Auto Temp Control AC Button Not Working (CJ2, HVAC,15855848)

[trottida]

2005 Chevrolet Suburban Z71 (290K KM) w/RPO CJ2 (HVAC auto temp control)

 

I've been having issues with my A/C where it will blow warm at idle and cold when moving. To add to this we have experienced loss of refrigerant over the last 2 winters while there are no leaks detected in the system. Also noted through the winter months the heat is not always working at full capacity; it takes a noticeable long time to heat up the interior in similar fashion that it takes a long time to cool the interior in the summer.

 

As far as the AC goes we've narrowed it down to the AC condenser fan not being turned on by the PCM. My mechanic figures that the refrigerant pressures are getting too high in the winter and the refrigerant is being blown out the pressure relief valve(?). After some head scratching and notation of the winter heat issue the HVAC controller in the dash was tested using a scanner and it was determined that when the AC button is pressed the command to the PCM occurs momentarily and then instantly shuts off. The AC on indicator remains lit on the controller. The electric condenser fan never turns on although when tested it works on command through the scanner.

 

So the recommendation is to replace the HVAC Controller part number 15855848 which costs about $230 CAD. My hesitation is that I also read that the HVAC controller works with input from several sensors (ambient temp, inside temp, sun sensor, etc...). I've read through multiple posts in multiple forums and I can't seem to find anybody with similar issues. Has anybody encountered this? Does anybody know the locations of the sensors and how to test them to ensure they are working properly? Any other things I should check?

 

I've read that the inside temp sensor located above the drivers seat for the front HVAC is the same as the rear so I can test that one by switching them out. I know the location of the ambient temp sensor and sun sensor but no idea how to test them. The ambient temperature is likely okay as it appears correct on the rear view mirror display. I'm not sure which other sensors come into play for the front HVAC controller.

 

I'm hoping that someone will be able to give me some advice before I start throwing parts at the issue.

 

[trottida]

Alright, yesterday I swapped in a different HVAC Auto Temp Controller (15855848) and there was no change after the unit calibrated. The AC condenser fan is still not being commanded by the BCM to turn on. I tried swapping the rear inside air temp sensor (aspirator, 22834330) to the front and still no change.

 

A couple notes I made...

• When not moving if I increased the RPM's the AC blows colder.
• When the digital controller is turned to 32 degrees Celsius (90 Fahrenheit) with the ambient temperate at 16 degrees I was not getting nearly sufficient heat. All the actuators seem to be working.

Any GM Tech's out there? Throw me a bone!

[trottida]

Still seeking answers. I was told today the the new HVAC Controller Module requires programming that can only be done with a GM Tech2 Scanner (dealer). They are not plug and play devices apparently. I haven't read that anywhere but seeing I'm not getting any feedback here I guess I'm a the mercy of the dealer. I have an appointment booked for tomorrow afternoon. Minimum charge is $109 which is 1 hour shop labour. I hope this is my issue.

[muddkatt]

I can't help you with that one. Bump to the top of the forum and maybe someone else knows the answer(s).

[trottida]

I can't help you with that one. Bump to the top of the forum and maybe someone else knows the answer(s).

Thanks for the bump muddkatt. I managed to find an electronic copy (torrent) of the factory service manual which confirmed that the HVAC Control Module should work out of the box. However, in my situation it did not calibrate properly at startup and needed to be reprogrammed. The dealer reprogrammed it with the most current configuration and while I haven't tested it yet they said the AC is working at idle. I'm out of town at the moment and will pick up the Suburban tomorrow.

[trottida]

Well it doesn't seem to be the Suburban's AC Control Module. It's all programmed with the latest updates from GM and the AC is still not cooling at idle. Heat seems to be working well again and now at least I don't have to replace some burned out bulbs in the old module.

The thought now is perhaps the AC Compressor is tired. Works fine with higher RPM's but as soon as it hits idle it stops cooling. Mechanic is pretty confident that there are no system blockages; he checked the orifice tube and it was clean. The known history indicates no catastrophic failures that would cause blockages. History includes leak in rear AC line which was replaced and a leak in the rear evaporator core which was also replaced.

They looked for a used compressor locally and there are none to be had in this area. New compressors are surprisingly not that expensive. I'm not sure where I'm going to go from here.

[muddkatt]

Things to check:

 

Is the compressor running when truck is idling?

Are the fans running when truck is idling?

What are pressure readings (both high and low sides) when compressor is on? when off?

