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How To Modify the 68-70 A body latching function for the AC Compressor hold-in relay
I thought I would share a modification I made to a 69 442 factory AC system to make the system function like the AC systems in modern cars. The appearance was not altered and someone would need to look very hard to see what was done. I’ll offer a few modifications that can be made and will cover these over several posts. It seems the function of the AC system in the 68-70 A body cars, especially as it pertains to the Hold-In Relay is misunderstood at times. Hopefully this will shed a little light on this topic and how the AC systems in these cars can be made to operate more efficiently. The modifications shown will allow the compressor to be switched off and on with the dash selector switch in AC mode; switch the compressor on and off through the Defrost setting and correcting the system operation when the Hold-In Relay is dead. These modifications are for the 68-70 A body cars only. I know some of the larger cars from 68-70 use a similar system but I have not studied their design so I don’t want to say if these modifications will be applicable to those cars or not. I'll follow this with several posts and hopefully I don't get you too bored with all of the text. I just want to make sure the info is clearly explained.
Those of you who have owned or currently own a 68-70 Cutlass/442 with factory AC know the compressor, once engaged by placing the dash selector in the AC mode, will continue to run despite the dash selector control being placed in the off position. The only way the compressor is disengaged is when the ignition is switched off. I’ve seen a few recommendations over the years on how to resolve this but none really seemed to do it correctly. A couple of years ago I began the restoration of a 69 442 with factory air and decided I would try to come up with a solution that would allow the AC system to function like the AC systems in our newer cars. I previously owned another 69 442 and wasn’t happy that I couldn’t turn off the compressor when I wanted to. The design of the original system is based on a single switch in the dash control to operate the AC compressor. There is a second switch that switches power on/off to the fan switch but that circuit is not relative to this discussion. The issue with a single switch for the compressor presents itself in that the compressor is required to function to provide cooling and is also desired in the Defrost mode to provide dehumidification. Most or all newer cars will operate the AC compressor when the Defrost mode is selected. Since there isn’t a switch in the dash control to activate the compressor for the Defrost setting, a Hold-In relay was added to make this possible. This relay is located under the hood on the passenger side fender near the center of the wheelhouse. This relay, by design, is the source of the problem with the compressor not turning off when the dash control is no longer in the AC or defrost (w/dehumidification) settings. The Hold-In relay is engaged when the dash selector is placed in the AC mode and this relay, by design, will continue to power the clutch on the compressor until the relay is de-energized. This design was done in this manner to allow the compressor to operate in the defrost mode, thus providing dehumidification. This is accomplished by sliding the dash control over to the AC mode and then back to Defrost. This operation is explained in the owner’s manual. Because of the Hold-In relay operation, the Oldsmobile engineers chose to place the AC mode in the dash control over to the far right. This allowed the heat, vent and defrost (w/o dehumidification) to be used without powering up the Hold-In Relay. Pictures will follow in the next posts.
Before going too far with anything, a few simple tests should be done to test the electrical integrity of the compressor switching circuit. This assumes the Freon is charged and there are no issues with the compressor. I have drawn a schematic that will show how the wiring allows the system to function. There are only 2 color wires for the compressor control circuit; brown and green. Brown is the main power source and becomes present when the ignition is switched to the RUN position. The green wire receives power from the brown lead once the AC switch in the dash control is closed. The first thing to do would be to make sure your relay has the terminals in the same place as the one in the photos. If not, you have one from another car, likely one of the big Olds models and I can’t say if that relay is the same. To start the testing exercise, start by checking for voltage to the brown wire that connects to terminal #3 on the Hold-In relay when the ignition is placed in the RUN position. If so, with the ignition still in the RUN position, place the dash selector in the AC mode and see if voltage becomes present to green wire on terminal #4. If everything is good, the dash control is working correctly. In order to check the Hold-In relay, an ohm meter can be used to measure resistance between terminal #4 and the rivet head that is the #5 location. Make sure the connections are clean and use a little sandpaper if necessary. I tested two different relays that I have and both measured 41 ohms. Given this ohm reading, it indicates that the coil windings are intact and not burned out and thus the relay should work. Lastly would be to put 12V to terminal #4 and ground to the outer housing (if the cover is on) and see if you hear the relay operate. If not, remove the cover and connect the ground wire directly to the copper ground tab riveted to #5. The outer cover is the ground path for grounding the relay circuit so it’s important that the connection to the fender is good. I have also seen corrosion inside the cover where the copper tab is unable to make a good connection to the cover so that needs to be resolved if it’s an issue. The arrow in one of the photos shows the copper ground tab. If the relay is dead, you can either purchase a replacement or follow along and I’ll explain how this can be overcome later in these posts.
