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Cadillac thermistor issues

Thermistorfied on Sun June 03, 2012 3:26 AM User is offline

Year: 1990
Make: Cadillac
Model: Eldorado
Engine Size: 4.5L
Refrigerant Type: R134A
Ambient Temp: 70
Country of Origin: United States

Help please! I know this has been covered in some old postings but I am batting zero so far.

I have a 1990 one owner 32K Eldo I bought in SC last summer. It is pretty much like new, but I am having issues with the AC. It was converted to R143A in SC in 2007 and appears to have been a professional job. AC blew cold when I bought it last July. About September of last year I started to get the "low ac refrigerant, service ac soon" warning (B446) as I have now come to know it. It was stored over the winter. This spring I was getting both B446 and B447 codes and it locks out the compressor. It appeared to have an adequate charge of refrigerant, so I began researching it a bit. I tried the combination of just about every resistor combination I read about on the low and high temp sensors, without being able to get rid of it throwing those two codes. I could clear the codes and it would operate and cool fine until about 3 miles at highway speeds, at which time it throws the codes and locks out the compressor until the codes were reset. I was getting frustrated to say the least!

I then took the car to a VERY reputable shop that does all of my mechanical work that I can't, or do not want to do. He is good at AC work. I had him evacuate the system, replace the stock orfice tube with the blue ford orfice tube, vacuum and leak check the system and recharge it with the proper amount of R143A (90% weight of the original R12 charge). It once again blew nice and cold and appeared to be cycling like it should. Until I drove about 3 miles, and again it throws the "low ac refrigerant, service ac soon) B446 warning!!! It did not throw the B447 code yet at this time. I took t home and tried the 2K resistor in parallel across the low temp sensor. After 3 miles it throws both B446 andB447 and locks out the compressor. I reset the codes moved the 2K resistor to just one of the wires in the low temp sensor, and it holds nice for about 12 miles and I think I may have it licked. Then once again it throws both B446 and B447 and locks out the compressor.

I am at my wits end and don't really know what to try next.

Any suggestions would be greatly appreciated!!

Thanks,

Lee Anderson
Durand, WI

NickD on Sun June 03, 2012 5:49 AM User is offline

Recently got rid of my 92 DeVille, had it a very long time, but kept it R-12, one common problem is that these compressors came with a single lip seal, a double seal is available. Best cure I have heard of when switching to R-134a is to change the orifice from a 0.072" size down to a Ford 0.067" size that helps to increase the low side pressure.

Problem with R-134a, just can't put enough of it in to bring the low side up to about 30 psi without blowing up the high side with that tube and fin condenser. Should have a parallel flow condenser. That smaller orifice helps to bring that low side up but with some decrease in performance. But still have foam.

If your system is cutting out, kicking out that 8 psi low pressure switch, so your low side is dropping below 8 psi and that is a switch. Should have installed an R-134a compatible accumulator and PAG after a very good flush.

I said the heck with all that stuff and just stuck with R-12, was cheaper for me and much prefer mineral oil over PAG. That's just me. That ban is 18 years old now, had friend with a bunch of R-12 cans they didn't know what to do with. Have any friends like that?

GM Tech on Sun June 03, 2012 6:53 AM User is offline

And you changed it from R-12 for what reason? This is why we don't convert- buy some R-12-- have that "ace" mechanic install it--he should know better--- YOU don't convert Caddies for the very issues you are having....

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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......

Thermistorfied on Sun June 03, 2012 7:01 AM User is offline

The accumulator and oil were properly changed when it was originally converted in 2007.I just had the smaller blue orfice tube installed a couple days ago when it was evacuated and vacuum checked for leaks and recharged. I was hopeful the smaller orfice tube would take care of the problem, but it did not. It appeared to have helped as the B447 code appeared to have been cleared up, but the B446 code returned. I don't know the history of the car from 2007 until last summer, but found it hard to believe it worked properly during that time and just started acting up now. Unfortunately I don't have any easy access to R12. At this point I would like to make it work with the R134A as all the work has been done. If you come up with any more suggestions I would appreciate the advice. Thanks for the reply!!

Thermistorfied on Sun June 03, 2012 7:03 AM User is offline

I did not convert it. That was done 5 years before I bought it. I'm trying to work with what I have. Thanks!

GM Tech on Sun June 03, 2012 7:36 AM User is offline

Well whomever converted it should have known better-- there are many more horror stories such as yours then then are success stories on Caddy coversions on the web! I've never seen one work right after converted! Go ahead- pursue the conversion- but if you want no more issues- change it back to R-12. R-12 is just about the same cost as 134a- and is available online. It is just that a/c shops don't want to deal with it. They don't have your best interest in mind. They want you in and out and your money and if you are not happy they want you to not come back.

Caddies are a different beast- We tried to convert them back in '94 and decided at that point it was futile- those thermistors feed an algorithm that is programmed to the properties of R-12, which ARE different from 134a. So the easiest, least expensive thing to do is to keep it R-12. If you insist on using 134a, you will forever have issues- It is truly my belief that no one talks about their successes converting Caddies because there are none-- maybe for a short while- but eventually they just drive hot, and don't talk about them anymore.

I've been doing a/c for 30 years- and am trying to give you the best advice-- it is your choice if you think I'm all wrong...

-------------------------
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......

GM Tech on Sun June 03, 2012 7:39 AM User is offline

NickD - Caddies respond by thermistor readings of refrigerant temps before and after evap- that is what determines cycling and code throwing- the low pressure cut-out switch is just there so to save the compressor from running low on lube when refrigerant level is extremely low.

-------------------------
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......

Thermistorfied on Sun June 03, 2012 9:46 AM User is offline

GM Tech,

Thanks for the advice, and I don't think your all wrong. I have just been trying to make the best of what I have if I can get it to work properly, and am beginning to seriously question if that is possible. My preferable route would be to switch it back to R12, but what kind of escapade is that going to be and what kind of pitfalls may I expect? Is it still possible to find a shop willing to do that these days for a reasonable price, and what should I expect them to do?? New accumulator, flush system thoroughly of all refrigerant/oil and recharge with R12 and mineral oil??

Thanks for your advice,

Lee

NickD on Sun June 03, 2012 10:25 AM User is offline

Feel I am aware of the high and low temperature thermistors. Self diagnosis even shows the temperatures in Celsius. But think in term of pressures so convert C to F, then look at a PT chart for the corresponding pressure. To check the accuracy, compare that derived pressure to my manifold gauge readings. Could have a defective or out of range thermistor.

But the AC system will still work as long as that mechanical cutoff low pressure switch doesn't open, if good, and can be tested, the low side pressure is dropping below 8 psi, and that is low. That is what kicks the AC system into Econ mode and kills the compressor. Excessive high side temperatures will also kick the ACC into Econ mode, this can be very easily done with R-134a because its temperature/pressure increases at a high rate above a certain point where R-12 increases proportionally.

Best bet is to connect manifold gauges and read in English exactly what the problem is.

One possible scenario is with these high pressures, bits and pieces of the compressor is breaking down and plugging up the orifice, it was already changed, how did the old one look?

Suppose if you can't find a good shop to convert back to R-12, do what I do, actually for all my MVAC work, do it yourself. Gets done right and only at a small fraction of the cost.

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