A/C refrig. temp. sensor question?
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HT4100 4.1, 4.5, 4.9 Discussion, A/C refrig. temp. sensor question? in Cadillac Engine Technical Discussion; I have a bad A/C refrig. Temp. sensor or so the Caddy dealer says. They can't order the part becuase ...
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    A/C refrig. temp. sensor question?

    I have a bad A/C refrig. Temp. sensor or so the Caddy dealer says. They can't order the part becuase it's discon and say it's impossible to find. five minutes at carquest and I had the part?? Anyho, it looks like the sensor is connected to the ac line next to the battery and I was wondering if you had to empty the refrig. to change the part. Kinda of hoping you don't since the whole ac system was replaced and six months ago and charging the lines is about 100$.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: A/C refrig. temp. sensor question?

    You probably have to evacuate the system--and maybe even change the accumulator. Why do they say its bad? Is this the low temp sensor that you're talking about? You might not have enough refrigerant in the system. Was it converted to R134a? If so, a leak is probably possible. R134a molecules are much smaller than R12 molecules, so they tend to squeeze out of old seals.

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    Re: A/C refrig. temp. sensor question?

    When the system was changed a new compressor, accum, oil, freon, expansion line and an orifice tube was put in. It was done by the same guy who did my fuel system you're helping me with in my other post. He put 134a in, but without the conversion kit, he said (and I've read here) that the 4.9 accum. tank is big enough that you can fill it 3/4 of the way and it will expand in the tank enough to still cool the vech. The problem started about a month ago when the a/c would only blow cold air under 30 mph, above that speed, outside air only. He ran new freon through it and couldn't figure out the problem, so he took it to the caddy dealer in Bradenton, where there is supposed to be one the best techs in the state, so he says. They ran freon through it (lack of communication) and determined that the ac refrig sensor was bad, but didn't say to what degree. Since it was a part that the dealer didn't carry anymore, they didn't seem to care about fixing the car anymore. Gave the car back and five minutes at carquest, I had the AC DELCO part. I haven't run the ac since. The part number the dealer gave me was ac delco 15-71881.

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    Re: A/C refrig. temp. sensor question?

    Eek. Mixing refrigerants is not good. I'm not so sure that your problem is actually the low temp sensor. The code may have been set for that condition, but I doubt it's bad. If your A/C was only working under 30 mph, it's likely that it was low on refrigerant. For future reference, try not to convert old systems to 134a. Leaks almost always creep in. I noticed that he changed everything but the evaporator--probably the biggest source of A/C leaks, and also the hardest part to change.

    When the refrigerant is low, A/C sometimes only works at low speeds and idle. This is because the compressor isn't spinning as fast, i.e. it's not pumping the refrigerant through the system as fast as it does when the engine is running at higher rpms. This is a bit hard to explain, but basically the A/C systems cycles on/off as the low side temperature fluctuates between -2 degrees C and 10 degrees C. When too little refrigerant is in the system, the compressor has to work less to compress the freon and the low side temperature drops very fast from 10 degrees C (with the compressor off) to -2 degrees C, at which point the compressor will kick off. So, the compressor is always kicking on and off, never staying on for more than 6 seconds at a time (6 seconds is mandated by the computer).

    With the proper amount of refrigerant, the low side temperature will hover around 0 degrees C, and not fluctuate. Thus, the compressor will run almost continuously, and the air will get nice and cold. My guess is that your A/C is low on refrigerant. Try recharging it first.

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    Re: A/C refrig. temp. sensor question?

    According to the ticket from the shop, he charged me $75 for sealant and $98 for ac performance test, adding sealant and recharge of ac system. (after some debate about not fixing a thing, he dropped the charges.) He also mentioned, that the computer was telling the ac system to pump freon out of the system. I may be car ignorant, but even I don't buy that one. I turned on the ac today and it blew cold for about 20 seconds and then outside air from then on. Since you're helping me with another post, I'll combine them together. I replaced the ISC and it fixed the hard, cold start perfectly, thank you for that, I wished I talked to you before spending $320 on parts and a would be charge of $198 in labor. (also dropped the labor charges, since that was not the problem and he insisted it was) The FDC though, is still acting up, it didn't turn on for 25 miles after reconnecting the battery. Now it will only turn on when the car is started and it's still showing code F31. If I recall, the ac problems started with the ac light turning on and the mech. saying he had to reset the computer and it worked fine for awhile after that, so could it be a BCM problem?

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    Re: A/C refrig. temp. sensor question?

    OK, yes, your A/C issue could very well be a BCM problem. The BCM controls the A/C compressor and climate control, along with the fuel data display. However, it could just be a BCM display issue--not an issue with the computer itself. Hard to pinpoint, but if your heat works fine (fan speeds up and down as you increase the temperature), it may just be a bad display. In any case, I'd buy a BCM off of eBay for around $20 and stick it in--after my following suggestions...

