Hi Folks, New to the forum with an AC issue looking for help. First let me tell you about my 78 Caddy. I purchased this Caddy about nine months ago and the only issues have been power antennae and lack of AC cooling. The antennae was an easy fix the AC not so much. The original compressor, A6 type had a lead at the rear head. I decided to replace the A6 with a reman from Four Season. The reman came with oil for R134 and 4S said to drain the oil and refill with the correct mineral oil for R12, which I did. After flushing several times with the new asnd filled with the new oil in the amount drained from the old compressor.
I replaced the compressor cycling switch, drier, orifice tube and recovered the remaining R12 from the system, about one pound. Installed new compressor, vacuumed the system and refilled with the prescribed 3.75 lbs of R12 by weight.
After troubleshooting the Programmer on and off the vehicle using the book published by CadillacTim, found it to function perfectly. Diagnosed and tested the dach control and found the vacuum and electrical to be working as well.
Now here's the problem. The compressor after sitting overnight wants to cycly about every 5 to 8 seconds with very little refrigerant going to the evaporator and of course no cooling. Both inside and outside sensors test OK. What I have discovered is when I jumper the two wires on the cycling switch for about 20 seconds with the clutch engaged, the system operated normally and the system cools as it should.
Is seems the compressor looses its R12 prime and after "Jump Starting" the compressor the cycling switch functions and cycles the compressor normally.
I'm at a loss where to go from here. I would appreciate some advice from you Guys.
Since jumping the compressor cycling switch corrects the problem AND there is no other connection to the ATC control system, I would suggest replacing the switch.The points inside might just be on their way out and the initial inrush on starting might just be enough to "chatter" the points, causing them to drop the switch out.
Greg Surfas
1977 RWD cadillac introduced cycling switches and an orifice tube, can't remember if eldorado retained the VIR thru 1978
... maybe the orifice tube is restricted?
Op said his system does have a compressor cycling switch
Greg surfas
Yes it does have a compressor cycling switch which is new. Does not mean it's not defective. What puzzles me is after jumping the cycling switch for about 25 or 30 seconds, the compressor starts cycling normally and the system cools great. after shutting down the engine for 5 or 10 minutes upon startup the compressor cycles normally. After shutting down for an hour or more is when it starts short cycling and will continue until I jump the cycling switch as described.
Is it possible the compressor is loosing its prime?
No such thing
Greg surfas
On systems with orifice tubes ... I try and verify they are clear
By removing them to inspect them.
I have ordered a new compressor cycling switch. It seems the most likely culprit if the compressor is not at fault. Probably take a week to receive it and then install it. I'll post the results here. Thanks everyone.
Could it be going off on high pressure? Or isn't there a high pressure switch in these?
Let's start at the basics. Connect your gauges and without the engine running, do both gauges read about the same pressure as the ambient temp? 75° = about 75 psi. Now, start the engine and turn on the AC. Watch your gauges. Does the high side go too high? Does the low side go below 30 psi and then it cycles the compressor off or is it cycling off with at 40-50 psi? What is the low side pressure when it cycles the compressor back on? Need to know this before throwing parts at it.
If the low side pressure is immediately dropping below the cycle pressure, you have a restriction and it probably is the the orfice tube. If there was debris in the system, it is now in the orfice tube.
If you have an OEM, there will be a temperature/pressure performance chart in it. If not, search some earlier AC posts. I have posted several times the charts in the past few months.
If nothing else, record the pressures, high and low sides, and the ambient temp. Post them and we can get a better idea of what is going on.
I believe his system cycles on temperature not pressure. He has stated that once stable operation has been established by jumping the compressor switch temporarily there is no issue. He has also stated that the system was thoroughly cleaned out.
Greg Surfas
Dureing my AC system rebuild, I replaced the OEM cycling switch with an aftermarket switch. Today after comparing the OEM switch with the one I installed they are different in appearance. The dash control and the MKII Programer seem to be functioning correctly as do the sensors. This leaves the cycling switch as the most likely culprit.
The switch I removed seems to be functioning correctly when testing on the bench with an ice cube and ohm meter. At least it switches on and off, not sure if the temps are correct. Since I have already ordered a new switch, I'll wait to install it and I'll test it before installing.
Quote from: J. Jackson on October 26, 2023, 02:55:30 PMDureing my AC system rebuild, I replaced the OEM cycling switch with an aftermarket switch. Today after comparing the OEM switch with the one I installed they are different in appearance. The dash control and the MKII Programer seem to be functioning correctly as do the sensors. This leaves the cycling switch as the most likely culprit.
The switch I removed seems to be functioning correctly when testing on the bench with an ice cube and ohm meter. At least it switches on and off, not sure if the temps are correct. Since I have already ordered a new switch, I'll wait to install it and I'll test it before installing.
Please let us know the outcome as I am also looking to get my AC functioning again with the original R12 refrigerant. I am trying to learn about this system as much as possible before I begin.
Rich,
Although some may find it a bit difficult to use for diagnosis,the Factory Service Manual is an excellent source of information on all the components and systems of your Automatic Temperature Control (ATC) system. Each component is described, its function explained and it's purpose int he component system.
There are basically 5 component systems: 1: the cooling (refrigeration) system, 2: the heating system , 3: the fan system, 4: the air distribution system, and 5: the control system.
When broken down this way it is not difficult to determine where any issues lie and how to remedy them
Greg Surfas
I agree, the GM Service Manual is invaluable when diagnosing and repairing these old cars. I have the 77 GM Manual and also the 78 Supplement manual. The Climate Control systems on these GM's can be very daunting. The troubleshooting book published by Cadillac Tim has been the most helpful with the issues on my '78 Eldorado Barritz.
Here's an update on my A/C issue for those that posted a reply. Even though I had replaced the cycling switch earlier, I purchased another and installed it. Compressor seems to cycle normally now and cabin temps are good. Low side pressure is 30 psi and Hi side pressure is 200 psi at 70 deg ambient. I think I'm good to go now. Thanks again to those who helped with their responses.