A/C bc27-28 readings

[2seater]

          I have searched using different terms but have come up empty. There was a thread in the not too distant past regarding the temperature indicators, as stated in the title, for determining the refrigerant charge level. The FSM has little to say about it aside from the readings for the points the compressor will turn on or trip out. I still have the original R12 system which seems to work reasonably well, and a complete gauge set, but the readings on the move seem like a simple check under actual conditions. Just for reference, it is warm here today 88*, with high humidity 78* dew point, so I thought it would be a good test. At 2000 rpm and 50mph, the high side, bc27 remains around 40*C and bc28 cycles between approx. -4*C and +10*C. I think I may have made a mistake on the desired temperature setting which was 65*F. I think this temperature range is wider than was suggested in the previous thread. Any insight?

[Ronnie]

You might find this helpful:   A/C ECM/BCM Data

[2seater]

Thanks Ronnie. It gives me the info. I need. The post I was looking for is newer but very similar. I suspected my readings indicate a slight undercharge, which this tends to confirm. The one downside to my turbo setup is the difficulty getting to the low side fitting. I made every effort to make it accessible but even so, it is asking for blisters if connecting while pipes are hot

 

Related to this: has anyone rebuilt, or resealed their own compressor? From the rash of bad rebuilds I remember from a while ago, I might try it on a donor compressor from the beater '89 I have.

Edited June 13, 2017 by 2seater (see edit history)

[DAVES89]

David [drtidmore] did a post that I think you are looking for. It is dated March 21, 2017

[2seater]

10-4 Dave. That sounds about right. Now if you guys would just teach me to do a better search, that would be golden. Thanks much for the guidance.

[DAVES89]

2seater I have been fortunate to remember who says things. I first start with Ronnie's site. Then when I have an A/C issue I look for Mc_Reatta or drtidmore. Mechanical is Ronnie. Electrical is Kdirk. I used to look at Padgett but he has posted way too many times to be practical. 

 In regards to how I found drtidmore's post, I went to "search" Topic was "range" poster was "drtidmore" Then I scrolled through his posts.

[89RedDarkGrey]

1 hour ago, 2seater said:

Related to this: has anyone rebuilt, or resealed their own compressor? From the rash of bad rebuilds I remember from a while ago, I might try it on a donor compressor from the beater '89 I have.

 

Straight from a certified HVAC Tech- Maybe not the same model- but you get the gist

 

https://www.youtube.com/watch?v=1zZyeEgDdyY

 

and charging then testing

 

https://www.youtube.com/watch?v=eadCGdwknTM

 

[2seater]

Perhaps there is a difference in the reaction of R134 and R12, but the information from McReatta, R12, and drtidmore, R134, is opposite regarding the charge level? The R12 thread mentions temperature cycling as undercharged, and the R134 indicates overcharged if cycling?? I guess I will put the gauges on it as it is then easy to add if I need to.

 

Thanks for the tips and I did watch the videos too. The price for the compressor in the Reatta store makes a rebuild less likely, just need to change plugs.

[Ronnie]

You should make sure the AC sensors are giving the BCM accurate readings before trying to get the system to cool by adding Freon etc. If a sensor is bad, especially the low TEMP sensor, you will not be able to get the system to cool properly. It took me a couple of years driving with a AC system not cooling properly to figure that out.

A/C Temp Sensor Troubleshooting

[89RedDarkGrey]

Is it true that the Thermocouple clipped to the Evaporator can come loose, giving erroneous readings? I would also change the accumulator too, R&R all O-rings and silicone paste them. Flush out all lines, evaporator, condenser with brake cleaner, then air, then vacuum, then install new parts. A new variable Orifice Tube will help. When I overhaul mine soon- I'm using the PC Duster refrigerant method. It's as good as R12, and way better than 134a.

 

https://www.youtube.com/watch?v=3wkBnhcyO3Y

https://www.youtube.com/watch?v=i4uAXDyICsA

 

FULL REBUILD AND CHARGE

https://www.youtube.com/watch?v=JVA5LPS34MA

https://www.youtube.com/watch?v=1S1wjsIGWZQ

 

FOLLOW UP RESULTS

https://www.youtube.com/watch?v=Eh0rsYnFM6U

 

[Ronnie]

The Reatta AC system doesn't have a thermocouple clipped to the evaporator to my knowledge.

[2seater]

I certainly understand checking the accuracy of the sensors, but that wasn't what I was noticing in the two reference threads. It appears the reaction to over and under charge are opposite to each other depending on which refrigerant is being used. That may be completely true, but could have you chasing your tail if following the wrong directions.

