1999 Suburban A/C Front vs Rear Temps

About my 1999 Suburban-5.7 motor. Any ideas why the front ducts blow much colder when the rear a/c is turned on? With normal high & low pressures on the system the front duct temp will drop by at least 15 degrees when the rear unit is activated. The pressures on the manifold guages do not change when the rear unit is activated. The rear ducts are always very cold (below 40 degrees in 85 ambient temp). The front ducts will not get much below 55- 60 degrees under similar ambient temp with the rear unit turned off. When the rear unit is activated, then the front ducts put out 45 degrees or so. Would too much oil in the system cause these symptoms? Thanks for any help.

The rear A/C is always part of the system (refrigerant flowing through rear evaporator) whenever the compressor is on. The only change when the rear is turned on is the rear blower runs and the rear mode can be changed. I would say this is fairly normal because with the front and rear blowers running, 2 evaporators are being used to remove the heat from the inside air instead of just the one in the front. This would make a big difference, especially in recirculate mode. There is a large volume of air inside a Suburban, and cooling the rear air would help the front cool more efficiently.

About my 1999 Suburban-5.7 motor. Any ideas why the front ducts blow much colder when the rear a/c is turned on? With normal high & low pressures on the system the front duct temp will drop by at least 15 degrees when the rear unit is activated. The pressures on the manifold guages do not change when the rear unit is activated. The rear ducts are always very cold (below 40 degrees in 85 ambient temp). The front ducts will not get much below 55- 60 degrees under similar ambient temp with the rear unit turned off. When the rear unit is activated, then the front ducts put out 45 degrees or so. Would too much oil in the system cause these symptoms? Thanks for any help.

I would check and confirm for full R134a charge and front orfice tube working good .
Low on R-134a or orfice tube or expansion valve can rob cooling .

Does it do that at idle or on the road or both?
What are the low and high side pressure readings at idle and at 2K RPM?

Here are the pressure/temp readings on a retest:
Ambient temp 80 degrees
Coldest setting-Fan on High:
Idle Speed-
22 lbs/160 lbs...Front on,Rear off-F=60 degrees...Front on,Rear on-F==50 degrees R=45 degrees
Motor @ 1500 RPM:
30 lbs/200 lbs...Front on,Rear off-F=55 degrees...Front on,Rear on-F=44 degrees R=40 degrees
Fan on Low: Front on,Rear on-F=40 degrees R=38 degrees
Car has 130,000 miles.

Here are the pressure/temp readings on a retest:
Ambient temp 80 degrees
Coldest setting-Fan on High:
Idle Speed-
22 lbs/160 lbs...Front on,Rear off-F=60 degrees...Front on,Rear on-F==50 degrees R=45 degrees
Motor @ 1500 RPM:
30 lbs/200 lbs...Front on,Rear off-F=55 degrees...Front on,Rear on-F=44 degrees R=40 degrees
Fan on Low: Front on,Rear on-F=40 degrees R=38 degrees
Car has 130,000 miles.
Motor @ 1500 RPM:
30 lbs/200 lbs...Front on,Rear off-F=55 degrees...Front on,Rear on-F=44 degrees R=40 degrees
Fan on Low: Front on,Rear on-F=40 degrees R=38 degrees

Has it ever been recharged or any service work done on ac?
I would pump it down and recharge with the set charge of R-134a.

I only use this car on road trips, but I do run the A/C twice a month to keep things moving around in the system.2 Summers ago (2500 miles) I had to replace the rear TXV. A measurable amount of oil was lost during this repair ('bout an ounce).I might have put back maybe a half ounce oil too much.The system worked great all that summer. This summer I noticed the front unit got colder when the rear was turned on.Don't think I ever noticed this symptom before.I thought maybe too much oil wound up in the accumulator when the rear unit was off and not circulating refrigerant, which caused the front unit to not blow as cold.The whole system works well when both front & rear are turned on,so I'll
probably leave it alone for now. I don't know how many miles these compressors are good for, but next time I have to break the system down I will likely replace the major components.

I only use this car on road trips, but I do run the A/C twice a month to keep things moving around in the system.2 Summers ago (2500 miles) I had to replace the rear TXV. A measurable amount of oil was lost during this repair ('bout an ounce).I might have put back maybe a half ounce oil too much.The system worked great all that summer. This summer I noticed the front unit got colder when the rear was turned on.Don't think I ever noticed this symptom before.I thought maybe too much oil wound up in the accumulator when the rear unit was off and not circulating refrigerant, which caused the front unit to not blow as cold.The whole system works well when both front & rear are turned on,so I'll
probably leave it alone for now. I don't know how many miles these compressors are good for, but next time I have to break the system down I will likely replace the major components.

If system has a proper charge the compressors have a lot longer life and system works a lot better.
And usually keeps people from thinking something is not working right.
Good luck.

With what you vent temperatures are: I wouldn't do anything but drive it. Many would love to have vent temperatures that low.

Pressures at idle are not really useful. It's the 1500 rpm pressures that are indicative.