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Misbehaving clutch cycling switch

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    My dads' 73 was a rust bucket, He ordered it new and the only options he ordered were H 78 15 tires (bigger than Std, but matched the other car we had) and Posi. It was a 3 on the tree and 250 CID 6. IIRC he had it Zebart'd but that didn't help much. He got it after his last car was stolen. He worked in Cambridge and figured a "stripper" would be less likely to attract thieves.


      OMG - Cambridge! What you don't realize is there's less salt than just 10 miles inland than there and no place to drive fast enough to really drive/spin salt soaked water up everywhere so they look better longer there, Boston has zero parking so all are garaged or not used at all, all season I think had the #1 theft rating in USA? I'm 22 miles inland from the state house in Boston so mix of office parks and expensive residential all around but quick highway access any direction. Commuters do rough the traffic and live this far out just not so many anymore. Biz just can't take the parking bull or the costs in either city very well nor most real close to those frequently would just say I'm from Boston which is a tiny bit of real estate doesn't matter if near it you are from there. Home to the "Big Dig" the most costly 1 mile of road in world history I think still stands was 18 BILLION BUCKS now long ago.

      Off topic but A/C: That was also the most refrigeration BTU power ever used in human history. Boston is filled ocean and muddy so had to freeze the whole mile solid to drill a tunnel or support buildings while working on that!

      The point is as you just said no A/C your Dad probably didn't need to go far enough if parked in sun and heat to make use of it. Sure do remember real vent wing windows and real floor vents not this crap that says "vent" and is 5 degrees warmer than real outdoor air - check that on almost any vehicle I do.

      Gotta love it you recall the tire size! Those were plain belted non radials with that exact # so was a real stripper. Radial tires would have cost more!
      MetroWest, Boston


        Originally posted by huntindog View Post
        My 73 Blazer has had a non functioniing clutch cycling switch.. It is new, as is everything else in the system.
        The new switch has never cycled off, even at really low vent temps of 32.5. After much discussion and thinking about it, I decided to leave it as is for awhile. Being in Phoenix AZ, it isn't a huge problem.
        This morning I was on the highway about 4AM, Ambient temp was about 88. The vent temp got to 37.8, and my clutch cycling status light stated flickering really fast. ( I installed this and a clutch shutoff switch some years back when I used to tow with it)
        It is a great way to see just what is going on with the clutch..
        I surmised that the switch is trying to work now. I think that as an R12 switch, it may have set on the shelf for so long that it became stuck, and is now working it's way loose.

        My concern is that it may over heat the clutch with the rapid on off it is doing, causing it to slip. I turned it off a couple of times with the manual switch, and it went right back to the flickering light.
        I am considering just jumping the switch terminals for a few days to see if the switch will settle down and start working normally. Since it's been running awhile with it non functioning, there would be no real harm right now. I also considered just replacing it, but it's replacement may well be in the same condition.

        I estimate that the evaporator temp was right around 32 this morning when this occurred...

        When you say flickering really fast, are you talking several times a second? Does it flicker when parked? I have a 1973 Chevrolet Truck manual. It shows a Discharge Pressure Switch. It doesn't describe its function but I presume that it works like the Clutch Cycling Switch in my Ford. The switch opens at about 24 PSI and closes when the pressure raises to about 42 PSI. This is how it controls the refrigerant temperature. Typical for CCOT.

        The first thing that should be done is measure the resistance of the switch when it is not running. It should be near zero ohms. If it is high it will mess with the electricity flow in the circuit. If there is a Shrader valve under the switch unscrew the switch. You should hear/feel it change states. The resistance should be infinity,an open circuit. Tighten the switch, you should hear/feel it change states. Measure the resistance. If there is high resistance, you can try loosening and tightening several times to see if it gets better. If the closed resistance is high, there is your problem. Is there a relay in the circuit? If yes, you need to measure the resistance there.

        If the resistance of the switch is good, measure the voltage at the clutch to see if it agrees with your indicator light. Note that digital multimeters samples for a certain amount of time before it displays the results. Auto ranging is slower yet. If the flickering is very fast, you wont see it, but it should show the average voltage. An analogue meter (with a needle) will respond quicker. If you use an incandescent bulb, the filament maybe broken and making interment contact.

