A/C Pressures; Cold Weather A/C

[mmahon04]

Hey all,

Before beginning, let me say I've read every post and thread on these forums relating to A/C work (including Jim Sims and Griffiths). I'm still at a loss. I'll try to be as heavy on data/setup as possible for clarity.

Car details:
1988 911 3.2
Complete R134a conversion, including:
- Griffiths/Kuehl evaporator
- Griffiths/Kuehl front condenser
- Griffiths/Kuehl high-side low/high pressure switch
- New barrier hoses
- New rear condenser
- New Sanden compressor
- New drier

So, a completely replaced system. All work completed roughly two years ago, with no alterations/adjustments since. Functionally cold temperatures to me, and easily seems to keep up after driving for awhile or at speed. ~44oF upper-center vent output temperatures (max cold, max fan speed) after having driven for ~20 minutes in 86oF/65% humidity last night.

I'm dealing with two issues (possibly related?):

1) Over the summer (daily highs of 94oF+, heat index >114oF), I would suddenly stop getting cold air in heavy stop and go (emphasis on stop) traffic. Front condenser blower would continue to run, and evaporator fan would continue to run. Then I would suddenly get cold air back after a few minutes, or after getting up to speed. I've finally gotten around to investigating, and checked the pressures last night (~78oF, ~65% humidity, rear deck closed, idling, no extra fans, oil temp @ 195oF, confirm front condenser fan functioning). Low side: ~45 PSI. High side: ~350 PSI.

Ouch.

That would explain the cutoff in stop and go traffic; probably triggering the high pressure switch. For the conditions last night, (and according to this chart here), I should've been 37-47 PSI low side, 162-190 PSI high side. Charlie Griffith has a post here indicating a high side of ~190 PSI would be appropriate; likewise, Jim Sims indicates here that 115-200 PSI high side would be appropriate.

I bled off enough from the high side to bring pressures down to low side: 37 PSI, high side: 300 PSI. I stuck two 'Vornado' type fans blowing down on the rear deck (still closed, still idling); this dropped pressures to low side: 35 PSI, high side: 250 PSI. Barely in range on the low side (according to chart linked above), and definitely high to me on the high side. Checking upper-center vent temperatures (doors wide open, so no re-circulation, max cold, max fan speed), I was getting low 62-64oF on output, indicating a temperature drop of 14-16oF for the system. Took the car for a 12 minute drive, and still had the same temperatures I had before venting: ~44oF (max cold, max fan speed, re-circulated).

I brought up these pressures and concerns to my mechanic, who indicated to me that in his experience (decades worth, with only import German cars and heavily with aircooleds) the pressures I'm having are normal. I'm concerned that I have a partially clogged expansion valve, or other restriction. I could bleed more refrigerant from the system to get the high side in check, but then I'm worried about dropping my low side pressures out of range.

2) Last winter (lows in the 40's), and the last couple nights here (overnight lows in the mid 60's), my A/C compressor would not turn on when instructed. It will turn on after the car apparently gets warmed up: today, it took 20 minutes for it to turn on on the drive into work in 75oF morning weather. It doesn't matter what evaporator fan speed I choose, or temperature I select. The compressor just won't turn on. Evaporator fan and condenser fan, again, all work normally. Additionally, when it does finally activate, I do get cold air (see above).

I also brought these concerns to my mechanic, who stated that he most frequently sees it as an issue with the temperature switch going out. I guess in my mind they may be related to the concerns I have in 1).

Before I do anything, I'd like to get some extra thoughts on my concerns.

[DRACO A5OG]

1. most likely expansion valve failure. Buddy's 89 did the same thing, we swapped it in 2 hours and Voila he has cold air again when he wants it and keep it there. I was screaming at him to work quickly so he does not kill his new drier.

[T77911S]

i have been told that the hi side on the 911 is very hi. 300psi not uncommon.
it would have to do with condensor area and airflow.

air that was left in the system due to not pulling a good vacuum can make pressures go up but both would go up i believe.

[mmahon04]

Originally Posted by T77911S

air that was left in the system due to not pulling a good vacuum can make pressures go up but both would go up i believe.

That seems to be the thinking of Sanden themselves.

[wwest]

High side over-pressure would result in the clutch cycling on and off independent of the thermostatic switch, it should not result in a noticeable period of system shutdown, only lower cooling efficiency


60dF, Your pressure switch might be preventing A/C operation when/if the equalized system pressures are low enough.

I think that I have seen something indicating Kuehl's pressure switch high limit is 325 PSI.

You have 2 symptoms that might well point to an intermittent or developing pressure switch failure.

[kuehl]

1) The evaporator blower motor function is independent of the compressor clutch and the front condenser blower motor. However, the evap blower motor must be on for the system to operate. With a normally working thermostat, when the thermostat calls for demand, the compressor clutch engages and the front condenser blower motor runs. When the thermostat is satisfied the compressor clutch disengages and the front condenser blower motor turns off.

