The A/C compressor clutch won't engage unless I power it directly. System has been rebuilt and charged, so refrigerant is fine. It does blow cold air when powered directly. Fuses are good. Without DEF selected ('72 model), I only get heat on the floor. What would be the first things you'd check?
Lovehugs Rags
Sounds like the Thermal Fuse on the Evaporator box could be blown. This is designed to blow if the Compressor loses refrigerant, and the blown fuse saves the Compressor from self destruction.
Bruce. >:D
Plz C pictures
I tried to bypass it with no effect on coil,
Then I gave the center spade 12v and the blower motor relay clicked and fan started.
(https://i.postimg.cc/5yd13YZF/IMG-8192.jpg)
(https://i.postimg.cc/x124PMnp/IMG-8191.jpg)
That is the thermal fuse. If you were gonna jump it you would jump B and C. There is a small heating element between S and C (the outside terminals). I don't recall the ohms but it should be a fairly low value like under 100 ohms. If its good/not blown B and C should read zero ohms.
Power came in on B (middle terminal) from the control system and normally flowed out C to the clutch. If the system was in distress a pressure switch grounded out and sent a ground to the S terminal. Since the heating element was connected to C which had power it would begin to heat. If it was on for long enough, I think minute to minutes? it would literally melt the fuse so power would no longer be being sent to the clutch (or the internal heating element).
It was basically a time delay circuit because during normal operation its normal to get pressure blips and you didn't want the compressor to cut out every time this happened. With this delay it would only cut out when something was really wrong and at that point you needed a major service so after the repair was made, possibly just a recharge the tech would replace the fuse and you were good to go again.
Blower isn't kicking on either? That seems like it could be a control issue. Does a 72 have a vacuum switch where if there isn't a vacuum supply to the climate control system it kills a bunch of the electrical? Only getting air out the floor could be a clue too except I thought lack of vacuum it would default to the defrost to the windshield.
Maybe snoop around the firewall and engine looking close at vacuum lines that go through the firewall? I know some years the same vacuum that fed the auto parking brake release also did the climate control so people would just go eh, I never use the parking brake and not bother to connect or repair the vacuum hose that was over in that area.
I didle a bit with the AC between other tasks. Just ordered the Indoor outdoor sensors, switches and some what. I will get back to this topic. Thx again
There isn't much to the sensors and I don't think they are usually the issue. They are also easy to test. What usually goes on them as they got older is the amplifier and or the I think they called it a transducer. The transducer takes the signal from the amplifier and modulates a vacuum signal that then drives a vacuum servo that then runs a vacuum distributor valve sort of thing as well as the blower speed. That vacuum distributor runs other air doors and the water control valve if a 72 has one.
The systems varied a bit year to year but basically all the sensors and the control wheel as well as a feedback sensor hooked to that main servo are all just variable resistors in a big loop. The feed a signal into the amplifier which then gets booted strong enough to run that transducer. All those devices in the loop can be tested in the loop or individually with an ohm meter.
I hope i can continue the work on the AC this weekend. It's not hot at all so not first priority. Il order Tim's guide and a then a new controller if needed.
Tim's book is nice because it was written 20 some years later with the common issues at that age in mind and using more commonly available at the time test equipment and tools. The factory manuals may have been written based on pre production hardware that had not been in used and aged yet.
I know I have looked at factory service manuals for products and got a bit scared seeing many pages full of specs showing how to calibrate some subsystem using all sorts of tools then asking someone that has been working on them forever how often that system has actually needed calibration and they say never. That is probably when and where the value engineers get involved to make the next version faster and cheaper and just good enough to make it to the end of the warranty period.
If Tim is reading this or anyone has bought any of his manuals more recently have they been updated? I would imagine there may be different tools and products available now and with more age I would think some different sorts of failures may be happening.