NEED HELP Front HVAC Module Replacement Condsiderations

[southsidesmoka]

Well...my HVAC is malfunctioning like everybody else's - the . It's the front control module. I've reprogrammed it with the latest GM software in accordance with their Technical Service Bulletin which was supposed to fix this problem - all to no avail; as such, I wonder if a replacement module would render better results? Like...working the way it is intended to! Does anybody have any experience with and recommendations about replacing the front HVAC control module; i.e. brand to purchase and where to purchase from. Thank you for your time and admonition.

- southsidesmoka

 

[Mooseman]

What exactly is the problem and symptoms? Which type of system do you have? Auto or manual?

 

[southsidesmoka]

Damn...I forgot ALL of the details! It's hot and muggy outside, here, in Columbia, South Carolina and the heat is killing both myself and my manual-controlled HVAC module.

The compressor seems to cutoff at random points in time with absolutely no correlation to anything and refuses to return to working condition until a future crank - and it's not guaranteed that it'll be the next crank after the HVAC module craps out. I've found that completely removing the fuse and then replacing it resets the control module and it will work, after a failure, if that trick is performed.

According to a Bosch code reader I keep in the glove compartment, the "HVAC module has lost communication with the Diagnostic Circuit 2" or something very close to that phrase. When the snowflake indicator light goes off, I can tell that the actuator doors are returning to their neutral positions - I can feel the change in the point of air output as well as hear it.

Now, my trusty Tech 2 allowed me to update the software in the HVAC module on three separate occasions with an apparent fix - but even the best software can't fix broken hardware.

With that said, I am wondering if the pinnacle of repairs is actually replacement of the HVAC control module.

It's rather unfortunate that we, as loyal Chevy owners, have to foot the bill to fix their engineering and/or manufacturing defect.

- southsidesmoka

 

[Mooseman]

Have you tried troubleshooting with the Tech 2? It can basically read live data of the HVAC system and verify things like compressor on commands or reason for not commanding it on. Can also test actuators to see if they move to the commanded position.

You have THE tool that can probably troubleshoot this fairly easily.

 

[southsidesmoka]

Have you tried troubleshooting with the Tech 2? It can basically read live data of the HVAC system and verify things like compressor on commands or reason for not commanding it on. Can also test actuators to see if they move to the commanded position.

You have THE tool that can probably troubleshoot this fairly easily.

Well...I haphazardly replaced my HVAC Control Module in search of a quick and affordable fix that I could stand to install in the muggy South Carolina heat - and soon thereafter the problem persisted; still no communication through the Class 2 Serial Data Line which is preventing the compressor, temperature, air flow and rear defogger controls from functioning.

As such, I finally setup my Tech 2 tonight and diagnosed the HVAC system and have isolated the problem to what I believe to be a break in the Serial Data Line somewhere in the dash behind the HVAC Control Module. 'Tis time to break out my DMM and attempt to find the break in the ol' white wire running between the module's harness and its' corresponding splice pack in order to either repair or replace it. I'll report back with any progress as to success or failure after I perform the procedure in the morning (in order to dodge the damned heat). @Mooseman - thank you for the motivation and the service manuals!

- southsidesmoka

 

[mrrsm]

In Post #4 of this Link... I included images of ALL of the PCM Three Connector Pinouts worth Downloading and Warehousing JIK You need to Trace Search for any Broken or Separated Wires and contacts in these oft' fragile Harnesses in there:

https://gmtnation.com/forums/threads/bcm-pinout.16994/#post-533371

 

[southsidesmoka]

In Post #4 of this Link... I included images of ALL of the PCM Three Connector Pinouts worth Downloading and Warehousing JIK You need to Trace Search for any Broken or Separated Wires and contacts in the oft fragile Harnesses in there:

https://gmtnation.com/forums/threads/bcm-pinout.16994/#post-533371

Thank you, brother. I've already downloaded the images. I think I'm ready to handle this surgical task! I'll let everybody know how this goes...thanks, again.

- southsidesmoka

 

[mrrsm]

No 'bother'...Uhhh..."Brother"

 

[southsidesmoka]

No 'bother'...Uhhh..."Brother"

Whoops...I was a little tired ; the SC heat takes everything I have out of me...including my ability to spell! Thanks for bringing that to my attention. Now, it's time to go on a precision wiring repair mission in my TB, gentlemen.

