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2004 gmc envoy_slt
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Sorry to double post....on my phone and easier to just post then to edit......

I just thought of a reason mine is a little different.......the gm sensing voltage is 10 to 19 if im not mistaken and due to me running a 14volt system(charging around 16:5), the reason my clutch stays engauged is because I have electrically changed the set point for the clutch to engauge....just a thought....I need find out if its just a contact or if its electronically controlled
 

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2004 gmc
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The relay for the AC compressor clutch is mechanical. Only the EV fan clutch and low beam headlight circuits use solid state relays (with a FET as the main element) to allow PWM proportional control.
 

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2004 gmc envoy_slt
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The relay for the AC compressor clutch is mechanical. Only the EV fan clutch and low beam headlight circuits use solid state relays (with a FET as the main element) to allow PWM proportional control.
thanks..figured as much but who knows lol....i mean who would of thought me cycling the windows could get the clutch(compressor) to pull in....so the only circuits to be pcm varible controlled are the EV fan clutch and low beams(im assuming for daylight running)...the other circuits are simply mechanical as in N.O. contacts and relays that the PCM controls(signal)? controlled almost like a DI? I will print off the 1 lines and find which circuits i actually will need....seems that the pcm induces to much voltage to keep a complete diff of potential and that causes the intermittent problems.....and you can't just run 1 new ground as running it in parallel would make everything work at the same time.....either a lot of diodes or jumpers for each signal.....also need to ground the PCM and BCM....find which circuit is leaking or producing a field to induct and isolate it........thats gonna mean pulling the dash out im sure...i do have most of it out already though...im sure i can easily track down why my clutch wont engage but i want to find these nonsense problems....its ridiculous to see the amount of money people spend at the dealership only to have no resolution. And not to mention safety on the road when dash/headlights fail or when the vehicle stalls or leaves you stranded when it drains your battery......


if the grounding is bad or not as good as it could be, then any solid state relay could be "failing"(even though its working) and could be bleeding off the signal voltage....
 

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..the other circuits are simply mechanical as in N.O. contacts and relays that the PCM controls(signal)? controlled almost like a DI? ...
You got it. Actually, the BCM controls a few circuits with PWM signals, like the dimmable interior lights. There are two dimmable circuits, one meant for incandescent and one for LED illumination, because the PWM duty cycle proportionality is different for incandescent lamps than LEDs. And the HVAC controller on digital/automatic systems sends PWM to the blower motor speed control module, for smooth variable fan speed as opposed to the manual 5-speed HVAC control system that uses a traditional resistor pack that fails often. And the rear HVAC blower speed control is ALWAYS PWM controlled because all EXT/XL vehicles have that kind of control in the rear no matter what kind of HVAC control they have up front.

DRL is indeed the reason for the solid state relay on the low beams. 70% intensity is what the shop manual says they were aiming for, but that means a bit more than 70% duty cycle because of the tungsten curve.
 

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2004 gmc envoy_slt_xl
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No A/C @ idle

I have had this problem since purchase. There are two problems at hand here. First; most V*'s only idle around 600 rpm, so the compressor isn't turning fast enough to stsrt with. 2nd; you MUST!! have airflow across the condenser out there in front of the radiator. Mine has a viscuous fan clutch with electric override. The fan will speed up when the Hi side pressure on the A/C hits about 400 or more. I personally think that the fan should lock in earlier than that ( say around 300 ). This would help tremendously with lo idle cooling. That is why it does so much better at road speed, you have increased flow due to engine rpm along with increased air flow due to vehicle speed. I hope this helps.
 

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2004 gmc envoy_sle
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2004 vehicles would benefit from a PCM reflash to improve AC performance at idle.
Roadie, do you know if there is a TSB or anything about this reflash? Spoke to the service manager at my dealer the other day and they couldn't find anything using my VIN.
 

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I seem to remember one being mentioned, but I don't understand why your dealer should need a VIN to look for a problem description. Anyway, are they saying your PCM calibrations are up to date? A 2004 should benefit from something they have in their bag of tricks.
 