2) What is missing in your data above is what you had 2 years ago: How much refrigerant, and what the low and high side pressures were at idle at a given ambient temperature. Look up your records from when you charged the system 2 years ago. Without those 2 year old data points you could have been overcharged 2 years ago.

3) If you are seeing 30 to 40 psi on the low side I would not point to the TEV (expansion valve) yet. Most new TEV's do not fail because of a mechanical defect in the TEV, rather failure is more related to moisture in the system or debris in the system getting in the TEV. Seeing 30-40 psi on the low side while operating says your TEV is opening and most likely throttling (the valve opening moving with demand). If the valve was closed you'd have 0 to - low side pressures. If the TEV was stuck open you might have have pressures higher than 40 or it might suggest you are overcharged or have air in the system. A stock 1988 takes about 47 oz of R12 and anywhere from 80 to 90% of that 47oz in R134a. If you had much less of either refrigerant in the system and you noted 350 psi on the high you could have air in the system.

4) If you suspect the low-high binary switch is not allowing the compressor to turn on, you test that by disconnecting the wire between the compressor clutch and the PS and check for voltage and then check for voltage at the original AC power wire that leads to the PS. Pressures switches, like TEV's, are subject to issues when there is contamination in the system as well, or when low voltage levels pit their contact points.

5) Depending upon who's thermostat you have in the car, the minimum "on" state might be 55F, if this is the case then if your cockpit is the same temperature as the outside ambient air temp, 40's or even 60's, the thermostat's probe could still be seeing laden temps below its operating threshold. If you want to run tests when the ambient temps are low... turn on your heat in the car and use digital thermometer to record your data points.

PS, ignore west

[mmahon04]

Regarding my issue 2):

Originally Posted by kuehl

4) If you suspect the low-high binary switch is not allowing the compressor to turn on, you test that by disconnecting the wire between the compressor clutch and the PS and check for voltage and then check for voltage at the original AC power wire that leads to the PS. Pressures switches, like TEV's, are subject to issues when there is contamination in the system as well, or when low voltage levels pit their contact points.

Didn't see your post last night, but set up a test last night after reading the Bentley manual, ran a trial run to double check measurements while I knew the AC was working, and checked them again this morning.

Conditions/tests last night:
- A/C had been functioning normally on drive home.
- Car not running, but ignition turned.
- A/C cabin fan set to max speed.
- A/C temperature set to lowest.
- Confirmed front condenser fan functioning.
- Indicated ~12v on wire leading to high/low pressure switch.
- Indicated ~12v on wire leading from high/low pressure switch.
- Turned car over, A/C still functioning.

Conditions/tests this morning:
- Overnight low of 64oF.
- Tested at indicated ambient of 72oF.
- Car not running, but ignition turned.
- A/C cabin fan set to max speed.
- A/C temperature set to lowest.
- Confirmed front condenser fan functioning.
- Indicated ~12v on wire leading to high/low pressure switch.
- Indicated 0v on wire leading from high/low pressure switch.
- Indicated zero continuity across pressure switch leads.
- Indicated 75 PSI static high-side pressure.
- Turned car over, AC non-functioning.
- A/C turned on ~10 minutes into morning drive.

So, that's one issue diagnosed. a bad/failing high/low pressure switch. I'll probably be giving you a call within the next week or two, Charlie, to get a replacement switch. I'm currently in the process of relocating to the Atlanta area, so something that requires evacuating and recharging the A/C system will have to wait.

Regarding 1):

Originally Posted by kuehl

2) What is missing in your data above is what you had 2 years ago: How much refrigerant, and what the low and high side pressures were at idle at a given ambient temperature. Look up your records from when you charged the system 2 years ago. Without those 2 year old data points you could have been overcharged 2 years ago.

I didn't do the work myself (otherwise I'd remember the pressures, at least vaguely); mechanic mentioned above did. I'll see if I can grab them, but wouldn't put much faith in it. Additionally, considering it hasn't been serviced since, if pressures are high now I'd assume pressures were high then.

Originally Posted by kuehl

3) If you are seeing 30 to 40 psi on the low side I would not point to the TEV (expansion valve) yet. Most new TEV's do not fail because of a mechanical defect in the TEV, rather failure is more related to moisture in the system or debris in the system getting in the TEV. Seeing 30-40 psi on the low side while operating says your TEV is opening and most likely throttling (the valve opening moving with demand). If the valve was closed you'd have 0 to - low side pressures. If the TEV was stuck open you might have have pressures higher than 40 or it might suggest you are overcharged or have air in the system. A stock 1988 takes about 47 oz of R12 and anywhere from 80 to 90% of that 47oz in R134a. If you had much less of either refrigerant in the system and you noted 350 psi on the high you could have air in the system.