- southsidesmoka

 

[Mooseman]

Also look for the datalink "hub". It's a taped over black connector with multiple wires and a "comb" inside that you would pull out to isolate each module from each other. It's somewhere under the driver's side.

 

[southsidesmoka]

Also look for the datalink "hub". It's a taped over black connector with multiple wires and a "comb" inside that you would pull out to isolate each module from each other. It's somewhere under the driver's side.

Thanks, @Mooseman ...I now know where to trace that Serial Data Line to. I think I may be able to accomplish this with a lot of effort, skill and, mostly, time. Thus far, I'm in the dash at the HVAC control harness. I've removed the comb and found that approximately three inches of the white wire have become ever so slightly discolored, leading me to believe that there may have been some excessive current draw. I'm thinking the Serial Data Line doesn't draw much current under normal conditions, though I can't say normal conditions have always existed; my TB has a long history of five previous owners (only 176k miles on the original odometer, though)...as such, that white wire may have been cooked in some unknown incident. The insulation of the wire remains flexible, though I haven't ruled out a line break. Thank you, again, for your time and for your assistance, @Mooseman . I'm going hub hunting!

- southsidesmoka

 

[Mektek]

Isn't the class 2 data bus just a serial data line connected in a "star" hub configuration? So you should be able to use an ohm meter and check for continuity between the data wire on the HVAC connector and the data line to the radio. (Disconnect both connectors first) If it's open then you can splice a wire from one to the other. .
Just a theory.......

 

[southsidesmoka]

Well, @*%#...there is NO discernable break or short in the Class 2 Serial Data Line leading to the front HVAC module. I successfully performed a continuity test from the end of the Serial Data Line at the HVAC harness to pin 2 of the OBD2 port under the dash.

I've already replaced the HVAC module and the condition persists regardless of which module, original or replacement, is put into service.

I've installed GM's software patch, which is supposed to fix this condition as noted in the TSB 02-01-39-005 I have attached to this post, into both HVAC modules to absolutely no apparent effect.

I am not deterred, just a little dismayed. I am scratching my head at the moment, gentleman...

- southsidesmoka

 

[Mektek]

What were we thinking?? If you could successfully install the firmware update, then of course the data connection is OK.

Could you have 2 defective modules????

Also strange that SOME are affected by this problem and others are not. I don't have the issue in the TSB even though I'm covered by it.

I'll go out on a limb and suggest that many electronics of this era were made with the earliest version of the lead free soldering process. The faults with the process are now well known and can cause intermittent failures like you describe.

My automatic HVAC control has the odd issue of not turning off the AC even though "AC off" is displayed. Hopefully when I switch to heat the system will have enough sense to do it

 

[southsidesmoka]

What were we thinking?? If you could successfully install the firmware update, then of course the data connection is OK.

Could you have 2 defective modules????

Also strange that SOME are affected by this problem and others are not. I don't have the issue in the TSB even though I'm covered by it.

I'll go out on a limb and suggest that many electronics of this era were made with the earliest version of the lead free soldering process. The faults with the process are now well known and can cause intermittent failures like you describe.

My automatic HVAC control has the odd issue of not turning off the AC even though "AC off" is displayed. Hopefully when I switch to heat the system will have enough sense to do it

@Mektek After checking for broken connections, finding none and, then, in direct succession, being unable to communicate with the HVAC module via Tech 2 Pass-Thru, I'm wondering if you may be onto something with your two defective modules theory because of the faults present in the lead-free solder process - one is the original and the other is a re-manufactured module - I'm thinking it's quite possible that both were made around the same time. It may be time to locate a new HVAC module with a manufacture date from a later year. Hindsight is always 20/20.

- southsidesmoka

 

[southsidesmoka]

I think I'm going to replace the Ambient Air Temperature Sensor under the hood before shelling out any more cash on HVAC modules...

- southsidesmoka

 

[m.mcmillen]

Are you still getting the loss of communication code? I worked on a Ranier once that the ac would work intermittently and then finally quit altogether.

I jumped the relay and it worked. I looked at my scan tool and saw that it was being commanded on by the PCM so I checked the wires from the relay to the PCM and they were good. I replaced the PCM and the problem was solved.

