if you still have R134a in the system and it is over about 90°F, I would not expect the compressor to cycle at all unless the refrigerant is low. The stock GM cycling switch cuts the compressor off at 26 psi. If you have the blower fan on high it will never get to the cut-off point around idle. With a good stiff fan clutch and a mechanical fan pulling a ton more air than the electrics ever can, I could get my dual evaporator system to cycle at ~2,000+ rpm on a 105°F day. I never have been able to get dual electric fans to cool as well a a good clutch fan anywhere above idle. I put a 16" 3,000 cfm electric puller behind the radiator and a duramax fan blade. Way more airflow than dual electrics even at idle and my compressor still does not cycle at idle. I run a better performing refrigerant than R134a and my compressor still does not cycle at idle unless I put the blowers on the 2nd notch and that is with more airflow across the condenser than the GM dual electric fans. On high blower on both ends I can get the compressor to cycle if I hold the engine at 1,500 rpm. Suction side pressure dropped below 26 psi and the compressor cycled, vent temps were 33°F at that point.


Duramax fan, cut down to 19" and a 16" puller. Shroud off for the picture.

Vent temps in 102°F from the left vent after idling for ~10 minutes with the puller fan running, both blowers on high and using recirculate. Center vent was 38°F.

AC Pressures with the puller fan running.

Center vent after 10 minutes with the puller switched off.

AC pressures after 10 minutes with the puller off.

I also stress the point on the refrigerant difference. At 39 psi R134a is boiling at 44°F, vent temps would be about 49°F. At 43 psi R134a is boiling at 48°F and vent temps would be about 53°F. At 39 psi R152a is boiling at 33°F resulting in the 38°F vent temps. At 43 PSI the R152a is boiling about 38°F resulting in the 43°F vent temps. The low side pressures on my system are about the same with R134a or R152a. R152a runs at substantially less high side pressure than R134a.

Personally I would want the fans on high the moment the compressor came on to help shed heat in the condenser. The system is going to reach pressure equilibrium by the heat of the condenser. More airflow cools the condenser better. The a/c pressures will stay lower with more airflow. You can watch the effect of improved condenser cooling, by misting the condenser with water. I do not understand why anyone would want to run low speed fans as long as GM does on a factory truck. I am sure GM delayed the high fan for noise concerns and possibly a smidge better fuel economy for the EPA certification at the sacrifice of ac performance. The newer house ac units have a massive condensing unit on them because the larger the condenser coil the better the SER rating and energy efficiency to a point. Since a vehicle is bound to a certain size condenser unless a remote mounted one is added like on a bus or limo, the only way to shed more heat is with more airflow.

When I had electrics in the van I had them set to come on low at 40°F with a/c and at 150 psi. I had the high speed fans set to come on at 190 psi. Both with a 20 psi spread in pressure. as for idle dipping concern when the fans come on. There are two airflow adder tables in the idle airflow tab on HP Tuners. Increase the airflow values until the idle no longer dips when the PCM commands on the fans. Stock values for an electric fan equipped truck would be a good starting point and tweak from there. Also most trucks without an ac pressure sensor use IATs to guess at the ac pressure and therefore compressor torque. Look at the same torque tables on an electric fan equipped truck with the ac pressure sensor. Most of the time the IAT table is zero'd out and GM used the ac pressure table instead. It calms down the engine noticeably when the compressor cycles. I used the torque tables out of a 4.3L truck with electric fans on my SBC and ended up multiplying them by 0.75 to compensate for my 25% underdrive crank pulley. I also enabled both idle and off-idle ac spark compensation. The ac then cycled on and off without once feeling the load of the compressor.

Also I would enable the cool down mode by allowing the fans to run 3-5 minutes at shutdown at 190-200°F. It will cool the engine down after shut down and help prevent the heat soak from cooking your electronics, wiring loom, boiling the fuel in the fuel rail, etc.

 

I will also give another good example for airflow. I had a black on black leather 2011 Infiniti M56S. Nissan engineers in their infinite wisdom decided that customers might object to fan noise. For a whole summer I dealt with subpar ac cooling and a hot running engine that would get so hot that it would go into a reduced power mode in stop and go DFW traffic above 90°F. Later I tuned the car using Uprev. Nissan had used a PWM based fan control. At 0 mph it had the fans set for no more than 60% fan speed regardless of ac pressure or engine temperature. At 20 mph it had the fans commanded at 100%. I brought the fans on at a slightly cooler engine temp and with lower ac pressure and gave them the ability to run 100% speed at ZERO MPH. You could hear the fans slightly more inside the car but the difference was negligable with the radio and the ac blower cranked up on high. My ac pressures dropped ~40 psi on the high side and the engine coolant temps dropped from 230°F to about 198°F with the factory 180°F thermostat. The ac vent temps dropped 10°F at the vents. It never limped the throttle again after that either. Under 20 mph when I say limped the throttle, it was like a Geo Metro 1.3L 3 cylinder trying to move the 4,600 lbs of car and scary if you ever wanted to actually pull out on a busy road.

 

if you wire the signal to EGR input. You need to enable EGR for it to monitor the incoming signal

 

EGR modifier tables need to be Zero'd out if he does that.

 

last night I decided to wire straight from the AC pressure sensor to the pcm, into the old EGR pins. I will reenabel the EGR.


I believe I already zeroed out that table, will go into the tune and give it a check.

thank you both for your help guys I really appreciate it. I’ll give this a **** and let you guys know.

 

Okay this may be a dumb question but how do you swap to a new OS? I currently have my 2001 on the stock OS, which is 12202088, I want to swap to the 12212156 OS. I downloaded a 2002 Silverado tune with this OS and put all of my setting and tune details into it and wrote entire to the PCM, it just cranks but doesn’t start now? Any help is appreciated

 

Is the vin number correct for your vehicle with the new OS? If not disable the VATS setting and see if it starts.

 

yeah I did the VIN wizard swap

 

what should I check in the tune for VATS? It cranks fires for a second then stumbles off, so it sounds like a VATS issue.

 

Look for security I think