    First of all, the computer can't "tell the A/C to pump freon out of the system." It's a closed system (if there are no leaks). The computer has no discretion over how much freon is actually in the system; it can only control the compressor. So, I wouldn't go back to that "mechanic." He has no clue about what he's working on. Second, once the computer senses a low refrigerant condition, it will not permit the compressor to engage in order to protect it. This can be temporary (when it senses a "somewhat low" refrigerant condition), or permanent--i.e. until the battery is disconnected--if it senses a "very low" refrigerant condition. Resetting the computer (as your "mechanic" did) will clear this code, and permit it to turn back on until it again senses low refrigerant. This code is triggered by the computer when it senses the low side temperature pressure moving too fast from 10 degrees C to -2 degrees C. As the temperature gets warmer, it will naturally take longer for the low side temp to decrease form 10 degrees to -2 degrees because warmer air is blowing over the evaporator. My guess is your A/C is "somewhat low."

    Anyway, here's what you need to do: 1) Pop your hood. 2) Turn the A/C on with the fan on low. 3) Listen for the A/C compressor engagement. It's on the accessory drive down by the radiator on the passenger side, below the P/S pump. 4) Now, count the amount of time it takes for the A/C compressor to kick off from the time it first engaged. Do this with the engine warm. If it only stays on for 10 seconds or so, you have a low refrigerant condition.

    To further verify this (with your fuel display working), first, turn your A/C on with your car idling in Park. Then, depress and hold the "Off" and "Warmer" buttons on the climate control until all digits illuminate. Wait for the FDC to display .7.0. Then, press the "outside temp" button. Press the "High" button until you see parameter 2.8. This is the parameter for the A/C low side temperature. Observe the values. Let me know if it cycles between 10 and -2, or stays around 3-4 at idle. Now, drive your vehicle observing the display. My guess is you'll notice it move from 10 to -2 very fast (after about 5 seconds). Also, watch the "Outside Temp" indicator on the climate control panel. When illuminated, this indicates the A/C compressor is on. Let me know if the light blinks off after 5 seconds from the time it first illuminates when you're driving.

    In short, your low side temp sensor likely isn't bad. You just need some more refrigerant. If that's the case, I'll guide you through how to add some.

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    Re: A/C refrig. temp. sensor question?

    Thank you for helping me with this. I have to drive to FT. Myers for the weekend, I'll take notes and post as soon as I can.

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    Re: A/C refrig. temp. sensor question?

    Sounds good! Don't operate the compressor too much on your drive. It's fine to turn it on for a few minutes, but I wouldn't leave it on for more than that.

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    Re: A/C refrig. temp. sensor question?

    OK, here's what I got. The compressor turns off 8 seconds after it turns on. On the FDC display in park, at idle, it goes from 9 to -2/-3 in under five seconds, within that time the "outside temp" light is lite. It takes about 10 seconds for it to cycle from -2/-3 to 9 and the "outside temp" light is off during that time. When driving, it stayed at 8 for 4/10ths of a mile, then went to 9 and shot to -2/-3 and took forever to climb back to 9. I looked at ebay and saw a BCM pretty cheap and since I'm still having FDC and F31 problems, I'd figure I'd throw one in, to see if it makes a difference with those issues and the A/C cycling problem. By the way thank you for all your help, I appericate it, esp. the ISC, it's soo nice to have the car start the first time!

    P.S. How bad is this? back around 15 April when I first talked to the mech., he walked me through a reset code for the A/C. It envoled setting the A/C to high, going into the diagnostic mode, after 7.0 and doing a few other things I really can't recall. Anywho, he told me to leave the A/C in high and not turn it off till I saw him on Monday evening, this was on a Thursday evening. After that Monday, I haven't used the A/C, except to trouble shoot.

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    Re: A/C refrig. temp. sensor question?

    Yeah, your A/C is definitely low. The critical time is when the A/C first engages. It sounds like it's only taking 5 seconds to go from 9 or so down to -2/-3. This should take much longer than 5 seconds. It makes no difference how long it takes to climb from -2/-3 up to 9, but if it's "shooting down" to -2/-3 as you say, you're low on freon.

    Here's what you need to do. Go to an auto parts store an buy an Interdynamics-brand refrigerant gun--one with a gauge on it. I think they're around $15. Then, but 2 cans of R134a refrigerant. It doesn't matter what brand. You can buy one with a sealer in it. When you get home, quickly screw the refrigerant can into the bottom of the gun. Give it a quick squeeze to purge the line of any dust. Remove the dust cover on the low side of your A/C system. This is the cold side when your A/C is on (cold=low pressure, hot=high pressure). On my Deville, the recharge nipple is up by the firewall. Start your A/C and turn it on max. If the compressor does not engage, you'll need to clear the code by entering diagnostics, then holding "Off" and "Lo" until F.0.0 is displayed. Then, exit diagnostics and turn the A/C on.

