1971 Cutlass A/C repair

Old November 11th, 2018, 01:27 PM
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1971 Cutlass A/C repair

1971 Cutlass Supreme Convertible Air Conditioner problems


When I got the 1971 Cutlass Supreme Convertible the A/C clutch switch in the dash had been bypassed by the previous owner as the switch had apparently gone bad. The A/C system was empty and void of R134a oil. The blower motor speed control switch only worked on low, med, med high and when placed on high it would shut the motor off. The power and ground wire to the A/C compressor clutch was missing. The replacement and remanufactured master control switch were obtained from the ParrtsGuy.com, these guys are a good source for refurbished parts for the vintage autos. Even though the refurbished unit had arrived months ago, I finally got around to installing it. The master switch in the cabin has 7 components.

1) The base (the A/C heater control and control lever). It is the part that attaches to the dash board head and has attached the following parts.
2) The fan speed control switch.
3) The A/C compressor clutch switch
4) The master switch
5) The vacuum hose control valve
6) The water vacuum valve
7) The dashboard mounting bezel.

Photo attached.

Troubles shooting and repair

While waiting for the refurbished switch, I built the missing wires from the Ambient control switch to the A/C clutch and a ground from the A/C clutch to a bolt next to the compressor. The compressor clutch has two pins. On this car one is for power and one is for ground and on this car the power is the pin closest to the passenger side fender on the ground is the closest to the engine.The old master control switch was removed from the dash and replaced with the refurbished unit from the Partsguy.com. A ground wire was attached to the dangling unit. I wanted the unit free and visible so I could get to the unit for trouble shooting. The problems began. Once the unit was installed the unit blew the heater motor fuse. I checked the wiring on the new unit to be sure the error was not mine. All the wires were installed correctly and no signs of bare wire were seen on the heater control. I traced all wires and found nothing that would cause the short. I removed the ground off of the unit and everything worked fine but the motor was dragging. Thinking that the motor was bad and the dragging could cause a load on the 20 amp fuse I removed the motor for testing.The video posted by ant hony is an excellent video and if you have any questions you should watch this video as it is exactly how to remove the blower motor. Even though it is for a Monte Carlo it is very detailed and is the same procedure for the 1971 Cutlass. Follow this link
a. Procedure to remove the motor
i. Remove the right front tire
ii. Pull the inner wheel well housing
iii. Remove the motor
iv. At the bench, I attached the motor to a ground and then then 12 volts to the power pin, the motor speed should go immediately to
top speed without reservation. If it does not, replace the motor. This motor tested fine. What I did find when I was removing the motor
was that the ground wire for the motor was corroded and dirty. I suspect it was not getting a good ground to the fire wall and is the reason
the motor was dragging. I cleaned that ground area and replaced the ground wire and reinstalled the motor. Be sure to use a good silicone sealer on the motor to keep
the water out.

After the motor was re-installed I again tested the control switch with the ground attached. Again; the heater fuse blew. I searched the internet and shop manuals for suggestions. I found this video posted by Tyler’s Neighborhood Garage at
. This video is invaluable to understand all the components that make the A/C and heater work on your vintage car. This video helped me expedite my trouble shooting.

b. Looking for the short, even though I had low speeds at the switch, I then checked the connections on the low speed blower motor resistor and pulled the resister from the air box near the passenger side fender. The resister looked good and no shorts were found. All the coils on the resister were intact. I tested the high-speed circuit just behind the master cylinder. The unit tested fine but I noticed that the unit had 4 pins instead of three and the manufacturer part number did not match the part number required for this car. I replaced the high-speed circuit. After replacing the circuit, I had speed at all four levels at the master switch. I inspected the wires and no shorts were found.
c. With the master control grounded to the dash the unit still blew fuses. I had now checked wires, low speed and high-speed circuits and the blower motor. The remaining items to check were the ambient circuit and the A/C clutch connections. As before these wires and circuits were all good.

I tested the unit and it now worked flawlessly. What I had not noticed is that during the testing of the A/C clutch switch and Ambient control circuit is that I accidentally knocked the jumper wire to ground off the dash board ground. I was elated that the unit worked flawlessly. Now that I had a working master switch, I installed the controller back into the dash and tested the unit. Immediately the fuse blew.I pulled the unit out of the dash again and let the unit dangle. I tested the unit grounded and it blew the fuse again. I took the ground off and the unit worked perfectly. I put a test light to the frame of the master control unit and it lit up. The short was on the Master Control unit.

Further testing found the master control switch was shorted. I pulled the master switch from my old unit and replaced the master switch on the refurbished unit and the short was gone. If you ever have to remove or replace the master control power switch be sure to adjust the unit so that it shuts the motor off when you slide the controllers to off. The switch is slotted to allow adjustment. After the switch was adjusted and the unit tested again, the unit was mounted in the dash and now works perfectly.

Recharging the unit.

The unit did not need to be evacuated as the R-138a oil from the system had leaked out a long time ago and before I got the car. Chrisfix posted this video at
and is a good tutorial for charging an A/C system.

1) Installed a vacuum pump and pulled 30 microns of vacuum on the system. Turned off the valves to the pump and the system slowly lost the vacuum. The vacuum at 30 microns on a sealed system will stay at 30 and will never drop.
2) I replaced the O-rings on the A/C lines on the system.

The O-rings were purchased from NAPA and resolved the issue of losing vacuum.

1) After staying at 30 microns of vacuum for over 30 minutes I turned the vacuum pump on and let it run all night for 9 hours to dry the system.
2) I Installed 3.5 pound of R134-a oil and I have a working A/C system.

Parts left over

With the job completed I have the following parts available to anybody who wants them. Just pay for shipping. the Heater control front bezel It is in really good shape, the controller *****, the vacuum controller, the water vacuum valve and the control levers.

Special thanks to, the partguy.com, Tyler’s Neighborhood Garage, ant hony, chrisfix, the 1971 Oldsmobile Chassis service manual and an enlarged full-size color wiring diagram.Hope this post helps somebody in the future.

Last edited by covert689; November 11th, 2018 at 01:35 PM.
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Old November 12th, 2018, 08:09 AM
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Location: Northern VA
Posts: 31,646
Two suggestions.

First, if the motor runs on ANY speed, the motor is NOT the problem. You don't need to waste your time R&R'ing it.

Second, I suggest you pay the money and buy an original (not reprint or PDF version) of a Chassis Service Manual for your car. Interweb info is notoriously inaccurate.
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