Your and my thinking is in the same spot. I think it has air in the system and is in need of a full evacuation, especially after reading the Sanden diagnosis guide (linked above). That should hopefully happen at the same time I get the pressure switch replaced.

I'll follow up when I have more progress to report.

Thanks again.

[wwest]

May I suggest that you use a Red Dot Trinary pressure switch with a high side pressure limit of (more safe) 325 PSI.

https://www.google.com/webhp?sourceid=chrome-instant&rlz=1C1KMZB_enUS523US524&ion=1&espv=2&ie=UTF-8#q=red+dot+trinary+switch&tbm=shop&spd=4140211850411058282

Check and be sure that you have a voltage spike "damping" diode in/at the compressor clutch. Without that in the circuit the switch points might weld shut over time, or become so pitted that the switch "range" changes.

Also, you might consider using a relay to indirectly switch the clutch, the relay points will also wear but no need to open the system to replace.

Since the system operated satisfactorally 2 years ago it much more likely that you have lost refrigerant rather that getting air into the system.

If Kuehl's pressure switch high side limit is 325oF then PS failure is highly probable and thus pressure may then have at some point gone high enough to compromise/displace an o-ring or seal.

"- A/C turned on ~10 minutes into the morning drive.." Probably resulting from engine/exhaust/CatCon heating the refrigerant enough to trigger the failed/failing PS.

[kuehl]

Usually, before you mark a PS as the culprit you do a continuity test on the switch terminals and test voltage level as well.
You noted you have 75psi static which is sufficient for the low side contacts.

1) Remove the 'hood' switch cover.
Use an ohm meter or continuity check and advise whether
you have continuity between the two male spades on the switch.

2) Turn on ign switch to 'on', or accessory, engine not running.
Thermostat set to max cold (ambient should be above 60F).
Set evap fan speed switch to '1' or slow.
At the original AC power wire by the oil filter unplug the PS switch and advise the
voltage you are getting at the wire.

You can respond directly to us when you get settled in, you have a phone number and
email contact information.

PS, ignore west.

[wwest]

CG, OP already tested the PS in the manner you described...

Unless you mean that the checks/measurement should have been DIRECTLY on/at the physical PS connections.

It's not clear where, EXACTLY, OP did the testing.

And assuming, as you have said, the high side limit is 325 PSI, there is a second indication of switch failure.

[kuehl]

ignore west

[T77911S]

Originally Posted by kuehl

PS, ignore west

thsi just absolutely KILLS me. LMAO!!!!

[Ronnie's.930]

Originally Posted by kuehl

ignore west

Sound advice right thar.

[mmahon04]

Originally Posted by wwest

It's not clear where, EXACTLY, OP did the testing.

For voltage testing, there are two white lines/wires coming off the pressure switch itself. There is a connection on the end of each. One connects to a wire from the A/C compressor. The other connects to a wire from the car.

Setup:
- Wire leading from car will be called wire A.
- White wire leading to connection for car wire will be called wire B.
- White wire leading to connection for A/C compressor wire will be called wire C.
- The pressure switch is between wires B & C.
- Wire coming off A/C compressor will be called wire D.

Electrical testing was done this morning as follows:
- Keys turned on, but car not turned over.
- A/C fan speed: full.
- A/C temperature setting: lowest.
- Tested at wire A. Result: ~12v.
- Connected wire A to wire B. Tested at connection for wire C. Result: 0v.
- Unplugged wire A from wire B. Tested continuity across wires B & C. Result: No continuity.

Thanks,
Matthew

[mmahon04]

Guys, I've read enough A/C threads to know how these go. Please, for the love of god, let's not let this devolve.

Wwest, I appreciate your concern. Thanks for your thoughts on the pressure switch. I'm not opting for a trinary switch when/if replacement comes. My call, but I'm satisfied.

Everything with Charlie's components has been exceptional. Let me make this clear: I am in no way dissatisfied. This is merely an issue with what's looking to be a temperamental (wow, there's an 'a' in the middle of temperamental there; never knew) pressure switch and possibly a poor initial evacuation/charge.

I try to keep my posts as much for a community body of knowledge as I do my own clarity of thought; can we attempt to keep it that way? Like I said, I'll follow up when I have more progress to report.

[wwest]

Okay with the switch issue, but I suggest you take care to be sure whatever switch you choose has a high limit of no more than 325 PSI.

[mmahon04]

Wrong thread wwest? I'd appreciate it if you deleted the above post (#17). It's irrelevant for my purposes, and its quoted text doesn't come from this thread.

Thanks,
Matthew

*Edit* This post was previously post 18. Mark removed the one I requested. Thanks again Mark.

[Mark at Pelican Parts]

I removed the post Matthew. Looking forward to hearing your progress.

Thanks
Mark

[SilberUrS6]

Originally Posted by Mark at Pelican Parts

I removed the post Matthew. Looking forward to hearing your progress.

Thanks
Mark

Thank you!