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2004 gmc envoy_sle
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I seem to remember one being mentioned, but I don't understand why your dealer should need a VIN to look for a problem description. Anyway, are they saying your PCM calibrations are up to date? A 2004 should benefit from something they have in their bag of tricks.
They wanted the VIN to pull up any recalls and TSB's to my understanding. Unfortunately, you sometimes have to spoon feed some of these service managers since you don't get to speak directly to the tech's themselves.
 

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2005 chevy trailblazer_lt
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i remember there being a tsb in where a person mentioned having to have the pcm re flashed and a sensor replced for rough idle. i think it was a cps sensor......
 

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2003 chevy trailblazer_ltz
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I have the same problems as everybody else here, my solution is to add 2 electric fans and it's on my list. I'll be adding 2 12" electric fans to the front of my condensor/radiator that I will switch manually. In the fire service, chasing, whatever else I do a LOT of idling in my truck. It doesn't seem to take much extra air flow and I figure adding 2 1000+ CFM fans to the front will significantly increase my cooling at idle.

One thing you can do is manually turn your fan down, I do that with mine and leave it on the third or fourth 'notch', seems to cool better that way if you have to be idling.
 

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2005 gmc envoy_sle_xl
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Roadie, do you know if there is a TSB or anything about this reflash? Spoke to the service manager at my dealer the other day and they couldn't find anything using my VIN.
There is a TSB. It was very hot today and my AC didn't work that well at idle. Looks like I need to apply this TSB to my truck. Many of us with a 2005 and prior may have this problem.

Bulletin No.: 04-01-38-019A

Date: June 07, 2005

TECHNICAL
Subject:
Intermittent Slow to Cool HVAC Performance Concerns in High Ambient Temperatures and/or High Humidity Conditions After Start Up, Especially at Low Engine Speeds (Install New Engine Cooling Fan Clutch)

Models:
2004-2005 Buick Rainier
2002-2005 Chevrolet TrailBlazer Models
2002-2005 GMC Envoy Models
2002-2004 Oldsmobile Bravada
2005 Saab 9-7X

Supercede:

This bulletin is being revised to provide repair information for this customer concern. Please discard Corporate Bulletin Number 04-01-38-019 (Section 01 - HVAC).

Condition

Some customers may comment on slow to cool Air Conditioning (A/C) performance or high A/C outlet temperatures after start up or that the A/C system is not cooling sufficiently, especially in high ambient temperatures. This may be an intermittent condition and the performance of the Heating, Ventilation and Air Conditioning (HVAC) system may not meet customer expectations.

Cause

This condition may be caused by the response of the Electro-Viscous (EV) fan clutch and can be related to airflow across the condenser. This low A/C system performance can occur for longer periods of time if the vehicle is allowed to idle or is driven in low speed conditions. The response of the EV fan clutch will affect A/C system performance. As higher road speeds are attained, the increased airflow across the condenser will usually improve A/C system performance and the response of the EV fan clutch. This condition may be more evident after the vehicle has set for a period of time and can be difficult to duplicate. The condition may occur after the vehicle has set for as little as 10 minutes or as long as overnight. It is most prevalent intermittently during the first 15 minutes of operation.

Correction

Technicians are to replace the EV fan clutch with a new design part, P/N 15293048, on vehicles built before June 2005. Vehicles built in June 2005 may already have the updated EV fan clutch and it will not need to be replaced. For 2004 model year vehicles and older, update the Powertrain Control Module (PCM) with a software calibration that was developed to improve coolant temperature gauge readings. This new calibration will cause the PCM to command small amounts of additional fan engagement to better control engine coolant temperatures. Model year 2005 vehicles already have this calibration installed. Refer to Corporate Service Bulletin Number 04-06-02-005 for more information about this calibration.

