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At the suggestion of Mrmerlin, I'm posting this to see if any folks here might have dealt with a similar experience as I. I believe that this issue is strictly related to 1978's (and maybe 79's, though I'm not sure).
I've continued to have issues with successfully getting the AC compressor engaged. Sometimes it will, most times it won't. I began the troubleshooting process by looking at a number of items: the freeze switch, the AC temp switch on the receiver drier, the AC temp switch on the radiator, the backside of the HVAC assembly, some of the vacuum lines, etc. Everything seemed to be in working order or was repaired if it wasn't. But the issue remained despite all of that.
I took her out for a drive again today and the compressor wouldn't engage no matter what setting I tried on the HVAC nor how gentle or swiftly I tried moving the lower slider. I've always been able to feel the subtle differences in the internal slider assembly as it changes in depth and moves across the microswitch. Today was no different, but compressor engagement still wouldn't happen. I then did one last movement with the lower slider to Defrost while the upper slider was on full cold (left). The compressor engaged and cold air started blowing out of the defrost vents! It blowed cold for a few minutes and I then attempted to get the compressor to stay engaged while I move the lower slider to the regular AC position. But once I did so, the compressor turn off and the air warmed up.....
So I brought her back home and looked behind the HVAC from the passenger side footwell area. I had the idea of trying to depress the microswitch myself with a small screwdriver to see what would happen. I had to move the lower slider to the dash vent (arrow up) icon in order to get the internal slider assembly just far enough away from the microswitch so that I could stick the screwdriver in and press on the microswitch. Once I did so, the compressor engaged!
So here's the problem in summary: the internal slider assembly and/or the microswitch are worn just enough that the microswitch isn't being depressed as much as it needs to be. I'd say it's lacking about 1/32" to 1/16" of movement. It's such a simple problem but I imagine it is not such a simple solution. From what I can see while leaning under and watching everything move, there's no play or excess movement in any of the internal slider assembly or the red housing that the microswitch sits on.
the early cars have a huge vacuum line arrangement included with the head unit its completely different than the later units.
What we are looking for are pictures of the insides of this particular head unit to figure out how to fix it,
as its like they built the console then built the car around it so console and HVAC head unit removal are very difficult on the early cars
I currently have my whole center console apart and the HVAC head is loose, so I can take a picture of it tomorrow evening or Monday, if someone else hasn't been able to help.
I currently have my whole center console apart and the HVAC head is loose, so I can take a picture of it tomorrow evening or Monday, if someone else hasn't been able to help.
I envy that you have yours out but I don't envy what you probably went through to get it out.....
It would probably be very helpful if you could take pics of the switch as it might be difficult for me to get good ones due to its location. I'll also really only be able to get them from the viewpoint of looking up at it from the passenger footwell area (which I think offers a better viewpoint that the driver side footwell area).
You can fix this yourself. There's 3 issues at play:
1. The contacts in switches can become flakey and inconsistent in their operation.
2. The mounting of the switch (rivets) can get loose and also cause intermittent operation.
3. worn or broken plastic bushings in the slider/cams system.,
Pull the unit out. Be careful not to brake the housing where the cable clip is installed.
Manually button push and test the switch about 20 times in a row with an OHM meter. If inconsistent drill the rivets pull the switch apart and repair. you can put it back together with small screws and nuts.
If it works consistently with the manual button push, check that the switch isn't moving on the rivets...then check the action of the cam that's supposed to push the button at AC and DEF positions. various plastic bushings can be worn or broken causing uneven and inconsistent actuation.
Quite simple compared to electronic units, but most of the used ones I see have this problem and need this service.
Where did you source new plastic bushings? Mine appear intact but they might be worn just enough that they aren’t moving the cam assembly quite far enough.
I did try to wiggle the red box that contains the micro switch and there didn’t appear to be any movement. But that’s also not as accurate of an assessment of removing the HVAC unit and getting hands on with it.
I just really don’t want to have to remove the HVAC due to the difficulty involved. I’m also not confident that I can do it without jeopardizing the large fascia trim assembly on the console. The more invasive I go the more likely something is going to break.
Last edited by islaTurbine; 07-07-2019 at 12:48 PM.
If your car still has the 1978 console, it takes time to remove the unit, it screws in from the backside of the console.
the only way I know to "fix" it without removing the unit is to plug the switch wires into a separate toggle switch you can turn on and off manually. the only source I have for parts is from the other used units i've collected.
I had the same thing happening with my ‘78. It was a flakey a/c speed relay. Sometimes it worked, sometimes not. I replaced it with a reliable spare, works great now. I’ve heard you can just jumper it in the panel if you can’t get one. Only difference will be the a/c compressor will run during startup if left in a/c position on the hvac adding load to the starter, which I’ve heard isn’t a huge deal.
another test:
With your hvac in the a/c position put an alligator jumper between the jump post and the a/c compressor and start the car. Does the clutch engage now? This will tell you if your clutch is working properly
I had the same thing happening with my ‘78. It was a flakey a/c speed relay. Sometimes it worked, sometimes not. I replaced it with a reliable spare, works great now. I’ve heard you can just jumper it in the panel if you can’t get one. Only difference will be the a/c compressor will run during startup if left in a/c position on the hvac adding load to the starter, which I’ve heard isn’t a huge deal.
another test:
With your hvac in the a/c position put an alligator jumper between the jump post and the a/c compressor and start the car. Does the clutch engage now? This will tell you if your clutch is working properly
Did you mess with the microswitch at all before discovering that the relay was the issue?
Sorry, I guess I missed the fact you had already narrowed it down to the micro switch. Nah, I didn’t have to touch the micro switch. I was just having clutch engagement issues.
I dealt with this on my 79. In my case the AC compressor switch had a spade that broke off. I found a replacement switch assembly at Frys Electronics for $2 and drilled out the rivets and attached the new one with small bolts and nuts. has worked flawlessly for many years. See post 9.
Only way you are going to repair/replace the micro switch is by pulling the unit. This is a Royal PITA. You will need to remove or at least free up the center console. I would also recommend removing the Radio. You will need as much room as possible to get those screws in and out. I did this on my 78 about 8 months ago and once I finally got everything back together was very happy, everything now works perfectly. You can order the part from Digikey - here is a link to the one I ordered -
I dealt with this on my 79. In my case the AC compressor switch had a spade that broke off. I found a replacement switch assembly at Frys Electronics for $2 and drilled out the rivets and attached the new one with small bolts and nuts. has worked flawlessly for many years. See post 9.
Only way you are going to repair/replace the micro switch is by pulling the unit. This is a Royal PITA. You will need to remove or at least free up the center console. I would also recommend removing the Radio. You will need as much room as possible to get those screws in and out. I did this on my 78 about 8 months ago and once I finally got everything back together was very happy, everything now works perfectly. You can order the part from Digikey - here is a link to the one I ordered -
I have quite a few photos from when I repaired mine, just let me know if you need anything
Great info. Thank you.
This is sounds like a hearty Fall/Winter type of project.
For now now I think my solution is to cut the end off of a zip tie and slide it in between the cam arm and the microswitch. This should close the gap and will stay in place so long as I don’t move the lower slider away from the AC setting (which shouldn’t be a problem in Georgia!).
07-06-2019, 11:07 PM
78 OB HVAC Microswitch
