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Response to HTSN Message:
Help needed: 2004 Chevy Silverado A/C not cooling in hot, humid conditions

July/August 2004
© 2004 All Rights Reserved
Have you run into a brand new 2500 Crew Cab (V8, Not HD) whose A/C system allows passengers to cook during hot and humid conditions? This truck has dual controlled front air only.
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We got a call from an upset owner because the local GM dealer cannot make this new vehicle cool on days when outside air temp approaches 90 degrees. The dealer has admitted, "GM has problem in the southern parts of the US." They've already replaced the low-pressure switch in a fruitless effort.

Additional information:
1. The cooling drops off only at idle or low speed, city driving. Once the truck resumes freeway speeds, cooling is adequate.

2. Turning up the blower speed, as suggested by the dealer, does nothing but reward the owner with a higher speed blast of hot air.

¤ Possible Fix: Having seen this problem on 2003 models, Ed Watts of Entropy Limited, San Jacinto, California, has already called in with what he thinks is the cause: evaporator freeze up.
He has yet to develop a solution, other than running the blower speed constantly at Max. (The idea is to not let the ice even begin to build up.)

¤ Possible Fix: Dick Kozinski, inventor of the Smart VOV believes that replacing the fixed OT with the Smart VOV might help this application. He's researching it at this time.

Tech-To-Tech and HTSN collaborate to deliver lots of answers:
2004 Chevy 2500 Silverado Crew Cab: Is there a problem with the A/C system?

By Chris Bede and John R. Hess
We very recently got a phone call from an upset northwest Louisiana truck owner. The local GM dealer cannot get the A/C system in her new truck to cool when outside air approaches 90°F and humidity is high. Slowing in stop and go traffic turns the vent air to “warm and wet.” The dealer, advising that “GM has cooling problems with this vehicle in the southern parts of the US,” replaced the low-pressure switch in a fruitless effort to correct it. Right now they seem to be stuck and are waiting for GM, or someone, to come up with a solution.

Additional details:
1. The vehicle was bought in February ‘04. The A/C (dual controls but for front seat only), which was not really used until April, has never cooled properly in hot, humid, slow-moving conditions.
2. During hot, humid conditions, the cooling drops off only at idle or low speed city driving. Once the truck resumes highway speeds, cabin cooling is adequate.
3. Turning up the blower speed, as suggested by the dealer, does nothing but reward her with a faster blast of hot, moist air.
4. Owner reports that at times when the climate control system is totally shut off, she still feels strong flow of air coming from the vents.

Time for HTSN!
As part of a July 21 HTSN (Heat Transfer Service Newsletter), we emailed this problem to the 10,000+ list recipients and requested:
“Have you developed, or do you know of a solution for this problem? If so, please let Cool Profit$ know. (Email: imcool@imcool.com or call/fax: 800-883-8929 (707-769-8929 outside US)”
We started getting calls and return email within a half hour of release. Here are some of the responses we received. (Note that responses have been edited for clarity, brevity, spelling, redundancy, grammar, etc.)

¤ Possible Fix: Having seen this problem on 2003 models, Ed Watts of Entropy Limited, San Jacinto, CA, was the first to respond with what he thinks is the cause: evaporator freeze up. He has yet to develop a solution, other than running the blower speed constantly at Max (to stop the ice forming).

¤ Possible Fix: Dick Kozinski, inventor of the Smart VOV believes that replacing the fixed OT with his VOV might help in this application. He’s researching it now.

¤ There might be icing going on. If they have an adjustable pressure switch, like some older ones, I'd make it cut out at 10 psi higher just to see. TG, Massachusetts

¤ Many times I have overcharged systems with R-134 on test benches to see how various compressors stand up to overloads and wear and tear. And, the evaporator does freeze up. Try 1/2 lb less and work from there.

