docmirror

Compressor cycling is a mix of the temp slider position, the position of the HVAC selector, and the sampled outside air, and sampled interior air from that little round dash vent. The AC will run the compressor so that the desired temp on the slider is maintained. There is a delta temp to determine how often, and how long the compressor runs, but I do not know the parameters of that delta(change) in temp. I would guess, and this is only a guess that when the set temp of say 72F is reached, the head unit will shut the comp off, and the sampled air inside is allowed to rise to maybe 75 when the head unit restarts the comp and it runs until 71F is reached, and the comp is cycled off. Sort of like your home thermostat.

in TX we just move the slider all the way to the blue, and pray the compressor doesn't give out before Oct.

merchauser

thank you again, for the schooling; still curious about what actually cuts the 12v to the compressor. guessing the freeze and pressure switches are there for safety concerns and are not the components that cut the power to the clutch? so if power comes from the head unit relay, is it the relay that "cycles" the compressor?

docmirror

Yes, and yes. an overpressure will damage components of the AC system, and underpressure same. There is a comparitor circuit inside the control unit which reads the various sensor inputs and determines the schedule for the AC comp. When the signal is sent to the head unit, the AC relay will be picked(energized) and the 12V from the contacts to the comp clutch is then energized. The orig relay is undersized for the application, and the contacts fail in common failure mode. Hence the need for ext larger relay.

merchauser

^^^that's what I was looking for. thank you.

in my daily cars, I never notice that the compressor is cycling, both in sound, but more importantly with cabin temperature. is that the same with the 928?
asking because when my compressor has been shut down, I immediately feel a difference in the cabin temp. when operating properly, should the 928 AC
operate like my daily driver, and the air thru the dash vent will still feel cool even though, the clutch might not be engaged?

docmirror

Oye. You are testing the limits of my patience, son. Also testing the limits of my knowledge of the AC system on the 928.

So, the short answer is yes, but the longer answer is - it depends. Originally, the 928 HVAC system was setup to 'blend' air, just as modern cars do with a 'soft' change in AC cooling and water heating. However(always one of those), this blending of air temps likely worked well in a perfectly new, fully functional, clean 928 HVAC system. If the entire system were fully restored, and operating in optimal climate, neither boiling TX heat, nor freezing Fargo cold(fave movie!), you would not notice the soft change in temp.

Here's another difference. You DD has an 'auto' setting which also controls the blower speed. We do NOT have that in the 928 world. When you get in your DD in Aug after sitting in the sun, the blower will come on max, and the AC will come on max, and you will very well feel the difference right away. In the 928, you will move the temp slider all the way blue, and will set the blower manually to 4. Alternately, in a cold climate, the DD will not set the blower on high until the temp of the water has reached a certain temp. In the 928 again, you will move the slider to the red, and set the blower on 4, and after a while you'll get max heat, but with the 928 it will blow freezing cold air at you until the engine warms.

As for the cool feel with the clutch not engaged, this is residual thermal difference, where the evaporator is still cool, and the blower is still extracting cool air. There is a hysteresis effect in both cooling of AC and heating with the water valve closed. This also is affected by the blending system, and makes the temp change more subtle.

worf928

docmirror

Boy am I sorry to step on your thread. Please note if you will, I have not mentioned the freeze switch(found in grid F31), but made sure I referenced the overpressure and underpressure switch found in grid P32 of the print in my answer to the question of what controls the operation of the AC clutch. It certainly is true that the freeze switch MAY interrupt the power to the clutch, but I seriously doubt it is the control mechanism which is by design the way to manage the temperature in the car.

The way the question was asked begs interpretation, which is one reason I avoided using the freeze switch in my answer: " still curious about what actually cuts the 12v to the compressor. guessing the freeze and pressure switches are there for safety concerns and are not the components that cut the power to the clutch? The best way to answer this question is that yes, the freeze switch is there for safety(more accurately efficiency), and yes it is a component which WILL cut the power to the switch.

Having said that, the freeze switch is for protection of the efficiency of the AC system. Safety is not perfectly accurate, but it's close enough. By the nomenclature of the freeze switch, I'm fairly sure that Porsche designers set the pressures, evaporator size, air flow, thermal expansion valve, and fluid volume so that the evap coil will generally operate in the non-icing condition. "Thermo-switch evaporator freezing protection" indicates to me that it is a secondary protection mechanism in case the controller, sensors, evap thermo expansion valve, or other parameter fail to operate in a desired manner. The AC controller will operate the clutch within the parameters that prevent evap icing in NORMAL mode. If something goes wrong, and icing occurs, then yes, the freeze switch will cut power to the clutch, and stop the expansion of the fluid which causes the evap to ice up.

BTW, since we're offering up our curriculum vitae, while I don't have an HVAC certification, I've installed home AC systems in multiple rental homes, several vintage cars, and yes - I've worked on 928 AC systems even thought I don't like it. As an addendum to this, I've taught basic electronic theory, advanced math, including rate functions, physics, with the typical heat, light, thermodynamics, etc, and analog circuits, among other courses(the "doc" in my nom-de-guerre).

if you're really that put out, I will delete all my info here, including where I made some egregious error in technical detail. Please point out those offending entries, and I'll get right on it.

worf928

Doc, this isn't 'my' thread. My post was not in response to anything you wrote. I'm not put out by anything you've written. Not in the slightest. All of your stuff has been spot on. Others, in the future, may benefit from what you've posted. You have ZERO need to apologize to me and I have no issue with anything you've posted. (And in any case, we've not posted overlapping info.) The OP has a predilection (seen in other threads) to disbelieve answers he doesn't like.

