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How does capacity affect target pressures?

turbojimmy on Sat June 30, 2012 12:17 PM User is offlineView users profile

Year: 1994
Make: Cadillac
Model: Fleetwood Stretch
Engine Size: 5.7L
Refrigerant Type: r134a
Ambient Temp: 80
Pressure Low: 38
Pressure High: 248
Country of Origin: United States

I have a '94 Cadillac Fleetwood limo. It's a 5-foot stretch with a separate HVAC unit for the rear. The rear climate control has a decent-sized evap core and a strong blower. The stock system is T'd off the compressor and has an additional condenser, accumulator and 2 extra cooling fans. When the rear HVAC system is shut off, there is a solenoid that isolates the rear evap core. When I first got the car, this solenoid was defective. The compressor was also bad. I replaced the compressor with a used stock unit, replaced the soleniod, evacuated the system, vacuumed it down and recharged it.

This is my second stretch set up this way. The previous one worked fine, this one however isn't cooling very well in the rear and the front is marginal. One issue with these cars in stock configuration is that the a/c gets warm at idle. The manual has you check outlet temps and pressures at 2k RPM. Below that, the system doesn't function well. Poor design.

I know that the factory label indicating how much refrigerant the system takes is obviously not valid. I filled the system up using the pressure table in the factory service manual. I have it exactly where it needs to be. But, as I said, it's not as cool as it should be. If I shut off the rear A/C, the front cools very nicely. When I turn on the rear a/c, the front warms up a bit (but is still fairly acceptable) but the rear never really gets "cold". It gets cool, but not cold.

The rear unit has a lot of work to do. A lot of volume to fill and the ducts run along the hot roof line (the car is black with a black top and black interior). Even so, I would think it would cool down eventually. It doesn't. The blower is very strong back there - it moves plenty of air.

So I'm wondering if the volume of the system makes the target pressures in the manual moot. Should I have different targets? Higher? Lower? Maybe the rear evap core needs flushed? Dunno.

iceman2555 on Sat June 30, 2012 4:48 PM User is offlineView users profile

The system is undercharged. Forget the pressure/volume issue with a retro fit. The pressures are meaningless, since the system was never designed for the operational pressures of 134a. Also, pressures are not an indication of a fully charged system....pressures are a diagnostic tool for a fully charged system.
A suggestion would have been to completely clean the system..front and that all flush chemicals are removed and then begin the recharge method. If the refrigerant labels are present, do that indicate the charge for the OE front system only or for the adapted system. Currently, it appears that the system may contain excessive lubricant which can inhibit cooling, elevate pressures and cause serious system problems. Four factors for a complete and proper AC repair, CLEAN THE SYSTEM, LUBRICATE THE SYSTEM, CHARGE THE SYSTEM AND INSURE PROPER ENGINE COOLING. Removal of one aspect drastically decreases the performance and longevity of a system and components.
If it is desired not to start over with a totally clean system, then begin by adding refrigerant 2-4 oz at a time, until system cooling reaches a desired point. This may increase pressures and if so, it may be necessary to add engine cooling, ie. fans etc.
Also, it is noted that the OE pressures were utilized to achieve a perceived charge level....134a operates at app. 25-35 % higher head pressure than 12...was this factor taken into consideration? Caddies, are one of those recommended not to retro fit, due to possible ATC problems. It can be done...but usually requires serious attention of all details.
Good luck!

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mk378 on Sat June 30, 2012 6:15 PM User is offline

I agree, you should charge more and try again. Keep track of the total amount put in so you have a reference for next time. Systems get warm at idle for two reasons, inadequate condenser cooling from the engine fan (high side pressure would rise) or compressor too small (high side pressure would fall). This latter isssue is partly bad design (using a stock compressor for a much larger heat load), but if your used compressor is worn it won't work to full potential anyway.

turbojimmy on Sun July 01, 2012 7:59 AM User is offlineView users profile

Thanks for the replies! It actually is a factory r134a system. It wasn't retro-fitted. Given this, are the factory target pressures still useless?