Try getting some warm air to flow with regular air to prevent freeze up. Add an extra fan. Possible restricted airflow from malfunctioning flaps, dampeners, vacuum control door openers, computerized controls, like on some 96 Caravans; some need reprogrammed. May be helpful. SAL, Compunova 2001

¤ One solution would be to evacuate the R-134a refrigerant and charge with Hot Shot (R-414B). R-134a has a relatively low critical temperature of approximately 215°F. In a hot climate with the heat generated under the hood, from the pavement, and if the vehicle is idling it is not uncommon to exceed the critical temperature.

Hot Shot has a critical temperature of 266° and is much more capable of handling the high temperatures of the South. With a much lower boiling point than R-134a (-21.6° to -15°), Hot Shot also cools down much quicker at start up. MR

¤ Not enough airflow; I know how to fix it, but GM refuses to pay me for it! Want my answer? GB

¤ Hello from Japan, I have modified a lot of units here by placing a 0° thermo switch in the evaporator coil. That way when the core temp reaches freezing it cuts out the compressor until the temp comes back up to +2°. Now you will always have cool air flowing from the outlets. We have very high humidity and this is the only way to control freezing. Hope this will be of some help. Ron, KJS Japan

¤ The solution can be determined by eliminating some of the theories. Attach a thermistor (10,000 ohm) to the evaporator coil to determine if the evaporator is actually warming, or if there is ice surrounding the coil. If the evaporator coil is operating at the designed temperature at 90°, but the air is warm, then there could be ice on the coils.

On the other hand, if the system was left to defrost and then restarted, the ice would melt and the air would be cool until the ice developed again. So, we could confirm or eliminate the ice theory easily.

With Ice NOT being the problem and the evaporator coil indicating a warm temperature at 90° ambient, we could assume a refrigeration system problem. To diagnose further we now have to look at the pressure switch and the condenser temperature. Also, check the thermal overload switch (if there is one it would be at or inside the compressor). With the pressure switch operating properly, and not affected by underhood temperatures, check the temperature at the condenser. If this temperature is normally high at lower ambient temperatures, it may prove that the condenser is undersized for 90°+ and humid conditions. To determine if the condenser is the culprit, try blowing air across the condenser more strongly than the factory fan. If this helps, then the condenser is under-engineered for the vehicle. Let's hope that the solution is a pressure switch problem, or a partially operative pressure switch affected by the higher temperatures.

Finally, I find it hard to accept that GM has grossly under engineered the system. I would check the mechanical elements, such as the temperature control inside the vehicle. If it's a climate control system, it could be shutting down the system at the higher temperatures - a malfunction. It it's a manual temperature control system, perhaps the interior temperature sensor is failing at the higher ambient temperatures and shutting down the compressor. This causes the air to blow warm since the evaporator is not chilling due to the shut down A/C compressor. I have not gotten into the receiver drier and clogs etc., because I would hope that these components would already have been suspect and changed. Well enough for now. I hope this has been helpful. I'd be interested in knowing what the end solution was. GFG, Scottsdale, AZ

¤ I live in hot and humid Costa Rica and ran into the same problem. Solutions: The fan may not be moving enough air thru the condenser. Fan clutch could be miscalculated for hot, humid, idle conditions. We blocked it and worked fine. Or, the thermistor at evaporator is doing a kinky job. It might be telling the compressor to remain on at all times, forming ice in evaporator due to high humidity, partially blocking it. Hope it helps, we had to come with solutions in the middle of nowhere. Cedric Liber, Golfito, Costa Rica
A/C Problesm with a 2004 Chevy Silverado CrewCab
Above: (Click for larger image) 2004 Chevy Crew Cab with Dual Control Front Air (no Rear Air). The A/C cools fine until the truck slows or stops during hot and humid outside conditions. Note that HD models of the same year do not seem to have this problem. What's causing the condition and why is it happening only on the non-HDs?

¤ 1. Could there be an inherent airflow restriction through the condenser, or perhaps just a fan not capable of the CFM required by a non-moving vehicle? I would think that the engine would overheat if this was the concern, but maybe it has a large enough cooling system to handle a short traffic stop.
2. Is the low-pressure switch the dealer installed adjustable? Perhaps a little tweak would do the trick.
3. Now for the doozy. If the evaporator is freezing, what would happen if the driver where to turn the temperature selector up just a bit off full cold, allowing a slight amount of air to move through the heater core and across the evap? If the HC is after the evaporator, what about modifying the plenum? 4XFord

Above: (Click for larger image) You can't tell by looking under the hood why the A/C system has problems. Of course, you can't see much under the hood anyway...except that the accumulator is insulated.