Proper diagnosis of any customer concern is essential. Refer to the Diagnostic Starting Point-Heating, Ventilation and Air Conditioning procedure in SI to begin a diagnosis of any HVAC concerns. Refer to the Diagnostic Starting Point-Engine Cooling procedure in SI to begin a diagnosis of any engine cooling or engine cooling fan concerns. If an attempt is made to diagnose an HVAC system concern, all parameters must be measured. As an example, both the low and high side refrigerant pressure readings must be measured. If only the low side pressure readings were measured, it would appear that the A/C compressor is disengaging at a higher pressure than expected. If both the low and high side pressures were being measured, it would be noted that the A/C compressor is probably disengaging due to the maximum high side pressure cut out point being reached. This occurs to protect the HVAC system from damage. The updated PCM software calibration along with the updated EV fan clutch will improve this condition due to increased air flow across the condenser. The new EV fan clutch has an increased minimum operating fan speed and an improved response time. Additional engine cooling fan noise can be expected with the updated PCM software calibrations.

The last area of customer concern is the operation of the HVAC system blower motor. For vehicles equipped with automatic control HVAC systems (RPO CJ2), the blower motor fan speed will be limited to 80% of maximum when the HVAC system is operating in the automatic mode. To obtain maximum blower motor speed, instruct the customer to manually adjust the blower motor speed to the highest setting.
 

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A/C not working at idle

I think it has to do with the fact that while sitting at a traffic light in the hot weather there is no air moving through the condenser that sits in front of the radiator. My 07 vette did that same thing in 95 degree weather when it was new. As soon as I started moving the a/c worked fine. Hope this helps...
 

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2003 chevy trailblazer_ltz
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I think it has to do with the fact that while sitting at a traffic light in the hot weather there is no air moving through the condenser that sits in front of the radiator. My 07 vette did that same thing in 95 degree weather when it was new. As soon as I started moving the a/c worked fine. Hope this helps...
Not trying to be an a$$ here, but that was established many posts back ;)

Now we're all just trying to figure out a solution :)
 

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2003 chevy trailblazer_ls
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Does the engine seem like it's "idling funny" or about to stall when it's idling with the A/C compressor on? :confused:

Also does this happen all the time? Or just at a 'cold start' idle, or idling at temp?
I noticed my A/C not working well at idle, now at the idle the engine runs rough and then stalls. Don't have a clue.
 

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I have an 04 trailblazer with 83K miles on it and have had the same issue with the low AC at idel. Will there be any cost if I bring this to the dealer to have them reflash the pcm?
 

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2004 gmc
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... Will there be any cost if I bring this to the dealer to have them reflash the pcm?
Out of warranty, nothing's free. Some dealers charge an hour minimum for a 10 minute job. :weird:

If Bill Gates charged for EVERY security and stability Windows update, civilization would cease. GM thinks their stuff's better.
 

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2003 chevy trailblazer_ls_ext
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......For 2004 model year vehicles and older, update the Powertrain Control Module (PCM) with a software calibration that was developed to improve coolant temperature gauge readings. This new calibration will cause the PCM to command small amounts of additional fan engagement to better control engine coolant temperatures. Model year 2005 vehicles already have this calibration installed. Refer to Corporate Service Bulletin Number 04-06-02-005 for more information about this calibration.[/B].....

So who, besides the dealership, can do this PCM update? Im new to this forum and just bought a 2003 Trailblazer EXT this past weekend with this same symptom. I purchased a can of R134a and after putting the guage on the system, noticed that it says that it is fully charged.

Any help would be appreciated.

Thanks!
 

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2004 gmc
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Nobody except PCMforless, if you buy a replacement PCM with a tune in it. Best bang for the buck mod if you're into mods. But if you don't want to fiddle with performance enhancements yet, the dealer update is cheaper. But call around, since prices can vary.
 

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2003 chevy trailblazer_ls_ext
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The only "enhancement" Im concerned with at the moment is MPGs! Brought this truck up from where I bought it and got 15.9 MPGs on the highway. Will the PCMforless tune help with that, as well as the A/C issue?
 
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