As for the freeze switch: It's common failure mode is always closed. Thus leading to icing and poor efficiency. OP's posted symptoms seem (to me at least, but maybe I mis-read) consistent with that failure mode (and/or intermittent head relay failure (which I have not once ever encountered but will not rule out)) but rather than testing he's decided that the switch cannot cause his symptoms because it can't stop the clutch from engaging and thus has led you down the relay replacement rabbit hole before confirming HVAC head death via a current-carrying test of the clutch circuit.

I would have been better off to just have posted an Amazon link to a book on automotive A/C or to have not posted in this thread at all.

docmirror

I prefer preventative replacement of the under-sized head relay. Seen plenty of failures, including two on my own cars. But - that's just my opinion.

As for the freeze switch, it should not control the operation of the AC clutch, unless some other sub-system of the AC is out of whack, which is all too common with the age and mileage on our 928s. The freeze switch is easy to test, but hard to get access. Buy a can of "Freeze-mist" aerosol or similar. Remove one wire(either pin) to the freeze switch. Attach multimeter to freeze switch pins, set the meter for ohms, lowest scale. The meter should read less than 1 ohm(dead short) in normal mode. Spray the Freeze-mist at the sensor probe of the freeze switch, it will drop temp, and at some point the multimeter will show an open circuit as the setpoint is reached. If the switch does not show an open circuit is defective and should be replaced.

Dave, apparently I misinterpreted the "if you can" statement right after my post #50. It read like a challenge to my ability to explain the workings of the AC, after providing theory, and some practical advice. We're all good.

worf928

It's not just your opinion. While I haven't prioritized the time to practice my soldering skills so that I can replaced the relay myself, every 928 that comes to me for work in the scope of a comprehensive "make it reliable" work plan is getting the 4A fuse and an HVAC rebuild. I've sent a bunch to Greg in the last year or so.

Maybe. I will not opine in any detail on the subject of the HVAC head cycling the AC clutch to maintain the slider-set temperature if all other parts of the control system's inputs are good. But, my observations of 928s suggest that on a hot sunny humid summer day (for example Florida), the AC system, even when in tip-top condition has a hard time keeping the cabin at refrigerator temperatures. If the refrigeration portion of the system (as opposed to the control part of the system) is functioning well (i.e. sub-freezing vent temps) the evaporator will ice up within minutes if the freeze switch is dead. On my '91, with 25-degree vent temps, I am the freeze switch because I've decided not to replace the POS freeze switch for the fourth time.... (which btw, need to be calibrated with ice water and a screwdriver when they come out of the box) . The 928 just doesn't seem to have enough heat exchange capacity to keep the cabin at ~70 when it's sunny and 105 outside. The natural reaction is to move the slider all the way to the left which - I would think - bypasses temp control such as it may exist in the head.

Usually easy to test. Access is hard? Last summer I had to fix an A/C system twice because the spade connectors on the freeze switch were making intermittent contact internal to the switch housing. The A/C functioned perfectly for 10-15 minutes then would cut out. Until you removed the connector on the freeze switch to check for voltage when it would miraculously fix itself. The first repair was a rebuilt head because the relay failed in the common 'shows 12v but doesn't light a test bulb' mode. (Which test, btw, the OP has not done, it seems.)

See above note on calibration. I've seen new switches that opened at 40 degrees while others at <30. I adjust them to open at ~34. The freeze can will tell you if it's dead but not if its opening at a reasonable temperature.

No. The challenge was to your patience not to your knowledge.

worf928

And, we haven't even touched upon the subject of the evap being full of mouse poo and leaves. If there's no air flow through the evap it will freeze if the refrigeration system is working (and the freeze switch is dead) .... 'cause there's no heat exchange.

merchauser

false: fake news. I have no automotive background, but love to learn from all of you who have an incredible wealth of knowledge. if my questions are redundant,
it may be age related, and any semblance of disbelief is merely my attempt to fully comprehend and gain a greater education. big thanks to everyone that help.

FredR

You should run for residency at 1600 Pennsylvania Avenue- your country desperately needs you!

merchauser

^^^ha ha thanks fred

merchauser

the relay that I found in my head unit has no blue (or yellow) cover on it and looks like it may have previously been replaced. it also has 6 pins.
I thought I read the original had 8? the base surrounding coil is grey in color. does this look correct?



when was the yellow relay introduced? they seem to have different configurations than the blue ones regarding how they function?

my board is dated 1989, and DOES SHARE the same part number as the one on Dwayne's (1988) write up, but looks different in the way
some of the foil is routed.

finally, I took my unit to a local trusted electronics shop to have the new relay installed. when I picked it up, I was told that they changed the pin mapping because the order
I provided "would have damaged the system." OK...…..so I got home and plugged it in, and there was no voltage at the compressor, and traveling backward, none at the pressure
switch, and none at the freeze switch. (fans worked properly) went back to the shop, and they reinstalled the old 6 pin relay. (also tested the new 10A relay, and it works fine.)
back in the car, and everything works as it should, with the old balky relay.

the electronics shop said they wired the new relay exactly like the function of the old relay, and it should be fine. its just a relay, so this should not be a big deal to wire up.

does the pin mapping from the rennlist write ups match the way the original relay works, or is it intentionally wired a different way?