 

[southsidesmoka]

Are you still getting the loss of communication code? I worked on a Ranier once that the ac would work intermittently and then finally quit altogether.

I jumped the relay and it worked. I looked at my scan tool and saw that it was being commanded on by the PCM so I checked the wires from the relay to the PCM and they were good. I replaced the PCM and the problem was solved.

@m.mcmillen I was thinking about replacing the BCM somewhere down the line, as I figure whatever is receiving and sending commands to and from the HVAC Control Module could be faulty. Thank you for letting me know it may be the PCM, @m.mcmillen .

- southsidesmoka

 

[southsidesmoka]

Ummm...I can't seem to locate the Ambient Air Temperature Sensor. I looked...and looked again...and gave it a third inspection. It's seriously not there. I had two other pairs of eyes try and locate the sensor and they were unsuccessful, as well.

So...front of driver's side of truck near the upper radiator support, according manuals and videos. Is the Ambient Temperature Sensor not a standard part? I damn sure bought a replacement for it, this morning. My grille's off and I had my ratchet in hand...did I waste time on some misinformation or is my truck missing something?

- southsidesmoka

 

[Blckshdw]

Is the Ambient Temperature Sensor not a standard part

No it's not. This was something I added to my 05 LS, when I replaced my plain Jane rear view mirror with a temperature and compass display mirror.

 

[southsidesmoka]

No it's not. This was something I added to my 05 LS, when I replaced my plain Jane rear view mirror with a temperature and compass display mirror.

D'oh! Thanks, brother.

- southsidesmoka

 

[southsidesmoka]

The HVAC was "in a good mood" this morning and decided to run while I was going to get the Ambient Air Sensor - which I have to return. When the engine was at idle, it couldn't breathe and would "choke out". While researching the Net, I found that this condition could, possibly, be eradicated with a good Throttle Body cleaning; so I decided to undertake said task.

Whilst cleaning the Throttle Body on my TB, I found over 15 years worth of carbon build-up! It took WAY TOO MUCH CRC Throttle Body Cleaner and about a half-hour of hard scrubbing with an old toothbrush and wiping with two old socks to remove about 98% of the carbon deposits. The other 2% are in my margin of error and remain in near-impossible to reach crevices and creases of the Throttle Body.

I'm going to reassemble my engine and perform a Throttle Body Position Sensor Relearn Procedure with my Tech 2. Nothin' like lazy Sundays...

- southsidesmoka

 

[southsidesmoka]

My Vortec 4200 is running much, MUCH more smoothly than prior to the throttle body cleaning. I skipped the Relearn Procedure. The truck ran fine. The HVAC worked right away and ran for an hour and a half (a new record) on the road; unfortunately, it got "moody" and cut-out for the last thirty minutes of operation. On to the next HVAC fix...

- southsidesmoka

 

[southsidesmoka]

Are you still getting the loss of communication code? I worked on a Ranier once that the ac would work intermittently and then finally quit altogether.

I jumped the relay and it worked. I looked at my scan tool and saw that it was being commanded on by the PCM so I checked the wires from the relay to the PCM and they were good. I replaced the PCM and the problem was solved.

I'm gonna give the ol' PCM a flash update and see how she does before I proceed with replacing anything else on my TB...seems like the brains of the operation needs a little electroconvulsive therapy, itself! Gon' Flashin'!

- southsidesmoka

 

[southsidesmoka]

Per GM's in-program advisory about reflashing with the same software version, I elected not to flash the PCM. While I had my Tech 2 hooked up, I checked all other module updates, as well; every module on my TB is running the latest version of available software as per GM's TIS2Web update mechanism. So...

I'm back to defects in the hardware...considering that the PCM is responsible for all the HVAC commands, a newly remanufactured PCM costs ~ $400! That's nothing to sneeze at...there is a $63 core-fee available - but that's a negligible amount, given how long it would take for recoupment. And then we have the salvage PCM route. I like the newly remanufactured option because of 1) the suppliers' access to parts and tools and 2) the warranty; however, with all layers peeled back, I'm thinking I can bring a salvage part to "newly remanufactured" condition myself! And a warranty is more of a hassle to have honored and redeemed than anything, else when the entity is not local. If I do replace the PCM, I think I'm better off going with the salvage option from a good Pull-A-Part!