        A good test would be to jumper the connector for the switch and see if the clutch cycles. Don't leave it on for more than five minutes. Since you know how to wire switches, you can route a switch into the cab. I suggest that you wire it in parallel so you can flip it when you see the flickering.

        If all this checks good, you need to know what what the pressures are doing. Hook up a gauge set and observe.

        After rereading the manual I learned that the temperature is controlled by the Thermostatic Switch. The Discharge Pressure Switch must be for low refrigerant protection. I don't know if your switch is a CC switch or just a low pressure switch.

        For the last thirty years I have lived on acreage on an dead end road close to town with three other houses around. It has been so peaceful and relaxing. The most noise I would hear is my refrigerator starting. The couple across the road moved into assisted living and sold their four acres to a trucking company who is going to remove all the trees and grass and replace it with concrete and asphalt. The construction trucks and equipment arrived this morning. It is so loud. The back-up alarms are the worst. The vibrators on the dump trucks are so loud and shake my house. I am so distraught. They want to use my property to have access to their property. I said no. They may get their way. There will be forty trucks a day and night will drive right next to my lawn, 20 feet from my front door. There is usually about six cars a day that passes by. All this has made me start smoking again.
        Last edited by I hate working on cars; 06-24-2017, 02:22 PM.


          Originally posted by Cornbinder89 View Post
          What I'm reading could be wrong. I just remember a lot of VIR back then and then the changeover to CCOT in the later 70's but my memory could be off and what I've seen is not OEM manuals, Can't locate my '73 Chevy truck manual, which I know would be accurate. I always thought the CCOT was a later design, but am willing to admit I don't have hard evidence for that. Our '73 Chevy Pick-up didn't have A/C, so I can say for sure.
          I have the same 1973 manual. It is not written well and the schematics don't show the components. It does show and discuss an orifice tube. The clutch cycling is controlled by evaporator temperature rather than low side pressures (as is in my 1993 F350).


            The two wire LPCO should be a 100% transfer of what power is there all the time on one side simply a switch put same exact power on to clutch. I personally never had one that dropped amount of current thru one but probably would thin OHM testing for 100%. Oh boy if that drop available power on to clutch it could cause odd behavior.

            Just other about 32F outputs. At some point to me that's just annoyingly cold. If you blow cold like that on you and can't turn it away it would be annoying to me personally.

            I'd much rather volumes of steady cool dry air more like home central heat and A/C does on purpose.

            Back - there should be a rise in thru the ductwork if just on vent of something like at best 2-4 degrees just lost to a warm firewall and uninsulated ducts to vents.

            There's the freeze up you don't want nor liquid flooding along to be compressed and can with failures lock up compressors, squeal belt it lucky or ruin it!

            The humidity may not build up enough water to freeze up if dry enough. If evap does ice up all I've found did the same things - airflow get lower can't pass thru and if you stop vehicle and shut down a huge puddle comes out the ice will melt fast.

            These switches GM or Ford are depressing a Schrader when installed and sealing that port. If you just turned it loose the threads alone are not going to seal refrigerant you would adjust it or if not a way inside them a brass screw get another for just that.

            I do wonder if power passed on if weaker than should be battery voltage with adequate amps delivered would cause clutch to cycle. Said here or somewhere you can test this that the warmer the metal the less magnetic it is.

            Try if you wish glow a piece of metal in a vise with just propane and a magnet pick up tool even will not stick to that but will when cooler,

            MetroWest, Boston


              Originally posted by Tom Greenleaf View Post
              Trivia: If clutches friction surface is hot from cycling it's losing magnetic force to pull it in or keep it engage may cool and be more magnetic! Funky but the hotter the metal the less magnetic! A maybe to rule out and might need checking function of the clutch of its coil,
              Kind of. There is a thing called the "Curie Temperature". This is the temperature when materials will change from Face-Centered-Cubic to Body-Centered-Cubic and become magnetized or not. There is no ramp as the temperature rises toward the Curie Point but rather, it is a sharp change. The temperature varies with different materials. Were are talking heat treat temperatures. Much higher than a clutch will ever see in normal operation. I can see that the strength of the magnetic field of the clutch could weaken as temps rise, but I don't know. I know that an alternators output is reduced as it gets hotter.


