SOLVED! A/C Permission Withheld

[NJTB]

Hi, all.
My 2004 TB with manual air works very intermittently. I know there's a charge in it ( maybe a little overcharged, added a can of dye to it), and all the buttons and switches work, it was tested with a Tech 2.
So here's my problem. While running through all the tests the Tech 2 has one line that says A/C permission withheld. If I command it on, the a/c blows cold, no noises or anything. As soon as I exit the program, warm air.
My question is this-why does it say that with all the controls operating properly, and how do I fix it?

 

[Mooseman]

Edited title for better responses.

If the PCM is not allowing A/C to function, it's not happy about something. There are a number of conditions it needs met to allow the A/C compressor to work. Some are low pressure switch, high pressure switch, above freezing ambient temp, engine not overheating and not at WOT, to name a few. There may be others.

You would need a set of A/C gauges to measure your pressures to be sure there is enough charge. The Tech 2 should tell you which condition is not met to allow A/C to work.

 

[mrrsm]

It sounds like the HVAC Controller is alerting One or More of the Blend Door Actuators to move between the Positions it MUST remain in for either Blowing Hot or Cold Air. I'm thinking that it is the one that's more or less positioned over the Transmission Hump under the Dash Panel.

IIANM... As soon as the Blend Door assumes either one position...Feeding the Blower Air over the AC Condenser Coil... or the Other feeding directly through the Heater Core, there would be a fairly rapid transition between the Cold or Hot Conditions.

Having the GM Digital Service Manual from @Mooseman to refer to will help to nail down which of the Blend Door Actuators is the possible Culprit:

Need service manuals? Get them here!

********* Please note that the FireFox browser has issues with the password handling and that these downloads are not under HTTPS, flagging them as security risks. Chrome and Edge work fine once you allow the download. ********* ATSG 4L60E Manual 4L60E Rebuild Manual 2005 GMC Envoy XUV...

gmtnation.com

 

[NJTB]

Mooseman
That's what I'm having trouble wrapping my brain around. If I use the Tech 2 to set it to permission granted, everything works and the A/C gets cold. The screen shows a bunch of 'normals and ok's', all the buttons check for on and off, blend doors change positions, etc. There's nothing that reads out of range or won't switch.
To me, that says the wiring, relays, etc. is ok. And yes, I'm figuring the PCM sees something it don't like, but damned if I can figure it out.
I spent some time yesterday on Google researching 'permission withheld', and read someone with a Sierra had an A/C clutch that was drawing too much amperage and causing the system to shut down. This was after he replaced almost all of the electrical components. Must have spent thousands.
By the way, if this helps anyone else out: some of you may remember my 'Service 4WD' post, where I had to use a 'T' body (in the Tech 2) to get the transfer case data to show up.
That didn't work for the A/C, the info was different for the radio. Went back to the 'S' body and got right in.
Thanks for the replies Mooseman and MRRSM, I appreciate the responses.
I'm really tired of fighting with this thing.

 

[Mooseman]

I spent some time yesterday on Google researching 'permission withheld', and read someone with a Sierra had an A/C clutch that was drawing too much amperage and causing the system to shut down. This was after he replaced almost all of the electrical components.

I can't see how the PCM would see the current draw as the A/C clutch is controlled by a relay.

Have you tried just jumping the low pressure switch? It could be defective or it's detecting a low pressure condition. Even a minimal amount of refrigerant would cool. In fact, an undercharge can cool too much at lower pressures with the switch bypassed.

There should be a chart or something in the manuals that gives the conditions to allow the A/C to work. I don't have access to my GM-SI laptop right now to search.

 

[Chickenhawk]

With the high failure rates of the low pressure clutch switch and the low cost of a new one, I would start there.

 

[NJTB]

Yes, I jumped the low pressure switch and the clutch didn't engage. That's when I knew I was screwed. Plus the Tech 2 says normal on operation.

 

[Hatchet]

Any problems with the fan clutch?

 

[Mooseman]

Or codes?

 

[budwich]

Mooseman
That's what I'm having trouble wrapping my brain around. If I use the Tech 2 to set it to permission granted, everything works and the A/C gets cold. The screen shows a bunch of 'normals and ok's', all the buttons check for on and off, blend doors change positions, etc. There's nothing that reads out of range or won't switch.
To me, that says the wiring, relays, etc. is ok. And yes, I'm figuring the PCM sees something it don't like, but damned if I can figure it out.
I spent some time yesterday on Google researching 'permission withheld', and read someone with a Sierra had an A/C clutch that was drawing too much amperage and causing the system to shut down. This was after he replaced almost all of the electrical components. Must have spent thousands.
By the way, if this helps anyone else out: some of you may remember my 'Service 4WD' post, where I had to use a 'T' body (in the Tech 2) to get the transfer case data to show up.
That didn't work for the A/C, the info was different for the radio. Went back to the 'S' body and got right in.
Thanks for the replies Mooseman and MRRSM, I appreciate the responses.
I'm really tired of fighting with this thing.

It would appear that perhaps your compressor has an issue... possibly. The system has a monitor lead for some check. Have you tried using a meter to measure resistances at the compressor of the pins that are there?

 

[budwich]

Yes, I jumped the low pressure switch and the clutch didn't engage. That's when I knew I was screwed. Plus the Tech 2 says normal on operation.

what does the AC switching parameter indicate when you select AC and then deselect. It should toggle. Maybe the switch is bust.