¤ My suggestion is to replace A/C condenser with one for Chevy Suburban with dual A/C. (Make sure it’s for dual A/C option as single A/C condenser for Suburban is the same as one for Silverado.) This condenser is compatible with Silverado condenser (i.e. same mounting points & same inlet/outlet connection), but about 20% larger in core surface area. Larger surface area will improve heat rejection from condenser, and especially effective in idle - low speed. Osamu Yamashita

¤ I have a 2002 Silverado, same problem. What did I do? I bought some NON-ACID Condenser coil cleaner (residential stuff) and mixed it per label in a simple garden sprayer. Removed the plastic cover from top of the radiator and sprayed the condenser, radiator, tranny cooler and oil cooler. Waited 5-8 minutes (per label) and hosed it off. Works REALLY GOOD. Problem solved. I do this about twice a year. My vent temp is 38-42°. FD, Jr, North Carolina

¤ More air flow across the condenser to allow greater sub cooling may be the answer since freeway speeds seem ok. Auxiliary fan, heavier duty fan clutch, more blade pitch, etc. BT, Iowa City, IA

¤ Is the heater control door operated by vacuum? Check for vacuum leaks at idle and low speeds. DM

¤ As for the vehicle freezing up, why not try and fit a mechanical thermostat either into the evap or onto the suction pipe. It will cycle the electrical clutch on the compressor once temps get down to about 2°C. Very few US cars use this method anymore, but the majority of Aussie cars use it very well, even in combination with the variable compressors used on GM vehicles. GR

¤ I'm no expert - but I can't see why you couldn't slightly undercharge the ac system and reduce the freeze up. A small radiator electric fan across the condenser may help also. TK

¤ The symptoms described are the results of poor condensing, not icing. It is also possible that insufficient compressor displacement could be a contributing factor, and might be alleviated to a some extent by a smaller diameter clutch. But what is being described are classic symptoms of poor phase change, the biggest field service problem that we face, and one which requires that we modify and improve factory designs in the field, a standard routine at our shop. One of those techniques is to install a 3" dia. x 9" long receiver in series with the liquid line to lower head pressure, subcool the refrigerant, and assure a liquid seal at the expansion device. In the case of a dual unit system, the liquid line tee to both evaporators should be installed downstream of the receiver. Replace the non-adjustable pressure cycling switch with an adjustable type for “ice point” adjustment, the controls set on “recirculate,” “low blower” and 600-800 engine rpm.

After performing this modification, dual unit Suburbans will achieve 35°F evaporator temps at idle at 95° ambient when converted back to R12. Requirements include (a) adjusting the charge level, (b) adjusting pressure switch for clutch “cycle-out” immediately after observing the formation of slight surface ice at the evaporator or accumulator. But even by staying with R134a, significant improvement can be achieved using this technique.

To have an efficient evaporator demands efficient condensing, otherwise there a constant war between the condenser and evaporator. But both of them can be winners by delivering properly subcooled liquid to the evaporator. Check the head pressure at 2000 rpm, high blower. Before modification, R12 system pressures shouldn't exceed 275 psi on the hottest days, and R134a systems shouldn't be more than 300 psi. After modification, the head pressure should flatten out to about 250 psi or lower. Try it. John Noble, Cool Flow, Inc.