@m.mcmillen About the defects in the first implementations of the lead-free soldering process you mentioned...might you have knowledge of how to check or verify a PCM's viability via a manufacture date or some other method, good sir? I'm thinking a salvage PCM from a 2005 TB would be the better option - if I do resort to such extreme ends...

Thank you for your time.

- southsidesmoka

 

[Mooseman]

Don't you have two modules? Try re-flashing one of them. Aren't both exhibiting the same symptoms?

 

[southsidesmoka]

Just for anyone reading or watching this thread, the Throttle Body cleaning seems to have seriously improved the HVAC system's reliability. It seems to be "in a better mood, MUCH more often". I may have fixed my problem, as the past few days have provided more run time from the HVAC system and less failure at the control module.

- southsidesmoka

 

[mrrsm]

Unless you have already plowed this field with your own sensitive equipment... Please consider running a Broken Wire Trace on the Three Wire Bundles from the Outer Connector areas at the PCM and "re-trace" the other areas on the Databus that you have already attempted with DVM and visually. The quality of the Copper Harness Wiring has been suspect on occasion... and using this inexpensive tool might find a problem that can then be fixed in a very straightforward manner.

This tool is worth the modest cost and has been documented on numerous Youtube videos by average owners finding breaks in their wiring ...even in places that would have otherwise been ...inaccessible. The sensitivity for this device has a low threshold of 6 Volts... and while the expected level for normal Databus communications is 0-5 volts... The GM OBD2 Class 2 Network design threshold is 0-7 VDC:

https://www.autoserviceworld.com/ca...neral-motors-communications-network-problems/

https://www.amazon.com/dp/B0743BVT6Q/?tag=gmtnation-20

 

[southsidesmoka]

Don't you have two modules? Try re-flashing one of them. Aren't both exhibiting the same symptoms?

Yes, sir; I do have two HVAC Control Modules. The newer, re-manufactured module is currently installed and seems to be working as expected. Would a reflash with the same software damage anything in any ECU or is that eratta in TIS2000 / TIS2Web just GM's lawyers covering all angles, @Mooseman ? Thoughts in general? Thanks you for your time, good sir.

- southsidesmoka

 

[southsidesmoka]

Unless you have already plowed this field with your own sensitive equipment... Please consider running a Broken Wire Trace on the Three Wire Bundles from the Outer Connector areas at the PCM and "re-trace" the other areas on the Databus that you have already attempted with DVM and visually. The quality of the Copper Harness Wiring has been suspect on occasion... and using this inexpensive tool might find a problem that can then be fixed in a very straightforward manner.

This tool is worth the modest cost and has been documented on numerous Youtube videos by average owners finding breaks in their wiring ...even in places that would have otherwise been ...inaccessible. The sensitivity for this device has a low threshold of 6 Volts... and while the expected level for normal Databus communications is 0-5 volts... The GM OBD2 Class 2 Network design threshold is 0-7 VDC:

https://www.amazon.com/dp/B0743BVT6Q/?tag=gmtnation-20

This tool would be a great asset to my existing inventory...I remember you mentioning it in prior discussion, but didn't give it enough consideration as to what and where it could diagnose. I do believe I'll obtain one of these...thanks, @MRRSM .

- southsidesmoka

 

[Mooseman]

I have re-flashed modules in the past with no I'll effects. But then, thinking about it, if it flashed successfully before, I don't see why a re-flash would help anything.

I could see the PCM turning off the compressor if the idle is struggling to remove the load. You did disconnect the battery during the cleaning?

 

[m.mcmillen]

I have PCM from a 2003 Trailblazer if you want it, just send me some $$ for shipping.

 

[southsidesmoka]

I just returned from a VERY uncomfortable ride, due to the fact that there was absolutely no cold air coming through the ducts; in fact, HOT air was being blown through the lower ducts aimed at the passenger and driver foot areas! The HVAC Control Module was 100% unresponsive to any requests involving the Serial Data Line; i.e. the blower worked - that's it. It's been thirty minutes. Just for ^$%@ and giggles, I started her up and the HVAC worked fine. Back to square one.

@m.mcmillen , I've messaged you, sir.

@Mooseman I did, unfortunately, forget to disconnect the battery during the Throttle Body cleaning. However, I did disconnect both battery terminals and then proceeded to reset all circuits by holding both battery leads together for approximately 30 seconds to drain any current and reset any non-volatile circuits. I then reconnected the battery.

