Automotive Air Conditioning Information Forum (Archives)

gmguy on Fri May 25, 2012 6:17 PM

Year: 2007
Make: GMC
Model: Yukon
Engine Size: 5.3
Refrigerant Type: R134a
Ambient Temp: 84
Pressure Low: 45
Pressure High: 187
Country of Origin: United States

Front evaporator freezes regularly with frost line starting at orifice tube to rubber portion of suction line. Decided to check orifice and found no debris. Changed orifice anyway. Pulled deep vacuum with JB Industries Platinum DV-200N 7 cfm pump for 2 hours, let sit for 2 hours, pulled again for 1.5 hours and let sit 1.5 hours and pulled again for 0.5 hours prior to charging with 2.5 lbs R134a per label and manual. Latest readings are 45 on low side, 187 on high side with 51 degrees front left center duct and 55 degrees left rear duct with 84 degrees ambient and 55% relative humidity at idle. At idle evaporator will not freeze. However in less than 20 minutes at highway speeds evaporator freezes starting at orifice tube all the way to rubber portion of suction tube. Changed low pressure switch on accumulator with no impact. Rear evaporator never freezes and continues to provide 55-60 degree air.
Why is front evaporator freezing?

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2007 GMC Yukon
5.3 liter
front and rear air
automatic temperature control

GM Tech on Fri May 25, 2012 8:53 PM

Does compressor ever cycle off? - yes the switch you changed may be okay- but the wiring harness may be crossed up- negating the switching action- put a suction gage on it and watch as you rev it up with blower motor on low speed, or even unplugged-- switch should click off- at 21 or so psi- you should be able to feel it and hear it--now does compressor also shut off when switch clicks off? Also look for a bad a/c relay in underhood fuse box if compressor never cycles.....

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

mk378 on Sat May 26, 2012 9:32 AM

To add to what GM Tech said, when you unplug the cycling switch the compressor should stop-- if it doesn't there is a wiring problem. There have been a few used cars where someone hotwired the compressor or jammed the relay closed to get the A/C to "work" for long enough to sell the car.

This is assuming it's not a variable compressor. On those the compressor controls low side pressure / temperature internally. The switch cuts out at 10 psi, it is only to protect the compressor in case of a total leak-out.

Edited: Sat May 26, 2012 at 9:38 AM by mk378

GM Tech on Sat May 26, 2012 11:11 AM

It's not a variable compressor-- it's a Sanden fixed displacement- hence the cycling switch on or near the accumulator.....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

gmguy on Mon May 28, 2012 10:46 PM

Disconnected plug for low pressure switch and reconnected. Compressor did not cycle. However, I pulled and reinstalled AC Compressor Relay and compressor cycled. One thing that happened is when we bought this vehicle used from a dealer we got them to replace the HVAC Module because of worn buttons. I took at least two months after we bought the vehicle before they replaced the HVAC module for several reasons. In either case the purchase and HVAC module replacement all took place during the winter between November and February. I don't remember precisely but I don't think we had this problem before the HVAC module replacement. I also did not have the shop manuals until about two months ago. I didn't believe the HVAC module could be the problem so the reason I went after the orifice tube and low pressure switch. After looking at the wiring diagrams I see where the low pressure switch goes through the HVAC module and the AC Compressor relay is tied to the ECM. It does not show it directly but from what I read apparently there is communication between the HVAC Module and the ECM. What is the likelihood that I have a problem with the programming of the HVAC Module that occurred during the replacement?

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2007 GMC Yukon
5.3 liter
front and rear air
automatic temperature control

GM Tech on Tue May 29, 2012 6:32 AM

So did you swap the relay with another one? like the horn? This will prove the worth of the relay...Other than that, I'd study the ground path supplied by the PCM to the relay leg- it should go on and off with the a/c request signal from the control head as you press the a/c button-- Sounds like your "ace" mechanic may have hot wired something other than replace that control head--faster and easier and got you out of their hair....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

gmguy on Tue May 29, 2012 9:00 AM

Relay is working correctly. When I pull the relay contacts to run compressor are open and I have continuity with appropriate resistance across coil portion of relay. Only way compressor would work when I put relay back in is for coil to close contacts. Also I know they changed the HVAC module since it has all new buttons and old one did not display outside air temperature where new one does.

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2007 GMC Yukon
5.3 liter
front and rear air
automatic temperature control

GM Tech on Tue May 29, 2012 9:04 AM

So basically they put in the WRONG HVAC control head....that might be the entire issue......better check it out-- I'd buy a used one off Ebay for $20 and test that out..

