1997 skylark A/C pressure

[steve5555]

I have a 1997 Buick skylark with air conditioning. It is running but I do not have a gauge to measure high pressure line. I have heard I can measure the high pressure line transducer voltage and get a rough idea of the pressure. I would like to find out if my AC is in the ball park, to low, or to high of pressure, as car is 20 yrs old.

 

Static, compressor not running, 85 degrees F, voltage reads 1,2 volts (1.2 volts). With AC running, engine idle, max air/fan, the voltage after 5 minutes reads 2.72 volts. Outside temp is 89 degrees.

 

I am wondering if the AC is fine, low pressure or too high of pressure. Just checking.

 

Sincerely,

 

steve5555

 

[Doug_Scott]

Easiest way to test system is to get a thermometer with a range of 30 degrees to 120 degrees and a long stem.  Insert the thermometer into the centre dash vent set a/c to full cold, max a/c and a medium fan speed.  Don't use high speed, it can actually give warmer temp air.  Run the car at an idle.  You should see a temp reading in the 40s.  Hot humid days will give warmer results due to the a/c gaining heat from removing the water from the air. 

 

You could also use the hand method, grab a hold of the smaller a/c line exiting the compressor, it should be hot.  The line just before entering firewall should be cold.  It's hard to define how hot or cold, but I think uncomfortably hot and cold would work. 

[Supreme Pizza]

1 hour ago, steve5555 said:

I have a 1997 Buick skylark with air conditioning. It is running but I do not have a gauge to measure high pressure line. I have heard I can measure the high pressure line transducer voltage and get a rough idea of the pressure. I would like to find out if my AC is in the ball park, to low, or to high of pressure, as car is 20 yrs old.

 

Static, compressor not running, 85 degrees F, voltage reads 1,2 volts (1.2 volts). With AC running, engine idle, max air/fan, the voltage after 5 minutes reads 2.72 volts. Outside temp is 89 degrees.

 

I am wondering if the AC is fine, low pressure or too high of pressure. Just checking.

 

Sincerely,

 

steve5555

 

 

 

   Sounds like you know a lot about air conditioning. Are you sure that you need assistance? 

 

  

https://www.ebay.com/itm/R134A-R22-R12-Manifold-Gauges-Set-A-C-Tester-Service-Diagnostic-Air-Conditioning/382489557005?hash=item590e27780d:g:6w0AAOSwvjBbIPIU

 

[steve5555]

Being so old, I am afraid of too much pressure and blowing a line/part. I had one tell me that a rough estimate is

10 psi for every tenth of voltage of the high pressure sensor. If that is fairly close, then 2.72 volts would be 270 psi.

 

What I don't know is what the Buick factory specs say is max and minimum; if my reading is normal, plenty low, or pretty high and will blow a line etc.

 

Last week, the A/C failed but it turned out the clutch relay failed, even though the digital meter measured the relay as being fine. Contact resistance was virtually zero, coil ok, relay clicked, yet the relay did not work. A new one solved the problem. Measurements are funny, sometimes look good but does not tell whole story.

 

I will try to find a ther and check the temp. Just hope the AC continues to work.

 

Cheers and thanks for responses. Hope for more.

 

steve5555

[Dnt]

Your AC issue probably was NOT the clutch relay, most likely your issue is the air gap is just a bit too wide on the AC clutch due to normal clutch pad wear.  Clutch can be re-shimmed easily. No need to reinvent the wheel here, an earlier poster added in a link for a cheap gauge set that is a much more accurate way to test your system rather than messing around with a transducer output.  Get you a set and post the low and high side pressure using proper testing procedures. Ambient temp above 85f AC set to Max Blower set to high Doors open or windows down and engine at 1500 rpm. Are you an engineer by trade per chance????

[Jsdirt]

You're way overthinking things. No need to go checking sensors if they're not broken. Thermometer in the center vent, doors open, fan on high - if you're in the 50's there, you're good, since this is a worst case scenario - about the highest load you can put on the system. Idle, doors open, fan high. If you're in the 40's there,  you will freeze once the car gets moving. Most I see run the mid 50's, and some even into the 60's, depending on the size of the condenser, and design of the system, or if it's an R12 system filled with 134a (doesn't apply to you, however - just an example).

 

Just because a relay clicks doesn't mean there's continuity through the load side. Here's a prime example of that: 

 

 

[steve5555]

Js, I guess the AC is fine. Thanks for all the responses.

 

A couple of quick points. I could not believe the the relay checked totally fine. The contact resistance measured zero ohms (can't measure down to 0,01 ohms though). Being an electronics engineer, I have had such rare weirdness happen before with electronics when a part checks good, but is bad. It is frustrating and very much time consuming. Rare but it does happen.

 

The second is that I was not checking the transducer for quality, but to check the high pressure, since the sensor output voltage is related to the pressure itself. Another mechanic had told me the relationship was approximately 10 psi for every 0,1 volt measured. I wanted to double check here, since that would mean approximately 270 psi on the high side, at 89 degrees.