[drtidmore]

The first thing is that you MUST be close to having a correct refrigerant charge to be able to tune using the low temp reading.  If the charge is way too low, the refrigerant tends to phase change from a liquid to a gas even before it reaches the orifice.  This is all due to pressure & temp in the closed system.  In a properly charged system, the refrigerant is a liquid until the orifice (or expansion valve for some systems) at which time the restriction creates a low pressure situation which forces the refrigerant to rapidly expand and in so doing dropping in temperature.  

 

Probably the MOST perfect car A/C system was the old suction throttling system (designed by Frigidaire when they were part of GM).  In that system, the suction-side pressure, and in turn the evaporator temp, was maintained by "throttling" in a small amount of high pressure refrigerant which maintained the evaporator temp right at freezing at all times (there is a direct correlation between the evaporator pressure and the temp). This system was so much better that Ford bought it and installed it on their Lincolns while other Ford products used a cycling compressor.  The downside was that the compressor ran constantly and when fuel economy standards came into being, suction throttling and its constant compressor operation went the way of the dinosaur.  

 

A perfect optimized charge level on our Reattas is the point where the compressor cycles the minimum (preferably no cycling) and the low temp sensor hangs around 0 to -1.   GM intended for the compressor to cycle from an economy perspective, but constant cycling is hard on the compressor and less than ideal for max cooling, especially when running R-134a.

 

If the system is constantly cycling the compressor the average temp at the evaporator is running several degrees above ideal, so the logic is to have a slightly overcharged, yes overcharged, system which does NOT allow the pressure on the low pressure side of the orifice to drop as low preventing the low temp from reaching -2, allowing the compressor to remain engaged and resulting in a lower average evaporator temperature.

 

If at highway speeds, with the A/C set to MAX COOL, you find that the low temp drops to -2, then cycles up to 9 then back to -2 and keeps repeating that indicates that the charge is just a TINY bit low for optimal operation, however it is NOT undercharged technically speaking.  The easiest way to reach optimal is to charge it to the point where the compressor cycles, then add in a bit more and test drive.  If the cycling is still occurring, add in a bit more and retest.  You will likely overshoot the ideal point, but it is easier to release a tiny bit from the manifold than to add a similar amount from the can.  It takes several attempts to find that sweet spot, but again, if at highway speeds the low temp reading is 0 to -1, you are THERE!  If you find that the temp is remaining above the 0 to -1 with NO compressor cycling, then you have a tad too MUCH refrigerant in the system.  Remember that the goal is to be JUST enough overcharged to keep sufficient low PRESSURE on the suction side of the system to prevent the refrigerant from dropping to the -2 trip point that forces the compressor to cycle.  

 

One last thing, as the interior of the car drops temperature and the Reatta begins to mix in a bit of warm air to maintain the climate control setting, the compressor WILL begin to cycle regardless.  The goal is prevent compressor cycling while maintaining the evaporator between 0 and -1 until such time as the interior reaches the climate control set temp.  In hot humid summer weather, the compressor may NEVER cycle when optimally charged.  

 

FYI, the low temp sensor is IMMEDIATELY after the orifice, so you can get an good idea if the low temp sensor is reading close by using an infrared thermometer (gun type) focused on the joint between the tube after the orifice and the low temp sensor.  The infrared reading will be a little higher as the actual temp sensor is IN the stream of the expanding refrigerant vs the outside of the tube where the infrared is getting its reading.  

 

Hope this clarifies things a little.

 

 

Edited June 13, 2017 by drtidmore (see edit history)

[2seater]

Thank you so much. That makes perfect sense and I think I now understand the goal. I did a little more investigation and unless I disassemble a few engine air intake items, items not shown in my signature photo, access to the low side fitting will be essentially impossible. I am preparing my other engine for installation and that seems like a good time to install a new compressor and sort of start from scratch. It will have a different turbo setup and should have better access to the low side fitting although I can operate the engine N/A until the a/c is completed. It appears the present compressor is seeping oil so I figure it is on borrowed time so I tend to limit a/c use until it will be changed.

[89RedDarkGrey]

4 hours ago, 2seater said:

It will have a different turbo setup and should have better access to the low side fitting although I can operate the engine N/A until the a/c is completed

 

It would be very easy to relocate another charge port, wherever you need it to be. Remove the Schrader valve, attach an AC hose with blue Loctite, New Schrader valve at end.

 

https://coldhose.com/

[89RedDarkGrey]

Using the website I posted- https://coldhose.com I figured a 12' hose, with appropriate fittings would be about $40.