                Another thought to check as just a dang switch has never been that much trouble to me? Is the correct current getting to the clutch via the wire you have? Hmmm on testing but maybe quick off the top of my head might try to chase the plug, wire itself for a soft spot or where it might fray inside not actually enough gauge really makes it - just a maybe.

                That may also show just with a voltage drop from switch to backprobing the plug at clutch. No way to know how much trauma the wire has been under without checking.

                This is found more often in a driver's door jam where wires bend and a power window or lock might be very erratic as it frays inside and unseen to a total disconnect then you really know. Those you can pull on each wire and usually find one that's springy only insulation left wire has frayed. Just a thought?
                MetroWest, Boston


                  Update: The switch is now fixed.

                  And I learned some things along the way.
                  The GM CCOT technically came out in 1977. Before that, instead of an OT, it was a sponge metal type affair.. I think it was brass. I actually removed that over 20 years ago when I did the first compressor job on the Blazer.
                  But it really worked pretty much the same way. Instead of a cc switch on the accumulator, a thermister was used in the evaporator to shut off the compressor.

                  Another thing. Freon capacities are listed as 44 or 48 oz. I know that there were two different condensers from the factory. My 73 Blazer was a serpentine affair with the tubes at a 45 degree angle winding back and forth. It was a very thick, stout piece. Later a thinner condenser with the tubes vertical and split in two was used... I think this was called a dual pass.
                  I have both types in my side yard, and it is pretty obvious that the 73 serpentine will have a higher capacity. So I believe the 48 oz spec was accurate for that condenser, and 44 oz for the later ones.
                  Now the replacement is an even smaller dual pass affair.

                  I had some troubles with that condenser, I accidently found that they were painting them during manfacture BEFORE putting the sealing caps on them. The paint ended up clogging the OT, My last trouble had me about to throw in the towel. As I disassembled the Blazer, I noticed that the green o rings were now silver. The color of the paint!
                  I took a Q tip and swabbed the inside of the tubes of a new condenser... It came out full of paint!

                  That is when I decided to make the switch to a PF generic condenser

                  So onto the CC switch. I read of a test where you unplug the blower and run the AC. With gauges hooked up, this will show the set points of the switch.

                  Mine was clicking off at 25 and on at 26! That accounts for the fast flickering pilot light I wrote about.
                  Interestingly, there IS a switch with those specs installed on the liquid line. It is not the same switch though. The threads are different, and it is all metal vs the plastic of the accumulator switch.

                  So I went and bought another new switch... It works perfectly according to specs. Off at 28, on at 45-48.

                  So I then took it for a short 300 yard drive with my gauges and their 10' lines hooked up. It short cycled pretty good . It was a pretty cool morning out around 78 degrees.
                  I hooked up a can of freon and went for a drive. I added freon a little at a time while driving until the cycling subsided to just an occasional cycle.

                  I then quit and removed the gauges. I took it for several drives as the day heated up to 108 and it never cycled. Vent temps were 41 to 44.

                  The next morning (3:30 AM) on the way to work. 82 degrees out. It cycled sporadically with vent temps at 41 to 43 degrees. On the way home, 105 or so, no cycling.

                  I ended up adding about 9 ozs of the freon to it. But I did not drain the lines back into the system at disconnect. So this amount needed to be accounted for, plus the little bit I purged the lines with when hooking the gauges up.

                  A little googling and i found the formula. A 10 foot liquid line holds 3.2 oz. Probably wasted another oz or so purging the lines. That makes for about 5 oz added to the system over what it was.
                  Originally I had probably 34 oz in it, so it is probably 39 now. Remember it now has a PF condenser, which is a much smaller physical size.

                  At this point I am happy with it.
                  Last edited by huntindog; 07-02-2017, 09:38 PM.


                    Good news, and thanks for letting us know how it turned out, that is how we all learn.