With the Hold In relay cover removed, we can see a white jumper wire that internally connects between terminals 2 and 4. The purpose of this jumper wire is to continuously power the relay once it is energized. When power is placed to terminal #4, energizing the relay, it also closes the contact on top and the 12V on terminal #3 is now sent to terminal 2 which will energize the compressor. When terminal #2 receives power, the voltage is carried back through the jumper wire to continuously supply power to #4, which contains the power lead for the coil winding. This is why this relay will not de-energize when the dash control is moved out of the AC mode. This is also how the engineers were able to energize the compressor and keep it engaged when dehumidification is needed for the Defrost mode. I have seen cases where the function of this relay was changed by connecting the 2 wire plug to the relay in a different manner. One of the photos show the plug rotated so the brown wire is no longer on terminal #3. While this will allow the relay to de-energize when the dash control is moved out of the AC mode (because the relay will no longer be energized without voltage to terminal #3) it also is carrying the current for the compressor through that smaller jumper wire which is much smaller in size that the green wires to the relay and compressor. I would tend to think this would eventually lead to the white wire overheating and failing. The other issue is the load for the compressor is now switched through the dash control switch. The dash control switch was not designed to switch operating loads but rather to switch low current power to a relay and the relay is intended to switch the larger current loads. Putting the load on the dash control switch will likely reduce the useful life of those contacts. With this understood, if the intent is to make the compressor switch on and off with the dash control AC mode only and there is no need to have dehumidification for the defrost mode, then simply remove the white jumper wire. Make sure to remove it rather than cut it so it can’t cause a short. Be careful not to cut the very small red winding wire that goes to the relay coil. With the jumper wire removed, the compressor will operate when the dash control is in the AC mode and as soon as the dash control is moved out of the AC mode, the compressor will disengaged.
The Defrost Mode Operation If there is a desire to have dehumidification for the defrost setting, because the vehicle is used when it rains or when the windows could fog, there is now a need to add another switch to activate the Hold-In Relay/compressor when the dash control is placed in the Defrost mode. I looked at trying add a switch that would be contacted in some manner when the dash control was in the Defrost mode but that looked to be too complicated and would require some effort (read that to be custom machining) so I took another approach. In the vacuum hose routing schematic photo taken from the CSM, I found that vacuum would be used to operate one of the vacuum diaphragms on the heater box when the dash control was placed in the Defrost mode. An arrow shows that diaphragm. I did a little searching and was able to find a miniature vacuum operated switch made by World Magnetics (model PSF109S w/optional 1/8” MNPT port) and found this could easily be located on the dash control. I’m sure there are others that will work. I don’t remember where I got the switch and it may have been sent as a free sample. Please note that I had to move the switch to the other side of the control as I found the switch hit on the dash structure when I installed the dash back into the car. Another photo will show where it ended up. It can also be located elsewhere if desired. To bolt it in place, I used 2 sections of #6 screws and used epoxy to attach them to the switch so they functioned like mounting studs. I also purchased a plastic 1/8” FNPT fitting with an 1/8” hose barb from McMaster Carr #5116K301 (pkg of 10..only $5.84 a year ago). A trip to Radio Shack provided the single-to-double spade connectors that can be seen in the photos. Bringing the brown wire to the “Common” on the switch and the green to the NC would switch the compressor when vacuum was present. Just make sure to keep brown with the wire harness brown and green with green. A small vacuum “Tee” fitting and section of hose was used to tap into the vacuum line for the diaphragm. Now when the Defrost mode is selected in the dash control, the compressor will be activated. Move it out of the Defrost mode and the compressor will disengage.
The next photos show the switch installed and the double contacts on the factory switch. The wiring is being added to connect through the vacuum switch and the Tee fitting has been installed to the vacuum switch. Lastly it will show ehere I had to relocate the vacuum witch due to the original location causing the switch to hit on the dash structure. The vacuum switch doesn't need to be on the factory control switch but I just put it there to keep everything together.
