A/C Vacuum Leak? Blend Doors?

[Ducky's Dad]

My '05 A/C is still not working right in hot weather. Problem started last August, and persists today. Basically, when the engine is under load (passing, climbing a hill, merging into traffic, etc.), the A/C starts blowing hot. Get into the throttle long enough and the A/C will blow hotter than outside ambient air temp. Here is a link to last year's thread:

viewtopic.php?f=2&t=5617&p=101064#p101064,

Refrigerant levels are fine, high side and low side pressures are fine, compressor runs fine in cool weather or when just cruising at moderate speed, already replaced the A/C fan and changed the shims in the A/C clutch. Installing a new pressure transducer this week, will probably add a test light to the dash to indicate when the compressor is running. May evacuate the system to check for metal bits or anything else that may be interfering with cooling. Talked to an oooooold mechanic yesterday and he thinks the compressor function is controlled by manifold vacuum, so that is consistent with A/C cutting out when throttle is opened and manifold pressure drops. So that means we need to start looking for vacuum leaks, but truck runs like a top and no other indication of vacuum leaks. Not throwing any codes, and truck going in for smog test end of this week.

Which brings me to the main question in this post:
These trucks are known to have blend door problems, and the blend doors are vacuum actuated. If the blend doors are causing my A/C problem, would I still be getting full air flow from the dash vents when the A/C starts blowing hot? Blend doors seem to be functioning OK, because I can switch from dash to floor to combined by moving the dash knob, and there is no apparent reduction in air volume when the A/C blows hot. When I say hot, I mean HOT. Worst was almost 104F at the vents last September, but I can get the A/C to blow hotter than outside air, even when set to max cold on the knobs. Anyone have direct knowledge of the blend door issues and whether they would cause my symptoms?

The other suggestion that the old mechanic made was to just power the A/C compressor directly from the battery, controlled by a switch on the dash. Might work as a stopgap in hot weather, which we now have.

Thanks all.

[trk4sale]

I thought it was to 06 trucks that had blend door problems

[cb1987]

i had a older dakota with vent door that had a mind of their own. when you get hard in the pedal going up hill the change in manifold pressure caused the vent doors to close then they would open when you let off the pedal. somewhere in the little vaccum lines there must of been a leak but i never bothered to track it down

[Ducky's Dad]

when you get hard in the pedal going up hill the change in manifold pressure caused the vent doors to close then they would open when you let off the pedal.

When this occurred, did the air volume at the dash vents change? When mine blows hot, I get the same volume of air but it's at a higher temperature.

[FirerescuePW]

'05 like yours.

I replaced my blend doors because they broke. Specifically, the Recirc door. I never had AC go completely hot. Sometimes it was warmer than it should be, but never as hot as you describe.

My memory tells me the door motors are electric. I think you need to determine if the compressor clutch is disengaging under acceleration.

Sent from my SM-G965U using Tapatalk

[Ducky's Dad]

My memory tells me the door motors are electric.

Your memory is correct. Figured that out yesterday, so have eliminated vacuum leak in blend doors as a possible cause. As of yesterday, my PW has a new Mopar A/C compressor and a new pressure transducer. Have not had it at highway speeds yet, but at idle and around town it is almost 10 degrees cooler at the vents. Have to drive 100 miles thru So Cal desert tomorrow afternoon and it's supposed to hit 100F, so will be able to test whether it warms up under engine load. Oil from old compressor came out clean, no visible shrapnel or residue, refrigerant was full, condenser passes air with no apparent restriction, blend doors still working fine. Went with a new compressor because clutch rebuild kit plus labor was almost as expensive as a complete compressor, and this eliminates one more variable. Fingers crossed for tomorrow.

[FirerescuePW]

This is a strange one, based on this and your other thread. I can't imagine you're too happy resorting to throwing dollars at it until it gives in.

I have a Tahoe that goes hot when sitting still, idling. Our shop is supposed to fix it this week. Quite cold while driving. I'm told they know what part it is this time. We'll see...

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[Ducky's Dad]

I can't imagine you're too happy resorting to throwing dollars at it until it gives in.

Not happy about it but resigned to it. I have too much money in this truck to give up on it, and I like the truck too much. I have even thought about skipping the factory A/C setup and installing a retro A/C from the street rod market, as if it were a PW from the late 40s.