The label shows 1 lb. 22 oz. of refrigerant. I have no idea how much I put in it, but it's more than that. I wasn't paying attention because I knew the label was useless anyway. On the other limo I had, the coach builder was kind enough to replace the factory sticker with a new one indicating the new volume of the system. But not on this one.

It could be that the compressor is over-worked, but this set up worked fine on the last one. That car was only a 42" stretch so the interior volume was a bit less. The other car had the HVAC module in the trunk and drew air in from the package shelf. This one has it mounted behind the passenger seat. It appears to be physically smaller than the trunk-mounted unit so maybe the core is smaller, too? Dunno. Maybe I'll put more refrigerant in it to see if it helps.

EDIT: The suggestion to charge 2-4 oz at a time, and the fact that this will raise pressures, got me thinking. There are 4 electric fans in total. 2 stock "pullers" mounted to the radiator and 2 aftermarket "pushers". One pusher is mounted on the secondary/aux condenser, the other one is on the factory condenser. The pusher fans come on with a pressure switch mounted on the accumulator for the rear system. I don't know what the pressure threshold is, but they cycle on and off seemingly as they should - typically only when the rear a/c is turned on. There's also a manual switch to have them run constantly. I've found that at idle, if I have them run constantly that the outlet temps up front are noticeably cooler. But, here's the thought that your post triggered: one of the stock fans turns on at 225 PSI head pressure and the other at 248 PSI. I would think, then, that the one that turns on at 225 PSI would be running constantly when the a/c is on. It doesn't. It cycles, but it is also temperature activated (at 225 degrees) so it may just be cycling based on coolant temp. I've never seen the second stock fan run (the 248 psi fan). So, loonnng story short: the system probably is still undercharged and my cheap manifold gauges are probably inaccurate.

Edited: Sun July 01, 2012 at 8:28 AM by turbojimmy

mk378 on Sun July 01, 2012 11:15 AM User is offline

With fans that cycle on and off, the high side doesn't tell much about charge state. Even undercharged, it will rise up until the fans turn on.

turbojimmy on Sun July 01, 2012 1:01 PM User is offlineView users profile

Originally posted by: mk378
With fans that cycle on and off, the high side doesn't tell much about charge state. Even undercharged, it will rise up until the fans turn on.

Thanks. I had about 1/4 of a 12-oz. can left so I popped it in there this morning. It made a noticeable difference, but it's still not as cold as I think it should be and gets warmer (still cool, but not cold) at stoplights. I'm going to grab another can and put a little in at a time to see what happens. I have a really accurate digital thermometer somewhere with a probe. I'll think I'll fill it until the temps stop dropping.

turbojimmy on Sun July 01, 2012 4:26 PM User is offlineView users profile

Eh. I'm going to have to take the rear HVAC unit apart. I put another 6 oz or so in it and the front cooled down some more, but the back is still hot. So warm that I thought maybe the heater valve was allowing hot water back there. It wasn't. The shut-off valve works fine (though I made sure by clamping off the hot water hose). I verified that the solenoid is opening up the A/C line, too. It is - and I can see the refrigerant flowing through the sight glass on the second accumulator. At speed it cools down slightly, but not like it should. The other car (sorry to beat a dead horse, but it's the only comparison I have) would freeze the back of your head with the A/C back there.

I don't know of the rear evap core is plugged or dirty or what. I took some massive rodents' nests out of the stock HVAC system when I first got the car. Maybe there's crap in the rear one too?

turbojimmy on Tue July 03, 2012 8:23 AM User is offlineView users profile

Originally posted by: iceman2555
The system is undercharged.

Okay. This original diagnosis was correct.

I verified that the rear evap core wasn't blocked. In doing that, I noticed that the lines running to it were not even a little cold. I'm not totally familiar with the mechanics of this Frankenstein system, but I had assumed that since the front was cold, that the system was charged enough. Not so. I kept adding refrigerant until the rear lines started to get cold, and ultimately the air in the back got cold. It's not a deep-freeze type cold, but it's adequate. It took another 36 oz. of refrigerant. I guess the rear system adds quite a bit of capacity. The volume of the hoses alone has to be significant given the distance they run.

One day I might have it properly evacuated and cleaned. In the meantime, though, I'm happy with it.


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