 

Again, thanks to everyone for your time and responses.

 

steve5555

[Jsdirt]

If that figure the mechanic gave you was accurate, I wouldn't trust that on every vehicle - I'd bet they'd all be different one way or the other. We do check signal voltage on transducers to be sure everything is working, but I've never heard of a scale directly correlating psi to voltage. I'd be willing to bet, the next sensor that gets thrown in there would be different from the OE, as far as what voltage  you see - I've seen this personally many, many times over the years.

 

You just proved why resistance is not the be-all, end-all check. You can have perfect continuity through a relay ... but hook a 5A sealed beam headlight through it, and you'll either see the light is orange, or extremely dim. I've seen relay contacts just completely nuked after breaking a bad one open - happens alot with today's quality components. Stuff just doesn't last, and the small, modern rectangular relays are a prime example of that. 

 

Glad we could help. I really wish there was a direct correlation to psi from voltage - sure would make my life alot easier!!

[steve5555]

Thanks JS. Much appreciated. Excellent points and hope others understand the best test is to swap, or replace the relay for a check. I was wondering how accurate the psi comment was, reason for starting this string. Glad you commented on such. Need to actually measure the high and low pressure side.

 

Cheers to all and thanks again. Over and out.

 

steve5555

[Dnt]

Quote by Steve5555 "Being an electronics engineer"   I KNEW IT LOL   

 Certainly nothing wrong with being an engineer and I am glad that your cars AC is working for you. I believe that the transducer is simply reading temperature rather than line pressure. If you delve deep into the properties of refrigerants you will find a Pressure/Temperature relationship for refrigerants that is used to calculate systems needs. Now if you want your mind completely blown and your world turned upside down, study the concepts of Superheat and Subcooling in regards to refrigerants. When I first delved into this many moons ago I found it hard to believe that their was actually such substance's with these properties on the planet earth. Just seemed like alien technology to me.  

Note: I am still leaning towards the AC compressor air gap as your "real" problem. the new relay may have allowed a bit more voltage through for the clutch magnet but eventually it will still fail to pull in due to the clutch pad wear. Just my guess. Next time you have an issue with it just take a long handled screw driver and carefully tap the AC clutch front face, if it immediately kicks in BAM, problem diagnosed and easily repairable.   

[Jsdirt]

The transducer reads pressure. The computer sends a 5v reference to the sensor, and the sensor pulls that voltage down with pressure, and relays that pulled-down voltage back to the computer. 4.99v would be empty, and .1v would be very high pressure 250-350psi, depending on the system.

 

Yeah, subcooling and superheat can make your head spin. I get the concept of it, but for auto A/C repair, it wasn't really necessary to completely understand those concepts inside and out. I always charge systems by feel, and use the gauges as a reference - high side tells me what the condenser is doing (if it, and the fan, are doing their jobs ), low side can tell you if the low pressure switch has failed (no cycling), if you have the correct refrigerant in the system, or if it's air contaminated (in conjunction with the high side gauge). Can also spot blockages (can find those by feel, too) or a weak/failing compressor. My hands are my most used tool on A/C systems, but that will be changing. Most new systems these days are TXV systems - normally the TXV is in the dash, and not external and reachable under the hood. Those I charge using the thermometer in the center vent rather than my hands. Also have 1234YF showing up in nearly every new model today, throwing more curveballs into what has been essentially the same system for the past 22+ years. There's an extra line on those that uses the cold, low side refrigerant to aid in cooling the condensed refrigerant that's headed to the TXV. The good news is, this VERY expensive refrigerant can be replaced with 134a ... but you didn't hear that from me.

[Doug_Scott]

Chrysler and Ford used TXV systems since nearly forever.  When I went through a/c "school" in the late 70s/early 80s (don't recall exactly) I took notes for both systems, still have them in my toolbox's a/c tools cabinet.  They were pretty dependable, don't recall ever having one plug up like the fixed orifice tubes. I figured they all would switch over to fixed orifice tube simply for cost reduction.  Fixed orifice tubes were dirt cheap, TXV weren't. Sounds like I got out just in time. 

 

Auto mechanics didn't need to be engineers to be good at there job.  We know what was supposed to happen, and for the most part we had no interest in why or how.  It simply wasn't needed, and likely would just over-complicate things.  We operate on a different level.

[Jsdirt]

Yeah, I couldn't have said it better. Our job was to fix them - only needed to understand just enough to get that job done.

 

I had an aftermarket TXV system in my '72 El Camino - an under-dash unit. Loved that system. I could use nearly any kind of refrigerant I wanted,  set the system up for it (with the correct oil), and it would always work. One time I charged that system with BBQ propane out of a 20 lb. tank, and the the thing blew 35°F air at idle, fan on high. It would fog up the rear glass every time I used it! Was friggin great!