Should I be seeing a 20 deg. difference between Recirculate and Outside Air switch positions? Potential areas to check?

Scanned DTC codes, and only 4 History codes seen (all for the AO LDCM & AO RDCM - B2282H, B2283H, and U1064H for each door), and these 4 codes are "completely normal" due to added on door lock & window modules (don't ask, and no need for comment, since both doors function as intended).

2003 LS1 convertible automatic 44K miles

93 deg. day with 95% humdity

Temp setting to max cold. Pass side set to "center".

On "Auto" with Recirculating Air: Max cold reading at center vent = 40 deg. (nice and cold, with no humidity felt).

On "Auto" with Outside Air: Max cold reading at center vent = 60 deg. (and could feel "humidity").

Repeated the above (for about 5 minutes in each switch position), and both times, same results. Air flow / fan output out of each dash vent was considered to normal in both switch positions.

Thanks,
Plasticman

Note: Above temps have been corrected (thermometer reads 3 deg. F high).

 

I would think for those ambient temps that would be normal

 

Are you measuring your air out temperature at high blower? Are you driving down the road or at idle?

 

Yes, blower on "High", and yes driving at an average of 45 mph.
Allowing about 5 minutes between each change in settings to let the thermometer to stabilize.

 

What’s been the history with your car? Is this something you are only noticing for the first time this year? My past experience has been when I notice a change where it’s just not as comfortable as it used to be I find I’m low on refrigerant charge. Another tell tale is if I find I can hear the compressor a little more than I used to that can be a low charge condition. A low charge condition will lead to a portion of the evaporator being warmer and no longer effectively dehumidifying and cooling to hot humid incoming air.

 

I purchased it ~11 years ago @ 19K miles, and have put on ~25 K miles since then.
AC up till last year was running properly.
Last year, due to poor AC cooling output, I checked the AC charge, and found it to be low. I added sufficient amount to bring it back up to proper charge based on LOW/HIGH pressures, ambient temp, & humidity at that time (using a dye and sealer version of R134a).
2 weeks ago, I rechecked pressures (all still acceptable), and I checked for any signs of the dye (none).
2 days ago was 1st chance to actually verify that AC was working correctly (due to other projects / health / etc.).

The fact that the AC obtains 43 deg. at the vent in the "Recirculate" mode, and is very comfortable, implies that the AC portion of the HVAC system is operating correctly. It is only when the "Outside Air" mode is initiated, that it fails with a 20 deg. increase in vent temp, with added humidity felt immediately.

From that perspective, I would think that perhaps a HVAC blend door is improperly opening or an actuator is damaged?

I note your concern of: A low charge condition will lead to a portion of the evaporator being warmer and no longer effectively dehumidifying and cooling the hot humid incoming air. I will have to recheck the pressures again, but since the change is so "immediate", when switching from "Recirculate" to "Outside Air", or in the opposite direction, I feel it is not the AC portion of the system that is at fault.

Thanks,
Plasticman

 

Pressures may still look reasonable under a low charge condition. You have a variable displacement compressor which will adjust to hold the low side to a constant pressure. The high side will be reduced under low charge. The tell tale feel will be the suction line. Is it cold?

It is possible to have leaks without seeing dye or the amount of dye is quite small. I have a Chrysler that experience this. I don’t drive it much and it sits most of the winter. It experiences “phantom leaks”. I didn’t make this term up. It is something we used to describe customer complaints where they would bring their car in the Spring because the a/c was not cooling. No amount of leak sniffing can find the leak yet the charge leaked out over the winter.

As far as immediately feeling the change, keep in mind that when you are on recirc about 85% of the air is pulled in from your cool comfortable low humidity cabin. When you switch to outside air, you slam that evaporator with 100% hot humid air.

If it was my car, I’d try a half a can of r134a and see if it makes a difference.

 

Quick check today-94 degrees but only modest humidity here in Cincinnati. Manual transmission but I kept it in a high gear to keep the compressor rpm low and high blower.

39 degrees in recirc and 46 degrees on outside air.

 

Thanks for the update.... We have lot higher humidity here in sunny FL.

I will add a 1/2 can tomorrow, and hope for the best.

Plasticman

 

?
Perfectly normal operation. In recirculate, the system is refrigerating previously refrigerated (i.e. cabin) air. In fresh it's refrigerating the hotter -- sometimes significantly hotter -- outside air. Generally, recirculated will also have greater air flow for a given fan speed, as both input and output air are from/to the cabin. In the fresh air condition the system is adding more air to the closed cabin, creating a bit of pressure as the added air volume has to escape.
Now when you first fire up the car after it's been sitting in the sun, recirculate may well be less cool than fresh, because the cabin air may be much hotter than outside air.

 

Bookyoh,

Appears the 1/2 can did the trick!

Results from today: 95-97 F temps, with only 45% humidity, with same operating conditions as before (45 mph, Fan on full, temp on min., reading temp @ center vent).
Recirculate = 38 F Outside Air = 47 F

I say that was "success"!

Will have to see how it performs when the humidity goes back up to our normal 90-100%, but for now I am "happy"...

Thank you,
Plasticman

PS: Previous readings reported have been "corrected". Found that my thermometer reads high by 3 deg. F. Above temps are corrected..

 

Glad it worked out. I’m pretty comfortable backyard servicing my home and auto a/c just with Low side temperature and pressures, and I don’t have a dial a charge unit, so I add until I’m happy with my low side. A system can tolerate some level of overcharge and I’m pretty sure the way you described your cooling performance loss that half a can (6 ounces) is no where close to overcharge.

 

Appreciate all the help!

Still have not had the car out again, since the weather has not returned to our typical high temp & high humidity...

Will report back when I get it back out.

Thanks again,
Plasticman