¤ I have another solution to the problem. Install a VSV (12 volt Vacuum Switching Valve) and create a small air leak. This will cause a small increase in RPM and force more air thru the condenser. Also, modify the PROM on the car ECU for higher RPM under A/C, in idle. Once you turn it off, (A/C) idle goes back to normal. Cedric Liber, Golfito, Costa Rica

¤ The usual problem is low refrigerant circulation, which could be weak compressor, low refrigerant charge, restriction, etc. A brand new compressor is unlikely to be weak, admittedly, but symptom of evap ice-up is that the system cools, stops cooling (then ice-up melts), so when you turn it on again, it starts cooling again. Unless there really is a specific issue with those cars that the factory knows about, the limit to the cooling is one of the issues I mentioned. PW

¤ If they have changed the compressor, they should change the receiver drier and carry out a charge quantity optimization. I propose to perform REFRIGERANT charge quantity optimization by simulating same weather condition (same as HOT & HUMID described in your problem statement) in a wind tunnel. OR, carry out refrigerant charge quantity optimization in southern part of US, where GM is facing this problem. SP, Pune, India

¤ The quite low evaporation temperature at the evaporator; can you change parameters at the expansion device? Why similar systems worked with previous models and not with this one? What is new? The equipment? The work/service conditions? Check the differences. António Soares

¤ Have you tried a different fan clutch or fan? DRD

¤ If this were one of my trucks, I'd reduce the size of the compressor pulley. WOM

¤ I found a gm bulletin that came out in early July. The HD diesel engine, if all other a/c diagnostics and repairs fail (like low pressure switch, lowering the charge .2 lbs., recalibrate the actuators, etc.) there is a different accumulator to be installed. It requires the accumulator and line to the compressor be replaced. TS

¤ The problem almost suggests a low Freon charge at idle, which makes sense at low speed there would not be enough pressure to make a pressure drop in the evap. High speed driving would increase the pressure thus giving the evap the needed pressure for cooling. Have the dealership take a closer look at the charge using the pressure temp chart instead of just relying on the low temp switch.

The other side could be the condenser is not getting the right amount of airflow at low speeds, thus causing hotter gases to run though the fixed orifice. I have seen evaps freeze up due to too little refrigerate and bad air flow combined. The PT charts will help determine this, but the airflow should be compared to a working 2500 at idle with an anemometer, versus the one that is not. AK, AV Cooling

¤ I had a 2003 1500 HD crew cab in the shop with the same problem. The system was overcharged and had damaged the compressor. The owner had complained about this when new and the dealer had adjusted the charge. We discharged the system and put the recommended amount in but the damage had been done.

We also found that the tensioner is weak and will break at the stop. It has been upgraded with a heavier duty one. We replaced the compressor and put in the recommended charge. The system works fine now - better than when new.
I think overcharging at the factory could be a problem. If you look at the GM supplied specs for the systems, the charge amount is all over the place with a range of over 5 ounces. It is easy to overcharge a system with this much range. We looked at pressures and charged accordingly. This is probably just a fluke fix on my part, but I suspect hundreds left the factory this way. JH

¤ I would change the accumulator and pull a long vacuum. In my experience, looking at the orifice tube only tells you how the compressor life is going. I'm glad GM seems to only have this problem in the South and not in the North East. A weighed charge is a must too-no guess work. CP

¤ We install mid 90's low-pressure, adjustable cycling switches. You’ll have to change wire ends and use an electric 12” condenser fan with relay. They should have done this at factory for the very hot areas on our globe. The Ice Man, Peter

¤ The only problem we have encountered with these trucks is chronic failure of the low-pressure cycling switch. The failure mode is either freezing or an inoperative compressor. Frequent cycling was also observed in spite of a full charge.

I have noticed a problem with the Suburban that also affects the rest of the GMT800 line: pressures get high very quick, in spite of an almost constantly locked up fan clutch. You can't idle the car for more than 5 minutes because the air gets humid. Pressures even trigger the HPCOS. I'll add a couple of pusher fans to keep this from happening. We've reached 120°F a couple of times. Right now, we're at 101 and the humidity is 67%. Ignacio G. Corella G. (Nacho), Hermosillo, Mexico

