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Looking For AC/Heat Help on my '93 DeVille

BillT on Mon July 12, 2010 3:33 PM User is offline

Year: 1993
Make: Cadillac
Model: DeVille
Engine Size: 4.9
Refrigerant Type: R-12
Country of Origin: United States

To describe the problems briefly, I first had No Heat troubles in the spring, then AC troubles when the weather got warm. I brought it down to my local shop for an AC recharge, but he discovered that the Hi and Low Sides were pretty close to each other, which he determined was a Bad Compressor (Do you all agree?) I then checked my DTC's as recommended and one of the problems was a "F40" (Air Mix Door Problem). I'm about to order a new Compressor, Dryer and Orifice Tube, but my question is: will this automatically fix my "Air Mix Door Problem" once I install the new parts and have it recharged?

Here are all of the codes I was able to retrieve for additional info:

"F" means following are recent codes from the BCM
F40 -- Air Mix Door Problem
F46 -- Low Refrigerant Charge
F47 -- Low Refrigerant Charge
"FF" means following are historical codes
F47 -- Low Refrigerant Charge
"7.0" is ready prompt for next command

Do you all agree to now proceed with installing the new parts?

Any info would be appreciated.


mk378 on Mon July 12, 2010 3:52 PM User is offline

The air mix door is a separate system entirely from the compressor.

A code 46 causes the compressor to be "locked out"-- the PCM and / or BCM will not let it run until the code is reset. Then the compressor will come on and try again, but if the computer detects operating conditions that could mean an undercharge, code 46 is set again and another lockout.

System should be known to be fully charged by weight before trying to diagnose the compressor.

Edited: Mon July 12, 2010 at 4:01 PM by mk378

BillT on Mon July 12, 2010 5:16 PM User is offline

Appreciate the reply.

When the guy at the shop tried to recharge the system a short while back, he said it wouldn't take and said the Hi Side and the Low side were pretty equal. It seems like they were both at about 80 to 85 instead of 30 and 230. That is how he determined (his diagnose) that the Compressor was bad. It is still possible to fully charge the system at this point as far as you can tell?


GM Tech on Mon July 12, 2010 5:38 PM User is offline

Sounds like the compressor was not running--he'll put the new one on-- then fix the elcetricsl problem and charge you for the new one-- when it was the electrical problem all along---typical piss poor diagnosis.....Your present pump is probably okay....just needs a shaft seal.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

BillT on Tue July 13, 2010 8:56 AM User is offline

Appreciate the replies.

Do you all believe the Air Mix Door and the AC problems are related? or just a possible coincidence?

Also what would be the best way to tackle the Air Mix Door problem?


NickD on Tue July 13, 2010 9:51 AM User is offline

Since I have the same kind of car, the blend door is completely independent system, the heater core should be at 195*F and the evaporator at about 33*F giving a source temperature range of 33 to 195 degrees. In Auto mode, all the air, external or interior goes through the evaporator where it cooled, then either directly to the vents, or some of it is bypassed through the heater core depending the the position of this blend door. An in-car sensor that can be plugged up with debris senses the in car temperature and compares that with the numbers you have selected on the climate control, where if a difference occurs, sends a signal to the blend door servo motor, where this motor is contantly opening or closing this door in proportion to maintain that preset temperature.

This motor is located in a box under the right side of the dash called the programmer, shop manual does not tell you how to make repairs inside of this box, you can do it, but you are on your own. It also controls all the vent mode actuators and the speed signal to the blower control module.

The climate control is akin to a combination of your computers keyboard and monitor, it sends instructions via a data link to the body module computer, where this BCM is the brains, that sends visual data to your climate control and instructions to the programmer which is actually a bad name for this box, as it doesn't program, just follows instructions.

In regards to the blend door, as this vehicle lacks an air filter, debris can be restricting the operation of the door, or can be problems with the servo motor, or the linkage that moves the blend door. Thing is, you don't know until you look. It is a complex system that makes a guy wonder when you run into problems, wouldn't life be easier with just a manually cable operated blend door? Its suppose to be for convenience, but not very convenient when you have problems with it and can either freeze or get cooked to death.

I actually preferred these automatic climate control systems from the 60's that were all electro-mechanical and you can see what's going on and I am an electronic engineer. Here you are dealing with microcontroller pulse trains that are not explained, so they don't make it easy to troubleshoot. With your climate control, can manually control your blend door over the entire range, tells you if it is not working properly, but you have to find out why. That requires some serious digging.

BillT on Tue July 20, 2010 3:14 PM User is offline


Appreciate the detailed info. Every bit will help as these cars are new to me.



Dave in Texas on Tue July 20, 2010 7:29 PM User is offlineView users profile

If your AC guy did not clear the codes, it can't be charged even to test with a small-dose charge.

At 2, I went home with a 10. At 10 I woke up with a 2 !!

Edited: Tue July 20, 2010 at 7:33 PM by Dave in Texas

BillT on Wed July 21, 2010 6:53 PM User is offline

He didn't clear the codes. Glad I wanted a second opinion.


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