Lastly, I wanted to show how to modify the system to deal with a dead Hold-In relay if a new relay wasn’t available. I based this modification on using an aftermarket headlight relay that would work in combination with the stock dash control and a slight modification to the Hold-In relay. The vacuum switch in the electrical circuit for the Defrost mode control of the compressor can still be used. I have attached a schematic showing how the headlight relay would be used in the electrical circuit. The green wire that runs to terminal #4 on the Hold-In relay will be cut at a point under the dash as will be brown wire that runs to terminal #3. The brown wire coming from the dash control switch(s) will connect to Power Feed contact on the headlight relay and the other section of brown wire that continues out to terminal #3 will connect to the NC contact. The green wire coming from the dash control switch(s) will connect to the headlight relay terminal required to power it. The remaining section of green wire that runs out to terminal #4 on the Hold-In relay will no longer be used electrically but will be retained for appearance only. A ground wire will need to be added to connect the headlight relay ground terminal for the coil to the car’s ground. The last step will be to add a jumper to the Hold-In relay to permanently tie terminals #3 and #2 together. With that jumper installed, the non functional Hold-In relay will serve as an electrical pathway for power from the headlight relay to the compressor. In the enclosed photo, I show an orange wire and where it connects to tie #3 and #2 together. My wire is not soldered in place because the relay is still function and does not need the headlight relay modification. Regardless of which modifications I detail is used, the external appearance of the Hold-In relay and connecting wires remains unchanged giving the impression that the function of the system is factory stock. I should point out that the modification removing the white jumper wire from the Hold-In relay was proven over a period of time on a car in TX. The vacuum switch and headlight modifications have only bench tested but should work fine in the car. Hopefully this sheds a little light on the function of the AC compressor circuit in these 68-70 A body cars and can show someone how to modify the system to operate in a more enjoyable manner. With gas prices once again on the rise, there is no need to operate the AC compressor any more than is necessary.
This is very interesting. I assume they changed this system in 1971. I have noticed the relay you speak of on my 70 Vista and its not there on my 71 Cutlass. You are very knowledgible about A/C. I have seen your posts before. I just started a thread asking for advice on getting my 71 back in operation. Would appreciate your input. Thanks Greg
Thanks Greg. Yes, in 1971 Olds eliminated the Hold In relay and changed the way the compressor was controlled. That change allowed the compressor to be swicthed off or on with the dash controls. The modifications I showed for the 68-70 A Body system was to allow it to function like a 71 and newer vehicle. I had a 69 442 with AC about 12 years ago and the AC function really bugged me. So when I took on the retoration of a 69 442 convertible with AC, I wanted to see what I could do to modifiy the system to work better without changing the outward appearance. I looked at the electrical schematics, did a little testing and came up with the modifications.
I saw your thread and like was mentioned, we really need to know what you want to do with your car. If it's restored or will be restored to stock specs, then you would need to use parts that give that look. If you are not concerned with a factory appearance, then you have options. Try to detail in your thread what you are looking to do with the car and with that info, you'll get some good input. I like a stock look so the plate to cover the factory holes in the firewall wouldn't work for me. But that's just me.
Brian, Yeah I like the stock look also, I'm leaning that way so far. I think I'll take out the evaporator and get cleaned out and checked-replace if needed just get flushed if ok. Then get new hoses,new original style compressor, get POA valve recalibrated for R134a. I'm not sure about the condenser. I have the original but I have heard its better to get a new one for 134?? -Get new drier,etc. I wish I knew some one nearby to talk to and do the work. I can replace the parts but I need someone to evacuate it and add refrigerant,etc. A buddy of mine had a aftermarket system put on his 40 Ford by a ARA shop in Lansing. I may talk to that guy. Thanks again, Greg.
An excellent post with a good resolution for those with the application. Now if someone could find a real solution to the A6 seizure phenomenon, many more of us could benefit.
How can we make this a sticky? Since it was brought up in the recent 1970 A/C hold-in relay thread(s), I really think it should be at the top full time.
Thoughts?
For a quick schematic representation of the white wire to cut inside the relay to get rid of the "hold-in" feature, but otherwise have operation on/off with the control head, here it is schematically
If you remove this wire, the hold in feature goes away, but otherwise works as before. Then you would implement the defog mod with a vacuum operated electric switch as outlined above. I am absolutely going to do this.
Thanks a bunch Brian!
FWIW, the official OEM Oldsmobile documentation, as identified in the Product Information Manual and accompanying diagrams is referred to as the A/C compressor hold in relay.
Since it does, in fact, have a name provided by GM Oldsmobile and a valid Part Number, my suggestion would be to identify the title by its Oldsmobile name.
FWIW, the official OEM Oldsmobile documentation, as identified in the Product Information Manual and accompanying diagrams is referred to as the A/C compressor hold in relay.
Done
Originally Posted by rfpowerdude
Thanks Eric!
Do you think it should be moved to the Electrical section rather than the Other section?
If anyone wants to do the modernized defog vacuum switch mod like Brian did, I just bought this vacuum switch from Amazon (actually Transmaxx online Amazon storefront) and it looks like it is readily available and not too expensive.
The instructions are wrong, however, as they have the wrong terminal connected. The one they have connected is the switch closed when NO VACUUM (atmospheric). The other one is the one to which you connect to switch once vacuum is more than the setpoint. I just verified it with my ohmmeter and vacuum pump. The N.C. terminal is closed at atmospheric and the N.O. terminal is closed under vacuum.
Last edited by rfpowerdude; July 20th, 2023 at 03:35 PM.
January 22nd, 2011, 07:54 AM
Now I have to go and restore one of my A/C cars...
The next challenge