 

Swapping parts will get expensive, finding the issue if its mechanical should be simple. Not knocking your mechanic, but he should know from the readings if you have a blockage. Looking for a used compressor? Really? If he suggested that, don't let him check the windshield washer fluid level because he is not qualified to raise the hood IMOP.

[MaverickZ71]

Most of these problems require a trip to the GM dealer, as they have to use their Tech2 or other special handheld scanner to calibrate everything in the system, including re-aligning the air control flapper door servos in the system. Apparently it can diagnose problems that other generic scanners cannot. I had to pay almost $400 US to have them install 2 new air-control servos and reset the system. And I think the parts were about $70, and I could've gotten them online for about $40.

[trottida]

Things to check:

 

Is the compressor running when truck is idling?

Are the fans running when truck is idling?

What are pressure readings (both high and low sides) when compressor is on? when off?

 

Swapping parts will get expensive, finding the issue if its mechanical should be simple. Not knocking your mechanic, but he should know from the readings if you have a blockage. Looking for a used compressor? Really? If he suggested that, don't let him check the windshield washer fluid level because he is not qualified to raise the hood IMOP.

 

 

Yes compressor is now running at idle

Fans are working

Low side is 50 and hi side is 150 (ambient temp was around 75 F (24 C))

 

Local parts dealers are asking over $500+ for OEM compressors so sometimes used compressors are an economical option. He also knows I'm frugal and has tailored his service to my needs. I can get better pricing for a new compressor on-line (even after shipping costs).

 

The Suburban was originally from Texas and I imported into Canada in 2013; I gather the AC would have been well used. Here in the North the AC only get used for a few months a year so used compressors are sometimes a good option.

 

We've pared down the problem to insufficient AC cooling when only when at idle. If you bring the RPM's up it cools as expected. If moving it cools as expected. As I understand it those pressures are on the borderline for faulty compressor diagnosis.

[txab]

I'd make sure the condenser is clean and check that air is not able to get by the condenser without passing through it. Check e-fans to make sure they are running on high as necessary. When charging unit with 134a make sure the charge is done by weight. There is a sticker showing the correct amount. A few ounces low or high can make a difference

[muddkatt]

Rockauto has new AC Delco compressors for about 140.00 USD for the front and rear air equipped Suburbans.

 

Txab is right, only a couple of ounces will make a huge difference.

[trottida]

Yes, I noted Rock Auto's price a few days ago which is $190.81 CAD. Cross border shopping can go bad if you ever have to return or warranty something but I may take that chance given the outrageous pricing in Canada. Through Rock Auto can get the compressor, orifice and receiver/drier with shipping and taxes in for $275 with the discount code applied.

 

I have to assume the shop has recharged the system with the correct amounts. They have discharged, flushed, vacuumed and recharged the system a few times now. It's an independent shop and the owner has a pretty good base of experience (25+ years). He works along side with a senior mechanic and a junior.

 

I actually looked up the GM FSM charts and the numbers are within acceptable range which is 31-54 psi on the low side and 113-180 on the high side. That's at 22C and above 40% humidity. That puts it in Diagnostics Pressure Zone A and suspect issues are Air Delivery Concern, Slight Refrigerant Under Charge or Refrigerant Contamination. All of those should have been confirmed and/or resolved with the actions taken so far.

[trottida]

Just a followup...

Given that the original compressor spent it's first 8 years (136K mi) in Texas I figured that it had a good amount of time logged on it and was probably tired and therefore I succumbed to replacing it even though it cooled at higher RPM's.

I ordered and replaced with OEM parts the AC Compressor, receiver dryer, orifice tube, AC belt and AC belt tensioner. While I think it is working notably better at idle, we haven't had any decent weather to stress test it yet. I'll give you an update weather permitting.

[trottida]

So we had 27 C weather yesterday (according to the ambient temp sensor) and I was able to stress test the AC System at 32% humidity. At idle the front center vents (driver and passenger) were blowing 27C whereas the front left AUX AC vent was blowing 20C. When I increased the RPM's to 3K the front center vent dropped down to 22C. I'd say that there is still something going on. Not sure where to go from here but because the condenser is working for the rear AC I wouldn't think that was suspect. The only other thing it could be is the front evaporator. What does everyone think?

 

I was told to measure the inlet and outlet temperature differential on the evaporator if accessible. It could be an internal bypass leak in the evaporator core.

[txab]

Are you sure you're properly charged? Have you checked to see if you're getting heat gain from the heater core?