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1970 Series 75 rear A/C issue

Started by mikanystrom, August 17, 2018, 12:42:07 AM

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mikanystrom

Hello Forum,

I did it again, I'm sure many of you know what I mean and have done it too... bought another Cadillac I mean...

I bought a 1970 Series 75 Formal Sedan with what seems like 25,000 original miles on it.  A bit of patina from age and some rust underneath from East Coast, but not too bad.  Interior is basically pristine and car looks and drives great.  The tires were marked LR78-15... one failed almost immediately, now it has some Hankook whitewalls in P-radial... rides smoothly at 85 mph, as fast as I dared take it with all the traffic on the freeway this evening.

It's a bit of a stripper (not the good(?) kind), it has very few options, but what the heck.. that's what the chauffeur is for, right?  (Things like opening the trunk and maintaining a constant speed... stuff that even my De Ville does for you... not to mention turning on and off the headlights and dimming them and playing stereo music...ah well)

Anyhow.... my issue... when I got the car it had very little charge in the A/C, but enough that I could tell that the system was tight.  I vacuumed it and charged it with my own special sauce, that is, about 90% Red-Tek 12a and about 10% propane.  (Red-Tek 12a is made with isobutane and propane to match the pressure curve of R134a, so the temperatures are a tad higher than R12, a bit of extra propane then brings it right into line with R12.  Heat capacity is a bit lower than R12, so on a really hot day it doesn't cool quite as well as original, but you'd really never tell.  It is also less strain on the compressor than R12 or R134a .. and best of all.. completely compatible with mineral oils as well as the modern "salad dressing" oils.)

Start engine, drive around... I have gauges on the low and high side, at a slow idle, showing 35 psig on the low, 210 psig on the high at an ambient of about 80-85F and 50% humidity.  Front vents blow quite cold.  Drive around town, front vents blow really quite cold, I'm shivering!

But.... the rear system isn't doing its thing.  The vacuum and electrical circuits all work, the system looks brand new to me.. all LOOKS great.  It responds correctly to running the dial at the right rear passenger up and down, also putting vacuum on the servo head switches it to heating just fine, and when you release the vacuum it does blow air through the ceiling vents, although not cold air (not warm either, just outside air).  The refrigerant suction hose isn't even getting cold (whereas the front one is dripping condensation like crazy).  There's just a tee just rear of the compressor suction port, so there's not really much to go wrong over there.  You can feel a bit of vibration in the liquid line to the rear evaporator with the engine running.

First diagnosis, then.. there's no flow of refrigerant going through the rear system.

The refrigerant circuit to the rear as far as I know consists of: liquid hose to the back of the car, expansion valve, evaporator, suction-throttling valve (STV), and suction hose back to the tee behind the  compressor.

It seems unlikely the hoses or evaporator would be plugged.  My conclusion is that it's either the STV (I didn't realize it had a service port, so I didn't even know to put a low-side R134a adapter port on it when I had the system open) OR the expansion valve... does anyone know which is more likely?  The STV is hard to find but rockauto claims that Four Seasons 38652 replaces the expansion valve.  It's cheap, but it's quite a bit of work to tear everything apart (the removal instructions for the rear expansion valve involve removing the entire rear A/C system from the car...!  29 screws just to split the air box apart, it says...)  Am I likely to fix the issue by replacing just the expansion valve?  Any ideas?  Or could it possibly possibly be anything else?? 

I am thinking just leaving it alone and letting the A/C do the work from the front only but it seems... wrong...

   Mika
1970 De Ville Convertible
1993 Fleetwood Brougham
1970 Series 75 Formal Sedan
1968 Eldorado "Purple Sister"
------------------------------------------------
1976 Pontiac Firebird Trans Am 455 4-spd
1999 Camaro SS Convertible 6-spd
1999 Honda CBR 1100XX :-)

Scot Minesinger

I agree, get the rear a/c unit working.  If you do not already have the factory shop manual that is going to be your best bet to repair it.  Probably something simple.  1970 is my favorite year, but have never worked on a 75 series.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

"Cadillac Kid" Greg Surfas 15364

#2
Mika,
While I don't personally agree with replacing R-12 in an R-12 system with anything else,. the Hydro Carbons, Propane and Iso Butane (R-600 and R-290) are becoming a "thing" and I am currently licensed for them.
That said, how did you charge the system, meaning how much "special brew" did you charge it with?
The rear system takes about another 1-1/2 pounds (or R-12).
IF the liquid line to the back is warm , the system is fully charged and you are not getting cooling or at least any refrigerant flow you have just two things left in the line as you say.  The POA and the expansion valve. I would suggest getting to the POA and with a small hammer tapping it on the solid part of the body (avoid damaging any of the tubes or connection points) with the engine and the system running WHILE you have a guage on the POA port.  See what the pressure (or vacuum) is and if it changes while you are tapping the valve.  While there check to see that the expansion valve thermal bulb and capillary have not been broken and the valve's charge lost.
It is all pretty straight forward, but as you say with the rear evaporator, a lot of work.
Greg Surfas

Going back and looking at the P/T chart for your "witches brew" I believe you are going to confuse the heck out of the expansion valves with that 11 degree "glide". You do of course realize that this mix of refrigerant with Mineral oil provides you with an explosive mix should you have any leaks in the system.
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

mikanystrom

Yes I know it could get exciting with open flame---I'll have to resist the temptation to smoke my cigars when I'm working on the system (even though the car has six ashtrays)!

