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1993 Coupe DeVille
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Hello all... I have a 1993 Coupe DeVille, 61K Miles. Having issue with refrigerant pressures. Vehicle has been converted to R134A, however, the service ac soon indicator comes on and pulls code F48, also pulls F11 on occasion. Can anyone please advise me on what pressure switches need to be replaced or updated or exactly what I need to do about the pressure diff between 12 and 134? I read another thread that suggested replacing the OT with a blue .067, but I do not know what that means. Any help is much appreciated. The Alabama heat is killing my grandmother and she has already spent 2K having the problem hastily addressed by the warranty work only pigs at the now-closed local dealership. Thanks again! greg
 

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1990 350 Brougham (The cruiser), 1988 Mark VII LSC (The DD)
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5,545 Posts
Refill the system with 75% of what is recommended with R-12. If it wants 3 lbs of R-12, refill it with 2.25 lbs of R-134.
 

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2005 Cadillac DeVille DTS
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150 Posts
These vehicles should not be converted, precisely because of the issues you're now dealing with. GM even issued TSB's on the subject.

The pressure transducer and ECM was calibrated for R-12 pressure curves, so there's two schools of thought on how to address the differential with R-134a: electrical modification, or pressure modification.

In my opinion, the best alternative is pressure modification, substituting the GM white OT at .072 with a Ford blue OT at .067 - you'll need to recover, pump the system down, and recharge following the repair. You’ll need to charge by pressure, since the original system capacity is no longer applicable.

The other alternative is electrical modification. You could try shunting the transducer on the low with a 2,200 ohm resistor, and 60K on the high (to kick the fans on faster). You might need to play with the resistance values though – variable pots would be your friend. Of course, this doesn’t take into account the P/T relationship, which should be considered (hence the .067 OT suggestion).

Joe
 

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2005 Cadillac DeVille DTS
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150 Posts
No expansion valve on the DeVille, the expansion device is an orifice tube (OT). If I recall correctly, it's located in the evaporator inlet. There's a special tool for removal, but typically it'll come out just fine with a needle-nose plier.

This isn't just a matter of swapping out OT's - you need to recover, evacuate (longer the better, or below 500 microns), and charge according to pressure, paying extra attention to discharge pressures. If acceptable discharge pressures cannot be obtained, you'll likely need more condenser and/or fan capacity.

Joe
 

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1990 350 Brougham (The cruiser), 1988 Mark VII LSC (The DD)
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The orifice tube is located in the AC line that runs from the condenser to the evaporator. It is in the middle of the line near a fitting. If you look at the tube you will see two small indents in it, like someone pinched it with a pair of pliers. That is where the orifice tube sits.
 
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