Thank you all for your time.

- southsidesmoka

 

[m.mcmillen]

The symptoms that you're saying now make it sound like the actuators are bad. I've heard that cycling the power to them will sometimes get them to work again. I'm not sure on these but, I've worked on the GM trucks that were having actuator problems and they would have trouble codes in the HVAC module for "out of range" or something of that nature.

Have you done the calibration procedure on them since you've replaced the module?

 

[southsidesmoka]

HVAC Actuator Door Calibration Procedure, huh? As of yet, the answer is, embarrassingly, no. I'll perform it this morning if the HVAC Control Module feels like communicating with the Tech 2...if not, as soon as said module starts feeling civil and talking to the rest of the Controller Area Network, I'll complete the calibration. Thank you, @m.mcmillen ! I'll report back with my results.

- southsidesmoka

 

[southsidesmoka]

Of course, the $*@<>?# HVAC won't communicate AT ALL this morning... I'm at a stand-still, at the moment, gentlemen. I'm cycling the ignition hoping to get a byte from fishing the Serial Data Line in that manner. 😁 So far, nothing...

- southsidesmoka

 

[Mooseman]

I don't think 30 seconds is enough to reset the PCM. I recommend that the battery be disconnected for 30 minutes to ensure all residual memory of previous idle parameters are gone. Whenever someone says the idle is wonky after a cleaning, it is because it wasn't reset properly.

For the HVAC, I see these things as possibly causing loss of communication to the module:
Defective module
No power to module (+ or -)
Data link line break

Just for the fun of it, have you tried running a jumper from the radio's data line to the HVAC? Sounds like the HVAC's data line has an intermittent. If the radio then also gets a comm loss, then that would be a definitive answer.

 

[southsidesmoka]

I don't think 30 seconds is enough to reset the PCM. I recommend that the battery be disconnected for 30 minutes to ensure all residual memory of previous idle parameters are gone. Whenever someone says the idle is wonky after a cleaning, it is because it wasn't reset properly.

For the HVAC, I see these things as possibly causing loss of communication to the module:
Defective module
No power to module (+ or -)
Data link line break

Just for the fun of it, have you tried running a jumper from the radio's data line to the HVAC? Sounds like the HVAC's data line has an intermittent. If the radio then also gets a comm loss, then that would be a definitive answer.

@Mooseman As for the wonky idle, my TB has, actually, never run better; the Throttle Body appeared as though it had NEVER been cleaned in its' 15 years of life.

Concerning the HVAC, the possibility of two bad HVAC Control Modules does exist, but I would think my remanufactured module should pass muster. I haven't ruled out a break in the power lines, yet. Also, if there is an intermittent break in the Data Line, I have yet to find its' location. I was thinking somewhere at the hub, but, again, I haven't checked.

I'm going to try a jumper wire from the SDL for the Factory Radio to the HVAC and see what I get as far as communication...I'm willing to bet I go out there and the darn thing is working. Trial and error - the Scientific Process. Be back later with results or the lack thereof.

Thank you for your time.

- southsidesmoka

 

[southsidesmoka]

All circuits have been reset as per a 45-minute circuit completion, followed by a two-minute engine run with no control manipulation. HVAC was then successfully tested for operation. HVAC Actuator Doors were then Re-calibrated.

HVAC is, apparently, thus far, operational...I have not road tested it, yet. I'll save the Eureka moment for a completed road test.

~ 24 Hours Later:

It's a good thing I didn't say Eureka...Unfortunately, my TB failed the road test this morning. Absolutely no response from the HVAC Control Module; however, it should be noted there was no hot air coming through lower vents upon this system failure.

Time to jump Data Lines from the Radio to the HVAC and back to troubleshooting...

- southsidesmoka

 

[southsidesmoka]

Damn HVAC still won't work for &*#@! So, to jump to the Radio's Serial Data Line, I would connect to the orange wire in pin slot A1 of the factory harness, correct? As per the schematics, is my deduction correct in assessment? Anyone? A little help would be greatly appreciated...I'm so SICK of removing the damned dashboard, I lament, rue and resent it, now...thank you for your time, gentlemen.

- southsidesmoka