I have let me see... I think enough refrigerant... 5 or 6 pounds "equivalent", the density is lower of the hydrocarbons so it comes out much less.  It works out to about 2 lbs of the "witches brew".  I think specs are 5 1/4 or 5 1/2 of R12.  I wonder if it would be a symptom of a low charge that the rear system cuts out first, or the front, or both at about the same time.

The liquid line... hmm... I wonder if it is warm enough, I should check that carefully.

I'll definitely try the suggestion of putting a gauge on the POA and tapping it also. 

Yes, I know what you mean about the glide.  It's not the first time I've used the witches brew, and what I've observed in the past is that what you get is a gradual loss in cooling performance (evaporator temperature) as the system gets closer to its limit.  Actually maybe a better way to put it is, if the system is lightly loaded you get "too much performance" (i.e., it's cold and dry outside... and you just set the A/C on AUTO, no "ECON" on pre-oil-crisis cars) and the expansion valve can start buzzing...(only the propane is boiling, the sensed pipe gets REAL cold, and the expansion valve "hunts" near its fully closed setting).  Or in other words, at low load, the evaporator gets a bit too cold and the expansion valve gets, as you say, confused.

Basically I think you can model the glide as a change in boiling point of the refrigerant with changing system load.  At low load, only the propane is boiling, the evaporator gets very cold, the sensed pipe gets colder than the set-point of the expansion valve, and then you get the hunting I described; but at high load, everything is boiling, the evaporator starts to reach the boiling point of the isobutane, so the sensed pipe is warmer than the set-point of the expansion valve, then the expansion valve opens to its max opening and just lets refrigerant through.  The low-load scenario isn't really desirable, but the high-load scenario ought to be OK (it's about the same as if the system were charged with R12 and operating out of control, beyond its performance capacity, like idling on a really hot day in Florida).

A question if anyone knows... the expansion valves nowadays, with the sensing bulb, what's IN the sensing bulb?  Is it R134a now?  I am guessing it used to be R12, and the POA (STV) is calibrated for R12, so it strictly speaking oughtn't be compatible with a R134a expansion valve...???

1970 De Ville Convertible
1993 Fleetwood Brougham
1970 Series 75 Formal Sedan
1968 Eldorado "Purple Sister"
------------------------------------------------
1976 Pontiac Firebird Trans Am 455 4-spd
1999 Camaro SS Convertible 6-spd
1999 Honda CBR 1100XX :-)

"Cadillac Kid" Greg Surfas 15364

Mika
A lot of questions result in a lot of answers.

Yes I know it could get exciting with open flame---I'll have to resist the temptation to smoke my cigars when I'm working on the system (even though the car has six ashtrays)!

The Hydorcarbon refrigerants are intended for use in small commercial and domestic appliances. Usually with a total refrigerant charge of less than a couple of ounces.  Propane and IsoButane are excellent solvents and "chew" right through the Mineral oil you need with them.  For a system in constant use that is no problem, but for a vehicle that is driven less frequently and then garaged for a spell, the oil in the compressor seal is slowly washed away, resulting in it leaking.  No problem other than these refrigerants are heavier than air and tend to creep along the floor until they find something like a water heater or furnace pilot flame on these appliances that are frequently installed in said garages

I have let me see... I think enough refrigerant... 5 or 6 pounds "equivalent", the density is lower of the hydrocarbons so it comes out much less.  It works out to about 2 lbs of the "witches brew".  I think specs are 5 1/4 or 5 1/2 of R12.  I wonder if it would be a symptom of a low charge that the rear system cuts out first, or the front, or both at about the same time.

This MIGHT be okay for a single system. On the dual system with the long liquid line to the back evaporator it is highly unlikely that ANY liquid refrigerant is reaching the TXV.

A question if anyone knows... the expansion valves nowadays, with the sensing bulb, what's IN the sensing bulb?  Is it R134a now?  I am guessing it used to be R12, and the POA (STV) is calibrated for R12, so it strictly speaking oughtn't be compatible with a R134a expansion valve...???

ALL expansion valves are "cross charged" meaning that the liquid/vapor in the thermal bulb is not the refrigerant being used and has a specific pressure/temperature curve intended to operate within the limits of the refrigerant being used.

Basically I think you can model the glide as a change in boiling point of the refrigerant with changing system load.  At low load, only the propane is boiling, the evaporator gets very cold, the sensed pipe gets colder than the set-point of the expansion valve, and then you get the hunting I described; but at high load, everything is boiling, the evaporator starts to reach the boiling point of the isobutane, so the sensed pipe is warmer than the set-point of the expansion valve, then the expansion valve opens to its max opening and just lets refrigerant through.


Glide is the difference between when a specific refrigerant blend starts to boil and when it is fully evaporated at a specific temperature.  Systems intended for HC refrigerants typically do not have TXVs but fixed orifice expansion devices.

Now my suggestion is that first of all you put a guage on the rear POA and see what you have withg the system running.  If it immediately pulls into a vacuum you have a component failure.  If it does not THE ISSUE IS WITH THAT REFRIGERANT MIX IN YOUR SYSTEM.

R-12 is still legal and available to someone who is licensed.  I just bought a 30 pound drum and although it cost my kid's inheritance, IT WORKS AS INTENDED.

Mika I hope this helps.  Look for the simple answers first before you start tearing into the system and replacing components.

For all you out there you have to know it is now 102 degrees with a heat index about 108 here, and I am doing anything I can to stay inside avoiding the work I need to do outside.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-