¤ We really need to know what is happening to gas pressures at the time of non-cooling, and is the evaporator indeed freezing up? If so, then the thermostat setting is too low. Is it adjustable? I cannot tell you - we don’t have too many Silverados in Sydney, Australia. Is it an accumulator with pressure switch operation or receiver drier and thermostat operation? Or, is it fully electronic with sensors? The accumulator pressure switches are sometimes adjustable; certainly, the thermostats are, but usually not the electronic systems. I DON’T believe, however, that the evap is freezing. It may be that there is an incompatibility problem (i.e. the capacity of the compressor may not suit the evap capacity or the condenser size). That’s why it cools only when the engine is revving (a manufacturing or engineering problem). Hope I got the story right. Some of this info may help. Mark, Sydney, Australia

¤ Just read the complaint about the 2004 Chevy Silverado and decided to add my input. If the unit blows cold then stops, that would be more symptomatic of evaporator freezing. The system could be slightly overcharged from the factory; even just a few ounces over can cause issues. Too much oil in the system can also cause poor cooling as the oil coats the inside of the system components and reduces heat conduction. Curious to hear other opinions. ARG

¤ How about a too small/inefficient condenser? Try auxiliary fan(s) in front of the condenser if it doesn't already have one. If it cools at freeway speeds, but not in traffic, the problem would seem to me to be airflow across the condenser.

Also, if the compressor is inadequate, along with low rpm of the idling motor, that may cause poor cooling in traffic, and not necessarily poor condenser performance at low speeds. Try putting a smaller pulley on the compressor so it turns faster. LO

¤ I have seen new Chevy pickups with defective fan clutches. Does a high pressure switch or other control shut off the compressor? If the compressor shuts off because of high pressure, probably at about 360 PSI, it is low airflow. BB

¤ Hey y'all. I saw a 2004 Cadillac doing the same thing. The local dealer replaced compressors on 40 Cadillacs under warranty to fix that problem.

I feel some models of Jeeps and GM pickups are overcharged. I’ve cut the charge on several of them and ended up with good cooling. The Cadillac had good pressures after recharging, and 38°F vent air.

But, I just drove a 2005 Venture van for a week. The only time you get comfortable is on the highway. Folks considering buying one had better drive it in town awhile. Keep up the good work gang. Best Wishes, BF, Amarillo, TX

Above: (Click for larger image) Is the compressor undersized for this application and causing the system to fail at high temps/humidity? We expect to have an answer in the next issue.


Inside information
When sending a specialized message to 10,000 plus voluntary (motivated) subscribers, some of the contents, especially if it’s somewhat startling, controversial and maybe a little contentious, are bound to end up in the hands of people who know more than most about that particular subject. (We often refer to them as insiders.) This message, especially when pleading for a fix to a problem affecting a brand new, popular, vehicle, was no exception. We got a couple of leads from folks “who should know,” and their info sounded logical and legitimate. As such, we immediately turned those leads over to Mole Snoopster, Automotive Spy par excellence.

Here’s what Mole learned:
“1. It’s quite possible that the compressor used in this application is slightly under capacity for the job. And confirming what the truck owner learned from the dealer, there are definite performance problems in hot, humid climates.

2. The truck owner really, really needs to pressure the dealer for a fix. The dealer HAS the wherewithal to get this warranty problem fixed ASAP!”


It would have been nice to have gotten more, but that’s all we could squeeze from Mole. (And that’s after threatening to take away his fake glasses and mustache!) But he must protect his sources!

Look for your next issue of Cool Profit$ because if we do learn of a fix by print time, you’ll certainly read about it!

Your turn, please speak up
Would you like to comment about this problem, or about any of the solutions given? Do you have a solution that was not mentioned? Please let us know.

Maybe you’d like to see Tech-To-Tech and HTSN cover a different problem? If so, here’s how to get that done:

1. Call or fax it to Cool Profit$ at 800-883-8929 (707-769-8929 outside US).

2. Email it in to imcool@imcool.com

Sign up for HTSN at: aircondition.com (the form is at the top of the page) or at: imcool.com. HTSN does not sell, trade or share your email address with anyone, and you can remove yourself from the list at anytime.      $$$

John Hess/Publisher
Cool Profit$ Magazine
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Email: imcool@imcool.com
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