91 AC starts cold then gets warm after 5 or so miles....

[akeyzoo]

Question for the AC pros. Rebuilt climate system on my 91. New Evaporator, new condensers, new dryer, new expansion valve and new compressor. NEW. ;-) I have gauges and a vacuum pump. Held vacuum for 30 minutes, so no leaks afaik.

I added 34oz of R134. Realized almost immediately that BOTH of my condenser fan motors were shot. Replaced them both. Also, CCU has been re-capped and appears to work fine. I may have inadvertently overfilled since I think you use less R134 than r12, right?

When I fire up car and turn on the AC it blows nice and cold for about the first 5 miles of my drive then it goes warm. Any idears? I dont recall the service manual having detailed expected pressures when operating for high and low side...

Found this in another thread, did I overcharge?
Then R-134a
NSX A/C 134a charge weight.
1.9 lb; 0.85 - 0.90kg
28.2 - 30oz
27-28 low side 179-175 high side

If so, how would I pull some refrigerant out... or can I..... Is stopping to cool after a few miles a symptom of overcharging?

Chris.

 

[e9coupe]

Usually if it's overcharged the compressor will complain. Vibrate or labor somewhat. If you have gauges can't you just add refrigerant until the pressure differences max out? Backyard mechanic here..

 

[drew]

Check the condenser fans.

Does freeway speeds change the efficiency?

 

[akeyzoo]

The condenser fans WERE shot and were replaced with new motors and work fine. Let me throw some gauges on it today and report what I see.... I promise not to vent off any R134 if too high...... pinkie promise.....

Chris

 

[akeyzoo]

OK, Hooked up gauges - Here is the "before" with AC running at idle.

Low side 55, High 137

With some R134 recaptured (use your imagination)

Low side 30, High 170 - left idling 5 minutes, continuous cold.

Then when I turn off AC/Car the readings are

Low side jumps to 65, High side 135. I presume I ignore this measurement, car off/compressor off has no meaning?

See pics below of 3 scenarios.

i will go take her fo a drive, but at lower low side above all works well, fans included. Should I bring that low-side down even further to 27-28PSI?
Chris

 

[tucsonsx]

Sounds like an overcharge to me - high press valve mat be shutting the compressor down. Taking a look at the sight glass and checking the temp of high and low press lines will give you some clues. Sight glass should have some bubbles when the compressor starts running and they will then go away. If there are no bubbles on start up that indicates an overcharge. R 134 is much more sensitive to over and under charge than R 12. I put less than the amount of refrigerant by weight and then slowly add some until the bubbles disappear from the sight glass. I have had better luck using the sight glass than the gauges to determine charge.

 

[Honcho]

The condenser fans WERE shot and were replaced with new motors and work fine. Let me throw some gauges on it today and report what I see.... I promise not to vent off any R134 if too high...... pinkie promise.....

Chris

Charge weight for the R134 system is 800-850g of refrigerant. Most people aim for the lower side to make it a little easier on the old O-rings and seals. It looks like you have the right range in ounces (~30oz). Dd you use a scale to weigh the R134 cans as you charged?

***EDIT*** Based on the posts that occurred as I was writing this, you are way overcharged and the pressure switch is cutting out the compressor to protect the system. The best thing to do is evacuate the entire system and charge exactly 800-850g of R134 using a digital scale to weigh the cans as you charge. You can't really go off of low side pressure to determine proper charge weight because it can vary depending on conditions.

Another thing to check is the triple pressure switch on the upper line in the front bay. This switch controls the compressor and is pressure-based. It's purpose is to make sure the compressor only runs if the correct charge pressure is present. If it is failing or getting a false signal, it may cut out the compressor at the wrong time.

 

[akeyzoo]

I have a new trinary-switch, all new o-rings. pretty much all new! Thanks. With slightly less refrigerant (was at 34oz, now less but unknown) will take her for a drive.

Thanks Honcho!

 

[Wild Turkey]

Sounds like an overcharge to me - high press valve my be shutting the compressor down. Taking a look at the sight glass and checking the temp of high and low press lines will give you some clues. Sight glass should some bubbles when the compressor starts running and they will then go away. If there are no bubbles on start up that indicates an overcharge. R 134 is much more sensitive to over and under charge than R 12. I put less than the amount of refrigerant by weight and then slowly add some until the bubbles disappear from the sight glass. I have had better luck using the sight glass than the gauges to determine charge.

Should the sight glass have some bubbles before the compressor kicks in and then disappear with R12 as well?

 

[akeyzoo]

@Honcho
I did find a chart to correlate low side pressure and ambient temp - and I am within that spec now... Let me go take her for a 10 mile jaunt and see how she does.

 

[drew]

Should the sight glass have some bubbles before the compressor kicks in and then disappear with R12 as well?

Yes. This is normal behavior.

 

[nis350]

Should the sight glass have some bubbles before the compressor kicks in and then disappear with R12 as well?

My NA1 R12 system is similar.... Some bubbles during the beginning then disappear, but I do see a little bubbles once in awhile. I was told that is OK. Someone mentioned that it is better a little under charged and over charged. The AC seems to be working OK.

 

[tucsonsx]

Yes, better undercharged than overcharged. Adding a small amount of refrigerant will make the bubbles go away. The biggest problem with low refrigerant is that compressor will run continuously instead of cycling. In super high humidity I have seen the evap ice up due to this which greatly reduces cooling. A sure clue of low refrigerant is the air being colder out of the vents on one side of the car than the other.

 

[nigel]

don't exceed the proper amount of freon or it it'll kill the compressor
800/ 850 ml. 134

nigel

 

[akeyzoo]

OK, no major progress here - hit an miss. Went to autoparts store to buy some refrigerant. AC worked like a charm there and back.

Got home and evacuated system, ran vacuum for 45 minutes. Went to charge and after I got the first can in (~340g), the compressor wasnt turning on. High side well within range. This is the symptom I have been having. Ran over and checked compressor feed (to turn it on at compressor) and 0 volts. Used my power probe and applied 12V and the clutch engaged and compressor ran normally. Was only able to put a lil over a can in and no bueno....

I have an electrical issue somewhere so as I start to diagnose I have a few questions...

I had CCU rebuilt by SoS, all controls work as expected...
My aspirator behind dash panel has a failed fan motor - i have a new one but havent taken dash apart yet...

Could the aspirator be fudging up the system? I am whipping up the troubleshoting guide and FSM, but appreciate any ideas.

Chris

 

[akeyzoo]

Update - fired up car and “played” with the relays (fans and compressor) and got compressor to fire up. Noticed blower and high blower relays were pretty warm. Regardless had system up and running and added 837g of R134. Will potentially get some spare relays. Is it typical of a failing relay to get warm and disengage? Why would blower relays be so warm (blower always worked)…

Alas. Some progress.

 

[drew]

Why would blower relays be so warm (blower always worked)…

Alas. Some progress.

Check the power draw on the blower motor.

Consider disassembling the blower motor to check brushes, bearings, and a general cleaning of the motor internals. It is amazing how much crud builds up. Also, I find that when the brushes get very worn, the motor can pull a lot of amps.