1973 Eldorado AC Issue

[Boltoncad41]

I have a 1973 Eldorado Convertible with 2 issues.
1 Cruise control needs a new servo as it is (diaphragm) shot. Where should I look to buy one?

2 Also - my AC (converted to 134 before I owned it) blows cool for about 1/2 hour and then cease to work. Got it recharged and mechanic (very trustworthy and has worked on many vintage cars but not these automatic climate sensing caddy's)said the compressor is not cycling, constantly on, so it ices up and stops blowing cold. Any idea for next steps. He did replace switch, but that is all so far.
Thanks in advance for any ideas. Mike

[MultipleCaddies]

Sounds like he's more familiar with modern vehicles that cycle and use a high pressure switch. Your system uses an expansion valve and the temperature sensing bulb and is not designed to cycle the compressor on and off. The expansion valve regulates the size of the orifice that the refrigerant passes through, thus controlling the phase change process, reducing or increasing it as necessary to maintain the ideal temperature. The temperature sensing bulb needs to be in the right place, and the expansion valve itself needs to be working properly. Odds are the expansion valve is the culprit, but an experienced A/C tech with a set of gauges and an IR thermometer will be able to quickly diagnose it. Expansion valve systems do not work the same as modern orifice tube systems, so if he's only used to working on newer cars, he has some learning to do.

[TJ Hopland]

73 could have been a VIR system and that could have been converted to a cycling system so we will need more information and or pictures to better guess what may be going on. 

On the cruise control the servo usually isn't what fails so I would think you could get one from any GM in the era and just change the bracket if it wasn't from a Cadillac.   Cadillac switched to a slightly different system in 78 but other GM's kept it till earlier 80's.   

["Cadillac Kid" Greg Surfas 15364]

Mike,-
IF the "conversion" consisted only of replacing the R-12 with R-134a, then the non cycling compressor would be correct operation for the Cadillac ATC (automatic temperature control).  The compressor runns full time, cooling all the air to 34 degrees F (+/-) then reheating the proper proportion of the cooled air to satisfy the demands of the user as determined by the setting of the temperature control dial.
If the suystem seems to work okay for a while and then stops cooling and air flow it is most likely short of refrigerant causing the coil to ice, blocking airflow and preventing any air from reaching the cabin.
R-134a has an inherant capacity loss in these A-6/R-12 systems of nearly 20%, so that must be taken into consideration in any diagnosis as well
Greg Surfas

[Boltoncad41]

Thanks TJ Multiple Caddies and Cadillac Kid.
On AC - New R134 compressor as well, so should cycle, correct? Looks like they bypassed a sensor fuse, back left corner of engine bay - as you look at front of car. We Replaced switch, behind air cleaner, to no avail. We disconnected at switch for time being, to turn off, because compressor is engaged, even when control on dash is shifted to off position.

On cruise - we think it may be servo diaphragm connected to chain - because cannot hold a vacuum.
All I know for now, thanks again in advance for any more ideas!
Mike

["Cadillac Kid" Greg Surfas 15364]

Mike,
It sounds like someone jury rigged the control system and hooked the compressor directly to 12 volts. I am going to say that all bets are off as to what else was done and recommend you get a Factory service manual and follow the electrical and vacuum line diagrams to see exactly what departures have been made from factory.
Greg Surfas

[TJ Hopland]

Some pictures may help us try and figure out what you have going on there.  If we can figure out which stock system was there and how much is left we can help you come up with options.  One system was just used for 3 1/2 years and its pushing 50 years since they quit using it and the other is even older.

On the cruise if that was all you had hooked up when you were testing then that is likely your issue.  If you were tapped in somewhere else there are other places you could have a leak.  There is a T between the transducer and servo that goes to the brake pedal.  When the pedal is at rest there is a valve that closes so the system can generate vacuum. If the pedal is pressed at all the valve opens to dump any vacuum in the system.  This is the safety feature so that even if there is some other electrical or mechanical malfunction in the system pressing the brake will release/prevent it from being able to yank on the gas.  Just like the servo its rare to have that valve fail but it is possible.

How are you doing the testing?   A hand operated vacuum pump/gauge?  That's how I would be testing the system.