[2seater]

Interesting. I will give that serious consideration.

[89RedDarkGrey]

Has anyone of our Electronic Gurus here determined the value of the OEM Unobtanium "Low Temperature Refrigerant Sensor" to come up with a suitable replacement- if anyone should need one?

 

( Part #9 in diagram)

[Ronnie]

23 hours ago, drtidmore said:

The first thing is that you MUST be close to having a correct refrigerant charge to be able to tune

 

That was a good write-up on optimizing the AC system. Unless you object I intend to use it to make a ROJ tutorial with the information it contains about how to fine tune the AC system..

[drtidmore]

Just now, Ronnie said:

 

That was a good write-up on optimizing the AC system. Unless you object I intend to use it to make a ROJ tutorial with the information it contains about how to fine tune the AC system..

That is fine with me.  I thought it time to explain the why as well as the how so that the process would make sense.  The reason I brought up the old GM suction throttling system is that it worked exactly the same way but dynamically and it worked extremely well. 

[Ronnie]

Thanks. It was my intention to leave out the part about the suction throttling systems since it doesn't apply to the Buick Reatta if that is OK. It is good background information.

 

 

Edited June 14, 2017 by Ronnie (see edit history)

[drtidmore]

8 hours ago, 89RedDarkGrey said:

Has anyone of our Electronic Gurus here determined the value of the OEM Unobtanium "Low Temperature Refrigerant Sensor" to come up with a suitable replacement- if anyone should need one?

To my knowledge, no one has found a suitable substitute.  However, in some cases the thermistor has NOT totally failed, but rather drifted in its resistance range.  The resistance of the thermistor rises as the temp falls and the typical drift makes the BCM think the low side temp is colder than actuality (resistance has drifted upwards).  A fix would be to lower the effective resistance slightly by attaching a trim resistor in PARALLEL to the sensor (1/4watt is adequate).   The value of the trim resistor will depend entirely on the amount of correction needed.  Ohms law for parallel resistors is all you need to determine the needed value (the product of the resistance values divided by the sum of the resistance values), assuming you know what it should read at a given ambient temp. 

 

The resistance of the thermistor in my Reatta measures 2624 ohm, at a temp of 80 degrees (measured with my infrared thermometer.)  As my A/C operates properly, I would assume that my thermistor has not experienced any significant drift. 

 

As an example, say your low side sensor reads a couple of hundred ohms HIGH at 80 degrees (2850 ohms).  Adding a 33K ohm resistor in parallel will drop the effective value to 2623 which is close to what mine reads under those conditions.  I don't have the temp vs resistance curve for the thermistor that GM used and thermistors are NOT linear devices. All the FSM gives us is that zero volts across the sensor is a short and 5V is an open (i.e. NOT MUCH INFO!).  No way to really determine the curve and predict the resistance over a range of temps without more data.  

 

[Ronnie]

1 hour ago, drtidmore said:

A fix would be to lower the effective resistance slightly by attaching a trim resistor in PARALLEL to the sensor (1/4watt is adequate).   The value of the trim resistor will depend entirely on the amount of correction needed.

 

Several years ago I had to do exactly that. It worked great but over time the sensor drifted out of range again. Selecting the correct resistor to add to the circuit was a matter of trial and error to get one that worked well.  I think i ended up with a 25 ohm resistor but I'm not sure about that. What I did is documented in a thread here on the forum if anyone is interested.

 

A year or so ago Daves89 supplied me with a good used sensor that I installed to get my AC working properly again. Thanks Dave!

 

Below is a photo of the low side temperature sensor removed in case anyone whats to know what the part that extends into the low pressure line looks like.

[Ronnie]

15 hours ago, drtidmore said:

That is fine with me.  I thought it time to explain the why as well as the how so that the process would make sense.  The reason I brought up the old GM suction throttling system is that it worked exactly the same way but dynamically and it worked extremely well. 

 

Thanks David for the information. The how-to guide is now posted on ROJ. It can be found here:  AC System Fine Tuning  

 

Let me know if changes or additions are needed.

[KDirk]

I've  not tackled the temp sensor issues yet as I've not had a need to. That said, the thermistor doesn't appear to be anything special. Provided a proper resistance range can be determined (probably need a NOS part to accurately test min and max resistance at temperature extremes) I can't  imagine a new bare thermistor couldn't be sourced and attached to the existing sensor body, thereby rebuilding the sensor.

