ATC & A/C... this is a wierd one.

[RJansen658]

Ok so today I finally got new o-rings for my compressor, put them on, charged up the system with 3 cans r134a. And I had A/C!

 

Had being the operative word.

 

I shut off the car, came back out 10 minutes later to bask in my accomplishment, and it was blowing hot air out the vents. I popped the hood, the compressor was on, the lines had condesation on them, and I was getting the telltale drips of condensation under the car. I sat back down in the car, still blowing hot. I was holding my hand to the vent, and it suddenly blew ice cold for about 2 minutes, then back to hot again. Then I remembered that in the winter, the opposite would happen with the heat suddenly going cold for a minute or two, then back to heat.

 

The car in question is a 93' Cutlass with the digital temperature controls. I am assuming this system somehow measures the cabin temperature, and diverts flow thru the blend door to change it, but it does it even when Auto control is off. I barely ever blows cool.

 

I pulled out the glove box, which allows good sight of the heater and A/C mechanisms, and I can see all the vacuum operated servos, but none of them seems to be used to divert air either thru the heater core or the A/C evaporator. It looks as if all the vacuum servos are for changing between vent/floor/defrost/etc. I see a small box mounted on the side of the HVAC box, that looks as if it may be used to operate a door, am I correct? Does anyone know if there is a sensor somewhere that feeds the ATC the cabin temp.?

 

KrazyK I could use some of that Cutlass knowledge....

[kuwaitliberation]

Does anyone know if there is a sensor somewhere that feeds the ATC the cabin temp.?

It should be located between the two vents on the passenger side. It is small and has a horizontal grill. You can see the sensor in the hole.

 

Hope that helps a little.

[RJansen658]

So thats what that little grille is eh? Thanks for the info, I'll check it out';p

[Brian P]

yeah, more than likely the heat-door "motor" is getting the signal to stay hot, from a faulty auto climate control unit. You could try removing the motor and operate the door by hand, to see if it moves freely and you get your AC.

[Pontiac6KSTEAWD]

I have one of those controls I do beleive. Take a pic of yours and I will see if it matchs. Then we can deal from there..

[Crazy K]

just to make sure the compressor is always working???

 

 

here's what I think you should do. get and find a view of the damper control motor... take the glove box insert out, and run the vehicle and see if the think moves back and forth mysteriously in conjunction with the temp fluctuating.

 

also.... check and see if the outside or inside temperature readings are fluctuating drastically. We once had a rental GM whose outside temp sensor would sporadically fail, and when if failed, it would read -30 degree and shut the A/C off in the middle of an arizona summer. it was a 2000ish lesabre I think??? (not that it matters)

[shyguy164]

Are you able to turn off the auto function of the climate control? I'm not familiar with the pre 95 auto climate controls. I dislike the auto feature. I dont like the computer deciding where the air blows, when the compressor runs, etc. etc. etc.

[RJansen658]

Ok well, I have done a lot of testing today. First I took the cabin temp sensor out and measured its resistance, then put it in front of the houses a/c and then blew it with a hair dryer while monitoring. It changed resistance correctly, the sensor was good. I put it back in. I found the blend door motor and removed it. The door operated freely, and when I moved it I suddenly had cold air. I tapped on the blend motor, it stayed in the "hot" position. I wiggled wires, it stayed in the hot position. I left it hanging under the dash and pushed a few button on the control. It moved to "cold". I put it all back together. I then changed it from cold to hot a few times. It worked correctly. I shut off the car. I started the car. It was stuck on hot again. At no time does the outside temp reading fluctuate, it is accurate within a few degrees always. The set temp never fluctuates, it stays at what I set it to. Ithink it is either the control head, or the blend motor, but once I got the blend motor to change once, it worked perfectly until I shut off the car. Also, if I manually put the blend motor in the "cold" position with the car off, it returns to "hot" once I start the car.

 

Thoughts?

[RJansen658]

Are you able to turn off the auto function of the climate control? I'm not familiar with the pre 95 auto climate controls. I dislike the auto feature. I dont like the computer deciding where the air blows, when the compressor runs, etc. etc. etc.

 

Yes, I can turn it off, but it makes no differnece. It still randomly blows cool for a second then back to hot.

[Brian P]

Also, if I manually put the blend motor in the "cold" position with the car off, it returns to "hot" once I start the car.

 

Thoughts?

 

When you did this, was the motor re-installed when it went back to "hot"?

[RJansen658]

correct, I tried it multiple times, with the motor installed and not installed

[badassoldspower]

Ok you said you used 3 cans of refrigerant, which is 36oz or 2.25 pounds, wheras to recall, these cars take less than 2Lbs, if you did overcharge the sysem, pressures are getting so high during the time that it blows cold that your high pressure switch is disabling the compressor to save you from ruining your car, or yourself, make sure that you added the correct amount of freon, and no more.

[Brian P]

It took 3 cans to get my system charged on my old Oldsmobile. The 2nd and 3rd can you end up wasting some freon anyway.

 

But heck, check to see if the compressor is shutting off. Look at the compressor clutch and see if it stops spinning at any time (with the AC on)

[shyguy164]

Ok you said you used 3 cans of refrigerant, which is 36oz or 2.25 pounds, wheras to recall, these cars take less than 2Lbs, if you did overcharge the sysem, pressures are getting so high during the time that it blows cold that your high pressure switch is disabling the compressor to save you from ruining your car, or yourself, make sure that you added the correct amount of freon, and no more.

 

Overcharging by a quarter pound will not cause the high pressure switch to open. What overcharging will do is cause the refrigerant temperature to be higher. R-134a and R-12 have almost identical temperature-pressure charts. Ideally, you're looking for about 29-30 psi on the low side with the system running on max cooling. At 29-30 psi, r-12 and r-134a have a temperature of about 35 degrees, which is exactly what you want, just above freezing. As you add more refrigerant, the pressure increases, as does the temperature of the refrigerant. This increase in temperature and pressure will give you a higher surface temperature on the evaporator which leads to a higher discharge temperature, etc. etc. etc. If you'd overcharge your car by say 5 lbs or so, then you'd probably be able to get the high pressure switch to open. I'm not exactly sure offhand what it's set at on a car, maybe 200 psi? I know on stationary a/c, it's around 375-400 psi, but r-22 runs at higher pressures than r-12 or 134a.

[shyguy164]

Are you able to turn off the auto function of the climate control? I'm not familiar with the pre 95 auto climate controls. I dislike the auto feature. I dont like the computer deciding where the air blows, when the compressor runs, etc. etc. etc.

 

Yes, I can turn it off, but it makes no differnece. It still randomly blows cool for a second then back to hot.

 

I'm pretty sure the temperature motor requires voltage to move either direction, it's not voltage to open and spring return to close or something like that. Therefore, I feel pretty confident the motor is fine and the climate control is faulty. Do you have a chassis manual? It should have the diagnostic procedures to determine if it's the control or the motor.

[rockfangd]

Sounds to me lke the control unit is bad.