 

[NJTB]

Budwich-the A/C select switch is a momentary switch, and yes it does toggle. Where is this monitor lead?
Hatchet-fan clutch works fine, according to the Tech 2. Always runs ~195, thermostat and coolant sensor replaced last fall.
Mooseman-now there you go. It had a something C506 that went away (I didn't clear it). Just for jollies I got the GM shop manual out and followed the trouble tree.
The great part was the manual sends you to body-A/C-grant permission, then sends you to the Powertrain screen to check a sensor in the A/C. Problem is as soon as you leave the HVAC screen, the A/C goes to permission denied, so the sensor reads 'off' and I can't command it on. Go figure.
I'm on vacation and have spent several hours a day since Monday screwing around with this.
So, my new plan is to start all over, go to the relay pins and jump them to see if it comes on, etc. At least that will give me an idea which end of the system it's on.
It's also real possible I'm just going to bend over and take it to the dealer. Hate spending the money on it, since the thing is 15 years old and getting pretty rusty, but hey, the wife's complaining and we all know how that goes. Running and driving is perfect, I hate having car payments.
Thanks to all that replied.

 

[budwich]

Budwich-the A/C select switch is a momentary switch, and yes it does toggle.
........
Thanks to all that replied.

Are you saying the switch toggles or that the data in the tech2 display toggles with the switch action?

 

[mrrsm]

You've probably already tried this when you went through the GM Manual Diagnostic Tree... But jik not... It would not hurt trying to use your Tech 2 to Command a Re-Calibration of the Blend Door Actuator(s) ... assuming of course that this Feature does come up on your Tech 2 Screen for your particular Year/Make/Model of Vehicle. If the Options are allowed, the Tech 2 Screen Steps should look like these:

 

[NJTB]

Finally found it. Spent several hours going through the shop manual for HVAC diagnosis, and there's one trouble tree that sends you to the 'Powertrain' screen. There's a whole bunch more A/C information in there, and the parameters they're supposed to operate at.
The high pressure switch, that I replaced about 2 years ago, was pegged at max, so the PCM withheld permission. I replaced it, and the A/C gets nice and cold now.
But again, it's not the end of the story. The new switch has a different connector (old one discontinued) so you need to buy the connector and harness with it. The new harness is 3 white wires, and working in that cramped space is a giant PIA. Of course I mis wired it and had to go back and do it over.
Budwich - When I said the switch was momentary I mean when you're in the Tech 2 testing the switches, the A/C switch (the one with the snowflake) always shows 'off' until you push it. Then, for a second the screen on the Tech 2 shows 'on', then goes to 'off' again.

 

[budwich]

OK... thanks... glad you found the issue and also the tech2 "path to fruition"... almost tempted to pick up one (the "copy").

On a side note, you mention that the new unit is different than the old ... but the number of wires is the same just the colors or lack there of is different... right? Further, the switch that you indicate is not a high pressure SWITCH, it is a pressure sensor... which the PCM reads and gets a range for operation. On my truck, torque app is able to get the pressure reading from the PID and I display it in my "dash" to watch what's up. Not sure why the PID appears to not work in other year vehicles but the trailblazer system has the sensor and PCM operation happening in all years as far as I have seen.

 

[Mooseman]

Good you found it.

 

[NJTB]

Yes, you're right, it's a sensor, not a switch. I'm so damn nuts with this thing.
At least the wifes happy.
An Mooseman, you were SO right about the PCM seeing something it didn't like. The suggestion put me on a different path to finding it.
Also, couldn't have done it without the GM service manual.

 

[mrrsm]

...and another one Bites the Dust!

Nicely , @NJTB

 

[TJBaker57]

Not sure why the PID appears to not work in other year vehicles

Just today I edited the Torque Pro extended pid for AC High Pressure so it now works on my 2002 4.2. I see you have an 08 and somewhere along the way the pid changed. For my 2002 the pid is 221144, the equation is (A*1.83)-15, Min/max is 0/400, OBD header is Auto.

 

[budwich]

Just today I edited the Torque Pro extended pid for AC High Pressure so it now works on my 2002 4.2. I see you have an 08 and somewhere along the way the pid changed. For my 2002 the pid is 221144, the equation is (A*1.83)-15, Min/max is 0/400, OBD header is Auto.

great info... hopefully this will help those without a full set of gages...

quite a difference from the 2008 pid which is at 221564 with an equation of just A*02.02 with min/max 100/0. I wonder if the formula is actually good (either one) and whether they should be the same and just the PID location is different?

 

[TJBaker57]

I wonder if the formula is actually good

Well I thought I might check the results against my gauge set.

 

[budwich]

It probably doesn't matter one way or the other in terms of the formula as if you look at it, the actual numbers are about "10% apart". Maybe a gage check will help although the movement and time delay may not readily correlate. Anyway, it was one of those "wonder if" type questions especially with the nature of torque where "pid discovery" is somewhat of an art as opposed to a science unless some one "snipes" an actual manufacturer's data definition / legend.

 

[TJBaker57]

So it turns out the pid & formula I found online is the correct one for a 2002 4.2 TrailBlazer as shown by the attached pictures. The erroneous pressure shown on the top right of the Torque screenshots is the 221144 pid with the formula as it comes with the GM extended pid set.

 

[budwich]

good stuff.... but just to be clear... what is the formula that "comes with the GM extended set"... A*02.02... I assume cause I don't think I have ever modified it.

Great... thanks again... it is probable that only the PID position changed and indeed the "torque discovery conscience" is off.