Temp Mixture(Hot, cold, or mid). I did couple of this on different vehicles. The motor inside the Actuator assembly is probably weak to open the door to cold side completey, thus allowing heat go through...
Check AC PRESSURES first(using AC MANIFOLD), can be rented(full refund) at autozone and alike.
 
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