 

Anyone know if there is a published spec  - perhaps in the FSM - on what the min/max resistance was on this part? That would make sourcing a new thermistor element for this purpose a lot easier. Or, if someone has a currently perfectly operating system on their car,  pull resistance readings from it at full cold and with engine at ambient temp (after car has not run in several  hours). My 88s both work but I  think both are slightly off spec, and thus probably not a good base reference point for this purpose.

 

KD

 

 

[drtidmore]

3 hours ago, KDirk said:

Anyone know if there is a published spec  - perhaps in the FSM - on what the min/max resistance was on this part? That would make sourcing a new thermistor element for this purpose a lot easier.

KD

I looked thru the FSM and it does NOT give any values outside of what the voltage drop is when shorted (zero) and open (5volts).  I agree that there is nothing special about the thermistor and if someone with a NOS unit could get us readings at 32F and say 80F, with a couple of points in between, I am sure we could find a suitable substitute to be soldered into the housing.  

 

 

[2seater]

On ‎6‎/‎14‎/‎2017 at 11:45 AM, Ronnie said:

 

Several years ago I had to do exactly that. It worked great but over time the sensor drifted out of range again. Selecting the correct resistor to add to the circuit was a matter of trial and error to get one that worked well.  I think i ended up with a 25 ohm resistor but I'm not sure about that. What I did is documented in a thread here on the forum if anyone is interested.

 

A year or so ago Daves89 supplied me with a good used sensor that I installed to get my AC working properly again. Thanks Dave!

 

Below is a photo of the low side temperature sensor removed in case anyone whats to know what the part that extends into the low pressure line looks like.

Unless I am measuring the wrong sensor, mine has the connections reversed? The sensor has a single female cavity with two pins and the plug is a single male w/two sockets and a lock on both sides? If it is the correct one, upside down just to the drivers side of the blower motor, I find 2562ohms @ 82*F. or 27*C. In or out of range is unknown and I will need a new battery for the temp. gun, the laser is iffy. I will measure the operational temperature relative to indicated as soon as I get a chance.

 

[Ronnie]

That is what my low side temp sensor looks like. It measures 2.40k ohm at 81 degrees measured with my inferred gun.

 

Edited June 16, 2017 by Ronnie (see edit history)

[Ronnie]

1 hour ago, 2seater said:

upside down just to the drivers side of the blower motor

 

I missed this on first reading. The low side temp sensor is on the passenger side.

 

 

Edited June 16, 2017 by Ronnie (see edit history)

[89RedDarkGrey]

http://www.omega.com/pptst/AD590.html  ???

 

http://www.sensorsci.com/small-bead-thermistors  TRANSLATE INTO ENGLISH

[2seater]

Interesting. As you can see by this diagram from my FSM, #6 is labeled as the low temp sensor. #1 is supposed to be the pressure sensor, but it appears identical, including the plug, to the sensor I measured before. I am sure your diagram is correct, as that location is after the expansion tube, so it makes perfect sense. The funny thing is, the connectors shown lower on the page, match the style you showed, but they most definitely are not. Apparently there is an error in my FSM, at least on this page

 

Aww, nuts, I cut part of the legend off.

 

 

Edited June 16, 2017 by 2seater (see edit history)

[KDirk]

An error in the FSM? I'm shocked, absolutely shocked, I tell you. I've noted a number of mess ups in there. For what they were, and th relative complexity of these manuals, I can cut some slack. But, accuracy - especially in some illustrations - could have been better.

 

[89RedDarkGrey]

I was just talking with my Friend (a Dealer Tech for Cadillac/Buick/Chevy/GMC here) about the FSM. The back cover of nearly every Haynes manual- shows a Tech, a note taker, and a Photographer.

 

There must have been only a few Artists who drew the Main diagrams, used for car manuals- because both contain many of the same hand-sketched illustrations- but the FSM does not contain one photo of anything..

 

Would it have broke the bank- to hire someone in house or professional- to snap a few B&W 35mm of something?

 

He just laughed- and said the newer "fly by wire" cars simply pull up the bay, they all have LAN, VPN, WIFI, and within seconds the vehicle is automatically paired, and prints out any C or H DTC's. They verify (if possible) the problem part or circuit, touch the screen- and the part is ordered. There's not much "diagnosis" with cars any more.

 

We're getting close to a time- where the Engineers will design vehicles to be also repaired by robots- and not just assembled by them.

 

He was deadpan serious and stone faced when he said that.