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

mk378 on Tue May 29, 2012 10:04 AM

Doesn't the GM CAN bus stuff have to be programmed at a dealer to match the serial number of your car? A great scheme by GM to limit the traffic in used parts.

It's like what they do with the "theftlock" radios. Who's going to steal a stock radio anyway? (well I suppose some people will steal anything) Compare that to how many people would legitimately want to buy a replacement one for their car from a junkyard.

Edited: Tue May 29, 2012 at 10:12 AM by mk378

gmguy on Wed May 30, 2012 2:10 PM

OK compressor turns off and on with button on dash and. This says HVAC Module to BCM and PCM communications are working. Also says compressor relay works which is also verified by compressor relay showing contacts open with meter when pulled and if I energize coils contacts close. However, if I pull connector on low pressure switch compressor continues to run. Probing ground lead on connector to low pressure switch I have good continuity. Only things left are wire from low pressure switch connector to connector at HVAC module and problem with new HVAC module or programming of HVAC module. Anyone have any ideas on how to narrow this down?

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2007 GMC Yukon
5.3 liter
front and rear air
automatic temperature control

rcolemanhd on Wed June 13, 2012 11:08 PM

On most newer GM cars the "low pressure" switch is a multi-function device. Unlike the low pressure switches of old, that simply interupted the flow of voltage when the pressure dropped below about 23 psi, the new style of switch is really a "sensor" that sends a pressure reading to the hvac computer. It then makes decisions based on the readings of this sensor along with other parameters that it monitors. The computer is making the decision to keep the AC relay energized even without a signal from the low pressure switch. It likely has a temperature sensor on the evaporator coil that would let the computer know that things are getting too cold. This could be a possible culprit.

I have seen car lot "mechanics" install jumper wires to fool the computer into thinking things are okay to avoid a costly repair. I don't know enough about the specifics of this vehicle to take you farther at this time but since my Yukon is doing the same thing, I may soon know more!

This is not vehicle specific but may help you in some way.

gmguy on Mon June 18, 2012 12:44 PM

Final answer is program for HVAC module was not correct when HVAC module was replaced due to cosmetic issue of worn buttons. Reprogrammed HVAC module and this thing works perfectly. Low Pressure accumulator switch on this unit is to prevent compressor from running on low pressure and damaging compressor and to prevent icing. My problem is even when switch hooked to wiring harness but with no pressure (not installed on accumulator) would still allow compressor to run. Key is when this first started I failed to go back to what was last changed which was HVAC module and instead started with if front evaporator is icing problem must be with refrigerant side of the house even though refrigerant parameters of pressure and temperature were in specification. Taking your time and proper problem solving is not always easy even though the answer is easy.

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2007 GMC Yukon
5.3 liter
front and rear air
automatic temperature control

rcolemanhd on Tue June 26, 2012 6:05 PM

rcolemanhd on Tue June 26, 2012 9:16 PM

I diagnosed the problem with my 2002 Yukon with climate control. I ran it idling with the rear air off and front fan on low and recirculation on (this places a very low thermal load on the sytem which will drive pressures and temps down). The low pressure dropped to about 25-30 psi where you would expect it, so the low pressure switch never cycled the compressor off. The low pressure lines did freeze over a bit but that's not that unusual with the fan on low speed on a humid night.

When I idled the engine up to 1500 RPM, however, the pressure gradually sank to 10-15 PSI which will definitely cause a freeze up (if the system is fully charged). (The higher RPM increases system flow and cooling capacity so things get even colder and pressures drop even more).

It turns out that my system uses the old-style clutch cycling switch that simply opens the circuit below about 21 PSI (1.8 KG/cm2 -I haven't done the math). So in my case, the switch was not opening at the proper low pressure which allowed the temps to fall way below freezing. On an extended drive this low pressure would allow the evaporator coil to get cold enough to freeze over completely and choke the flow of air through the vents entirely.

I replaced the switch and the compressor now cycles off and on as it should under low thermal load It now operates normally even on extended drives with fan on low and recirc on. The switch is around $20 and is literally a two-minute job to replace (requires no special tools or refrigerant leakage/recharge). This is the little round two-wire switch that screws onto the side of the accumulator near the firewall.

Note: There are apparently two types of switches on this model. One is the clutch-cycling switch as discussed in this post. The other is a "sensor" that sends a signal to the HVAC computer as mentioned in an earlier post. They are not interchangeable, but could both cause the same symptoms if they malfunction.