    Anyway, once you have the dust cover removed, you're ready to attach the refrigerant gun. The gun will snap onto the top of this recharge nipple. I find it easier if I pull the spring-loaded collar up on the gun, then push it down on the recharge nipple and release the collar. Once this is hooked up, and your A/C is on, squeeze the gun. You'll feel the refrigerant can getting cold. Advance the throttle a bit while you do this so that the engine is running around 2000 rpm. It's nearly impossible to overcharge the system.

    Once the can is about empty, start shaking the can until it's out. Then, pull up on the gun's collar and pull it away from the connection. Squeeze the refrigerant gun to release any pressure before unscrewing the can.

    Check the temperature at the A/C vents. It should be getting cold. My guess is you'll need more than 1 can to recharge the system. You'll know your A/C system is nearing recharge when you see the cooling fans come on. Repeat the procedure for the second can.

    Sadly, since you converted your system to R134a, this will probably be a yearly ritual.

    One thing bothers me about your previous posts: you say that the dealer mistakenly added freon to the system instead of R134a. However, this should be impossible to do. If your system was actually converted to R134a, then the recharge nipple will be the snap-on type. R12 recharge nipples have a "screw on" tip. The two are NOT compatible. So, before you buy the refrigerant gun, check the recharge nipple fitting. If it's threaded, your A/C mechanic didn't really convert the system to R134a, and I think you should have a "discussion" with him.

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    Re: A/C refrig. temp. sensor question?

    Thanks for the walk through, I believe i'm mis-using the word freon, I know the mech put 134a in, instead of R12, because he said he saved me about 200 dollars above what he charged me for the fill, instead of using R12. So I know he serviced it with 134a, He has a working relationship with the service manager at the dealer, so I would assume they also put 134a in it, when trouble shooting. My question is, with all this new 134a going in, why am I still low? I assume there is a leak somewhere. You are right with your comment about a yearly ordeal, it's been over a year (last March) since the system was changed out and about 2-3 months worth of problems. I appericate your time with this matter, I can't return the favor with car knowledge, but I can with medical issues (paramedic) or any type of Geograhical matter (getting my B.A. in Geography), feel free to ask and I'll check the fittings in the morning.

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    Re: A/C refrig. temp. sensor question?

    There's probably a leak somewhere--likely in one of the evaporator fittings. This almost always occurs when converting an A/C system from R12 to R134a. It's not really a problem if you don't mind the recharging procedure, which takes about 10 minutes.

    I took an online test to become a certified A/C technician so I could buy R12. I have 4 older GM cars, so it comes in handy. The test was $25 or so, and was very easy.

    Despite the fact that many people always want to replace the compressor, I've yet to encounter a bad one. Most of the time the vehicle just needs a recharge from sitting. If the compressor isn't making any noise, chances are it's in good working order. My suggestion would be to keep your systems running on R12 if possible in the future, since it's refrigerant properties are much better than R134a.

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    Re: A/C refrig. temp. sensor question?

    By the way, I doubt that the dealer truly "added any refrigerant" to your system when your mechanic took it there. My guess is they simply pulled the codes and told you that your low side temp. sensor was bad.

    However, by reading parameter P.2.8, the low side temp. sensor, (whose values were completely normal), you just proved that the low side temp sensor is, in fact, working correctly. If it wasn't--or if you had an open/short circuit--your temperature readings would be way off. So...

    1. The dealer was completely wrong about the temp sensor being bad.
    2. They probably just checked the codes and didn't really add any refrigerant.

    My guess is that after you add some, you'll be good to go. Then, you can go argue for more overpaid fees back.

    By the way, I'm leaving on a road trip for the weekend, so I might not be able to respond until then, but my guess is that you'll be just fine after recharging your system.

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    Re: A/C refrig. temp. sensor question?

    O.k., I put a new reman. BCM in, did nothing, the FDC is still interm. and shows a "C" atleast once a day (since the reman. units are around 170$, I can live with that) Still showing F.31 (shorted MAP sensor) even though it's within parameters and a new unit changed nothing. I put two cans of 134a in with sealant, still the same condition up to 9 race to -3 back and forth. The intial pressure on the low side, at 90 degress local, was 45. When both cans were discharging into the system, the A/C clutch would disengage and the pressure went to 100 (highest the built in gauge went to) and when the clutch turned on, the pressure would drop to 54, stabilize, then creep to 42 and then clutch would turn off and go back up to 100. I truely appericate the advice you have given me and the little problems with the car I can live with, but living in hell, I mean Florida, I NEED the A/C and this is beyond my abilities, so off the A/C shop it goes. I'll let you know the outcome.

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