[drtidmore]

9 hours ago, 2seater said:

Interesting. As you can see by this diagram from my FSM, #6 is labeled as the low temp sensor. #1 is supposed to be the pressure sensor, but it appears identical, including the plug, to the sensor I measured before. I am sure your diagram is correct, as that location is after the expansion tube, so it makes perfect sense. The funny thing is, the connectors shown lower on the page, match the style you showed, but they most definitely are not. Apparently there is an error in my FSM, at least on this page

 

Aww, nuts, I cut part of the legend off.

 

 

Grabbed my FSM and what is shown in mine is at odds with what is shown in your FSM.  What year and what version is your FSM?  Mine is 1989 Riv and Reatta Final Version.

[2seater]

I think 89RDG has the right idea, take a photo, it doesn't lie, but unfortunately, it too can be reproduced over and over, correct or not   I guess to use the manual requires a basic knowledge to know when they are correct or not. I definitely have a sensor in the liquid line, located as per my FSM diagram, which I must assume should be the pressure switch, but it is odd that it had a similar resistance readings to the others posted above?  I guess I will get my temp. gun charged up and do a little more in depth before I post back, which may be some days.

[2seater]

I replaced the battery in the temp. gun and ran a few tests. I locked the throttle at 1500 rpm and set the desired temp. to 60*. The 28 reading cycled as before -2* - 10* and back. The lowest observed temp. at the low temp. connection was 44*F. The center outlet temperature settled at 56*F and stayed almost constant. Despite the hot air from under the hood, the inlet air was only 92*F. After ten minutes or so the #28 low temp. reading would stabilize at zero for 15-20 seconds but mostly it continued to cycle. This at least gives me a baseline to see if this improves when the compressor is replaced and properly charged.

 

I checked the resistance at the two sensors, low pressure and temperature. I found 2967ohms @ 77.2*F on the temp. sensor and 2894ohms @ 77.3*F on what should be the pressure sensor. Strange that they look the same, have the same connector, and read so similarly if they have completely different functions.

[89RedDarkGrey]

1 hour ago, 2seater said:

Strange that they look the same, have the same connector, and read so similarly if they have completely different functions

 

Maybe you could think of it as a 2 O2 sensor ECM- where it takes the 2 values, and uses the slight differential readings to adjust? Isn't the Orifice Tube between the temp and pressure? That makes sense- as 1 effects the other. Engine at 190*F in a pressurized system- but open that 11 pound cap- and you'll get a geyser of +400*F shooting out.

 

Think how many "sensors" control our automatic climate control systems- and why a veteran HVAC Tech openly dislikes that method. Way too high signal-to-noise ratio. Outside temp, cabin temp, Sunload, line temp, line pressures low and high. All I would want is cold.

[drtidmore]

2seater,

I am still curious as to what version of the FSM you own?   The difference are far from minor between the one you posted and what I posted!  It would be great for future reference to know if it is mistaken page OR that there are variations in the A/C sensors/locations across the 4 model years.  I think it is safe to say that the '88/'89 model years were the same as Ronnie's '88 is clearly the same as my '89 and in agreement with my Final Version FSM.  Given the other changes that occurred in '90 followed by more in '91, it is very possible that GM changed up the A/C plumbing somewhat.

 

 

[Ronnie]

2 hours ago, 2seater said:

The lowest observed temp. at the low temp. connection was 44*F. The center outlet temperature settled at 56*F and stayed almost constant.

 

You should check to make sure the blend door isn't mixing warm air with the air conditioned air. If that is happening you won't get the vent temp really cold no mater how well you tweak the AC system.

 

2 hours ago, 2seater said:

I checked the resistance at the two sensors, low pressure and temperature. I found 2967ohms @ 77.2*F on the temp. sensor and 2894ohms @ 77.3*F on what should be the pressure sensor. Strange that they look the same, have the same connector, and read so similarly if they have completely different functions.

 

The terminology you are using doesn't seem to be correct. There is a low side temperature sensor, who's resistance changes with temperature -  and there is a low pressure switch that only contains a set of contacts. Those contacts are either open or closed. There is no "low pressure sensor" that I'm aware of that would have a variable resistance reading. Perhaps your '90 is different from my '88???

 

There is also a high side temperature sensor. That sensor and the low side temperature sensor are interchangeable parts if I remember correctly. It has been my experience that the high side sensor doesn't play a part in cycling the compressor on and off under normal operating conditions.

Edited June 16, 2017 by Ronnie (see edit history)

[KDirk]

FWIW, the 91 system is laid out differently with the accumulator mounted in front of the engine up near the coolant resovoir. Line set is different as a result, but functionally is identical.