In terms of just replacing parts, the pressure transducer would have been the next logical component, but I decided to throw a new compressor at it at the same time because I'm three weeks away from heading to southern AZ for a couple of weeks and running out of time to deal with the A/C. If the transducer by itself did not solve the problem, then I might not have time to do the compressor before departure. So I figured, what the hell, might as well give the old girl a new compressor.

[cb1987]

when you get hard in the pedal going up hill the change in manifold pressure caused the vent doors to close then they would open when you let off the pedal.

When this occurred, did the air volume at the dash vents change? When mine blows hot, I get the same volume of air but it's at a higher temperature.

no it would close off the doors under the dash. fan was still blowing however. once there was sufficient vacuum the door under the dash would open again. im sure my problem was just a simple leak in one of the small vacuum hoses. truck was nearly 25yr old and rubber pieces rot away.

[Ducky's Dad]

Drove the PW yesterday with new transducer and compressor and it was much better, but ambient was only low 90s. Heading out thru the Lucerne Valley tomorrow and it should be a lot hotter. Good news is that vent temps only varied a few degrees under engine load. Bad news is that A/C is not blowing as cold as it did before all this stuff started, but it's a whole lot better than it was last week.

[Ducky's Dad]

Got some highway drive time yesterday thru the Lucerne Valley, saw ambients from 102-111. A/C on recirc took a while to cool down at the vents but eventually settled in mid-high 40s at the vents. Pulling long hills or passing kicked vent temps up to 50+, but temp dropped back quickly when I got off the throttle. Can't know whether it was the pressure transducer or the compressor that solved the problem, but I'm glad to have functional A/C again. Truck will do a few thousand miles in southern AZ in a couple of weeks, so that will be the acid test.

[Retired BLM Rig]

[Ducky's Dad]

RetiredBLM, thanks for your input and advice during this process. Much appreciated.

[Retired BLM Rig]

[Ducky's Dad]

It's Baaaack!

Driving from Yuma to Temecula this past weekend, I was pulling the long grade from Plaster City up toward Jacumba on I-8. Rise is about 4,000 feet over 20-30 miles, but parts of it are steep so brief periods of full throttle operation. A/C set on max cold, recirc, blower on highest speed. Got to the top of the grade with outside air at 68F and dash vent temp at 73F, road speed above 70mph. As soon as I slowed down or road leveled out, A/C cooled right down. System has new compressor, pressure transducer, fan unit. All system pressures check out OK, refrigerant full. Same A/C performance as last year at this time, before it got the new compressor and transducer. Other than climbing that grade, A/C was OK on this trip but ambients never got above high 70s.

[Bill2014]

Assuming your compressor is now good and working correctly,
and your pressure transducer is now good,
and your refrigerant charge is correct (and clean),
and you don't see a change in air flow out of your vents (only temperature changes under engine load),
then everything is still pointing to the computer controlled blend door which is the only component in the system that regulates the temperature of the air flow. The computer gets signals from various sensors and uses the temp selector switch setting as a control input to adjust the electronic servo motor to manipulate the door position.

Since the AC works as expected except when under heavy engine load, then the blend door probably isn't broken. So... either some sensor that the computer is reading is bad or the lower vacuum that is inherent with wide open throttle is driving the computer to adjust the blend door. Maybe there's another possibility, but I can't think of it. Just because the doors are not directly vacuum controlled doesn't mean that a lower vacuum can't affect their positions through the computer.

[DamageWagon]

On 3rd gen trucks the radiator / condenser layout is kind of odd. Are you sure you aren’t just getting the condenser too hot?? If it’s on long pulls where things get hit, even if the engine thermostat is keeping the coolant happy, you might be keeping the refrigerant too hot in the condenser to let it provide enough cooling in the cab.

[Ducky's Dad]

Even with my crappy hearing I can usually hear the blend doors operating properly at idle, but at WOT up the mountain all I can hear is the exhaust. When I am having the problem, there is no noticeable change in volume through the dash vents, just a gradual change in vent temp as I modulate the throttle. I might have to go to the dark side and see if the dealer can pull any sensor codes. Engine coolant and trans temps were in normal range the whole time, and the truck has never run hot.